CVE-2025-53110

@@ -6156,7 +6156,7 @@
     },
     "src/filesystem": {
       "name": "@modelcontextprotocol/server-filesystem",
-      "version": "0.6.2",
+      "version": "0.6.3",
       "license": "MIT",
       "dependencies": {
         "@modelcontextprotocol/sdk": "^1.12.3",

@@ -10,10 +10,12 @@ import {
 import fs from "fs/promises";
 import path from "path";
 import os from 'os';
+import { randomBytes } from 'crypto';
 import { z } from "zod";
 import { zodToJsonSchema } from "zod-to-json-schema";
 import { diffLines, createTwoFilesPatch } from 'diff';
 import { minimatch } from 'minimatch';
+import { isPathWithinAllowedDirectories } from './path-validation.js';
 
 // Command line argument parsing
 const args = process.argv.slice(2);
@@ -34,9 +36,21 @@ function expandHome(filepath: string): string {
   return filepath;
 }
 
-// Store allowed directories in normalized form
-const allowedDirectories = args.map(dir =>
-  normalizePath(path.resolve(expandHome(dir)))
+// Store allowed directories in normalized and resolved form
+const allowedDirectories = await Promise.all(
+  args.map(async (dir) => {
+    const expanded = expandHome(dir);
+    const absolute = path.resolve(expanded);
+    try {
+      // Resolve symlinks in allowed directories during startup
+      const resolved = await fs.realpath(absolute);
+      return normalizePath(resolved);
+    } catch (error) {
+      // If we can't resolve (doesn't exist), use the normalized absolute path
+      // This allows configuring allowed dirs that will be created later
+      return normalizePath(absolute);
+    }
+  })
 );
 
 // Validate that all directories exist and are accessible
@@ -63,7 +77,7 @@ async function validatePath(requestedPath: string): Promise<string> {
   const normalizedRequested = normalizePath(absolute);
 
   // Check if path is within allowed directories
-  const isAllowed = allowedDirectories.some(dir => normalizedRequested.startsWith(dir));
+  const isAllowed = isPathWithinAllowedDirectories(normalizedRequested, allowedDirectories);
   if (!isAllowed) {
     throw new Error(`Access denied - path outside allowed directories: ${absolute} not in ${allowedDirectories.join(', ')}`);
   }
@@ -72,25 +86,26 @@ async function validatePath(requestedPath: string): Promise<string> {
   try {
     const realPath = await fs.realpath(absolute);
     const normalizedReal = normalizePath(realPath);
-    const isRealPathAllowed = allowedDirectories.some(dir => normalizedReal.startsWith(dir));
-    if (!isRealPathAllowed) {
-      throw new Error("Access denied - symlink target outside allowed directories");
+    if (!isPathWithinAllowedDirectories(normalizedReal, allowedDirectories)) {
+      throw new Error(`Access denied - symlink target outside allowed directories: ${realPath} not in ${allowedDirectories.join(', ')}`);
     }
     return realPath;
   } catch (error) {
     // For new files that don't exist yet, verify parent directory
-    const parentDir = path.dirname(absolute);
-    try {
-      const realParentPath = await fs.realpath(parentDir);
-      const normalizedParent = normalizePath(realParentPath);
-      const isParentAllowed = allowedDirectories.some(dir => normalizedParent.startsWith(dir));
-      if (!isParentAllowed) {
-        throw new Error("Access denied - parent directory outside allowed directories");
+    if ((error as NodeJS.ErrnoException).code === 'ENOENT') {
+      const parentDir = path.dirname(absolute);
+      try {
+        const realParentPath = await fs.realpath(parentDir);
+        const normalizedParent = normalizePath(realParentPath);
+        if (!isPathWithinAllowedDirectories(normalizedParent, allowedDirectories)) {
+          throw new Error(`Access denied - parent directory outside allowed directories: ${realParentPath} not in ${allowedDirectories.join(', ')}`);
+        }
+        return absolute;
+      } catch {
+        throw new Error(`Parent directory does not exist: ${parentDir}`);
       }
-      return absolute;
-    } catch {
-      throw new Error(`Parent directory does not exist: ${parentDir}`);
     }
+    throw error;
   }
 }
 
@@ -331,7 +346,19 @@ async function applyFileEdits(
   const formattedDiff = `${'`'.repeat(numBackticks)}diff\n${diff}${'`'.repeat(numBackticks)}\n\n`;
 
   if (!dryRun) {
-    await fs.writeFile(filePath, modifiedContent, 'utf-8');
+    // Security: Use atomic rename to prevent race conditions where symlinks
+    // could be created between validation and write. Rename operations
+    // replace the target file atomically and don't follow symlinks.
+    const tempPath = `${filePath}.${randomBytes(16).toString('hex')}.tmp`;
+    try {
+      await fs.writeFile(tempPath, modifiedContent, 'utf-8');
+      await fs.rename(tempPath, filePath);
+    } catch (error) {
+      try {
+        await fs.unlink(tempPath);
+      } catch {}
+      throw error;
+    }
   }
 
   return formattedDiff;
@@ -625,7 +652,31 @@ server.setRequestHandler(CallToolRequestSchema, async (request) => {
           throw new Error(`Invalid arguments for write_file: ${parsed.error}`);
         }
         const validPath = await validatePath(parsed.data.path);
-        await fs.writeFile(validPath, parsed.data.content, "utf-8");
+
+        try {
+          // Security: 'wx' flag ensures exclusive creation - fails if file/symlink exists,
+          // preventing writes through pre-existing symlinks
+          await fs.writeFile(validPath, parsed.data.content, { encoding: "utf-8", flag: 'wx' });
+        } catch (error) {
+          if ((error as NodeJS.ErrnoException).code === 'EEXIST') {
+            // Security: Use atomic rename to prevent race conditions where symlinks
+            // could be created between validation and write. Rename operations
+            // replace the target file atomically and don't follow symlinks.
+            const tempPath = `${validPath}.${randomBytes(16).toString('hex')}.tmp`;
+            try {
+              await fs.writeFile(tempPath, parsed.data.content, 'utf-8');
+              await fs.rename(tempPath, validPath);
+            } catch (renameError) {
+              try {
+                await fs.unlink(tempPath);
+              } catch {}
+              throw renameError;
+            }
+          } else {
+            throw error;
+          }
+        }
+
         return {
           content: [{ type: "text", text: `Successfully wrote to ${parsed.data.path}` }],
         };

@@ -0,0 +1,77 @@
+import path from 'path';
+
+/**
+ * Checks if an absolute path is within any of the allowed directories.
+ * 
+ * @param absolutePath - The absolute path to check (will be normalized)
+ * @param allowedDirectories - Array of absolute allowed directory paths (will be normalized)
+ * @returns true if the path is within an allowed directory, false otherwise
+ * @throws Error if given relative paths after normalization
+ */
+export function isPathWithinAllowedDirectories(absolutePath: string, allowedDirectories: string[]): boolean {
+  // Type validation
+  if (typeof absolutePath !== 'string' || !Array.isArray(allowedDirectories)) {
+    return false;
+  }
+
+  // Reject empty inputs
+  if (!absolutePath || allowedDirectories.length === 0) {
+    return false;
+  }
+
+  // Reject null bytes (forbidden in paths)
+  if (absolutePath.includes('\x00')) {
+    return false;
+  }
+
+  // Normalize the input path
+  let normalizedPath: string;
+  try {
+    normalizedPath = path.resolve(path.normalize(absolutePath));
+  } catch {
+    return false;
+  }
+
+  // Verify it's absolute after normalization
+  if (!path.isAbsolute(normalizedPath)) {
+    throw new Error('Path must be absolute after normalization');
+  }
+
+  // Check against each allowed directory
+  return allowedDirectories.some(dir => {
+    if (typeof dir !== 'string' || !dir) {
+      return false;
+    }
+
+    // Reject null bytes in allowed dirs
+    if (dir.includes('\x00')) {
+      return false;
+    }
+
+    // Normalize the allowed directory
+    let normalizedDir: string;
+    try {
+      normalizedDir = path.resolve(path.normalize(dir));
+    } catch {
+      return false;
+    }
+
+    // Verify allowed directory is absolute after normalization
+    if (!path.isAbsolute(normalizedDir)) {
+      throw new Error('Allowed directories must be absolute paths after normalization');
+    }
+
+    // Check if normalizedPath is within normalizedDir
+    // Path is inside if it's the same or a subdirectory
+    if (normalizedPath === normalizedDir) {
+      return true;
+    }
+    
+    // Special case for root directory to avoid double slash
+    if (normalizedDir === path.sep) {
+      return normalizedPath.startsWith(path.sep);
+    }
+    
+    return normalizedPath.startsWith(normalizedDir + path.sep);
+  });
+}
\ No newline at end of file

@@ -0,0 +1,844 @@
+import { describe, it, expect, beforeEach, afterEach } from '@jest/globals';
+import * as path from 'path';
+import * as fs from 'fs/promises';
+import * as os from 'os';
+import { isPathWithinAllowedDirectories } from '../path-validation.js';
+
+describe('Path Validation', () => {
+  it('allows exact directory match', () => {
+    const allowed = ['/home/user/project'];
+    expect(isPathWithinAllowedDirectories('/home/user/project', allowed)).toBe(true);
+  });
+
+  it('allows subdirectories', () => {
+    const allowed = ['/home/user/project'];
+    expect(isPathWithinAllowedDirectories('/home/user/project/src', allowed)).toBe(true);
+    expect(isPathWithinAllowedDirectories('/home/user/project/src/index.js', allowed)).toBe(true);
+    expect(isPathWithinAllowedDirectories('/home/user/project/deeply/nested/file.txt', allowed)).toBe(true);
+  });
+
+  it('blocks similar directory names (prefix vulnerability)', () => {
+    const allowed = ['/home/user/project'];
+    expect(isPathWithinAllowedDirectories('/home/user/project2', allowed)).toBe(false);
+    expect(isPathWithinAllowedDirectories('/home/user/project_backup', allowed)).toBe(false);
+    expect(isPathWithinAllowedDirectories('/home/user/project-old', allowed)).toBe(false);
+    expect(isPathWithinAllowedDirectories('/home/user/projectile', allowed)).toBe(false);
+    expect(isPathWithinAllowedDirectories('/home/user/project.bak', allowed)).toBe(false);
+  });
+
+  it('blocks paths outside allowed directories', () => {
+    const allowed = ['/home/user/project'];
+    expect(isPathWithinAllowedDirectories('/home/user/other', allowed)).toBe(false);
+    expect(isPathWithinAllowedDirectories('/etc/passwd', allowed)).toBe(false);
+    expect(isPathWithinAllowedDirectories('/home/user', allowed)).toBe(false);
+    expect(isPathWithinAllowedDirectories('/', allowed)).toBe(false);
+  });
+
+  it('handles multiple allowed directories', () => {
+    const allowed = ['/home/user/project1', '/home/user/project2'];
+    expect(isPathWithinAllowedDirectories('/home/user/project1/src', allowed)).toBe(true);
+    expect(isPathWithinAllowedDirectories('/home/user/project2/src', allowed)).toBe(true);
+    expect(isPathWithinAllowedDirectories('/home/user/project3', allowed)).toBe(false);
+    expect(isPathWithinAllowedDirectories('/home/user/project1_backup', allowed)).toBe(false);
+    expect(isPathWithinAllowedDirectories('/home/user/project2-old', allowed)).toBe(false);
+  });
+
+  it('blocks parent and sibling directories', () => {
+    const allowed = ['/test/allowed'];
+
+    // Parent directory
+    expect(isPathWithinAllowedDirectories('/test', allowed)).toBe(false);
+    expect(isPathWithinAllowedDirectories('/', allowed)).toBe(false);
+
+    // Sibling with common prefix
+    expect(isPathWithinAllowedDirectories('/test/allowed_sibling', allowed)).toBe(false);
+    expect(isPathWithinAllowedDirectories('/test/allowed2', allowed)).toBe(false);
+  });
+
+  it('handles paths with special characters', () => {
+    const allowed = ['/home/user/my-project (v2)'];
+
+    expect(isPathWithinAllowedDirectories('/home/user/my-project (v2)', allowed)).toBe(true);
+    expect(isPathWithinAllowedDirectories('/home/user/my-project (v2)/src', allowed)).toBe(true);
+    expect(isPathWithinAllowedDirectories('/home/user/my-project (v2)_backup', allowed)).toBe(false);
+    expect(isPathWithinAllowedDirectories('/home/user/my-project', allowed)).toBe(false);
+  });
+
+  describe('Input validation', () => {
+    it('rejects empty inputs', () => {
+      const allowed = ['/home/user/project'];
+
+      expect(isPathWithinAllowedDirectories('', allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories('/home/user/project', [])).toBe(false);
+    });
+
+    it('handles trailing separators correctly', () => {
+      const allowed = ['/home/user/project'];
+
+      // Path with trailing separator should still match
+      expect(isPathWithinAllowedDirectories('/home/user/project/', allowed)).toBe(true);
+
+      // Allowed directory with trailing separator
+      const allowedWithSep = ['/home/user/project/'];
+      expect(isPathWithinAllowedDirectories('/home/user/project', allowedWithSep)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/project/', allowedWithSep)).toBe(true);
+
+      // Should still block similar names with or without trailing separators
+      expect(isPathWithinAllowedDirectories('/home/user/project2', allowedWithSep)).toBe(false);
+      expect(isPathWithinAllowedDirectories('/home/user/project2', allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories('/home/user/project2/', allowed)).toBe(false);
+    });
+
+    it('skips empty directory entries in allowed list', () => {
+      const allowed = ['', '/home/user/project', ''];
+      expect(isPathWithinAllowedDirectories('/home/user/project', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/project/src', allowed)).toBe(true);
+
+      // Should still validate properly with empty entries
+      expect(isPathWithinAllowedDirectories('/home/user/other', allowed)).toBe(false);
+    });
+
+    it('handles Windows paths with trailing separators', () => {
+      if (path.sep === '\\') {
+        const allowed = ['C:\\Users\\project'];
+
+        // Path with trailing separator
+        expect(isPathWithinAllowedDirectories('C:\\Users\\project\\', allowed)).toBe(true);
+
+        // Allowed with trailing separator
+        const allowedWithSep = ['C:\\Users\\project\\'];
+        expect(isPathWithinAllowedDirectories('C:\\Users\\project', allowedWithSep)).toBe(true);
+        expect(isPathWithinAllowedDirectories('C:\\Users\\project\\', allowedWithSep)).toBe(true);
+
+        // Should still block similar names
+        expect(isPathWithinAllowedDirectories('C:\\Users\\project2\\', allowed)).toBe(false);
+      }
+    });
+  });
+
+  describe('Error handling', () => {
+    it('normalizes relative paths to absolute', () => {
+      const allowed = [process.cwd()];
+
+      // Relative paths get normalized to absolute paths based on cwd
+      expect(isPathWithinAllowedDirectories('relative/path', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('./file', allowed)).toBe(true);
+
+      // Parent directory references that escape allowed directory
+      const parentAllowed = ['/home/user/project'];
+      expect(isPathWithinAllowedDirectories('../parent', parentAllowed)).toBe(false);
+    });
+
+    it('returns false for relative paths in allowed directories', () => {
+      const badAllowed = ['relative/path', '/some/other/absolute/path'];
+
+      // Relative paths in allowed dirs are normalized to absolute based on cwd
+      // The normalized 'relative/path' won't match our test path
+      expect(isPathWithinAllowedDirectories('/some/other/absolute/path/file', badAllowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/absolute/path/file', badAllowed)).toBe(false);
+    });
+
+    it('handles null and undefined inputs gracefully', () => {
+      const allowed = ['/home/user/project'];
+
+      // Should return false, not crash
+      expect(isPathWithinAllowedDirectories(null as any, allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories(undefined as any, allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories('/path', null as any)).toBe(false);
+      expect(isPathWithinAllowedDirectories('/path', undefined as any)).toBe(false);
+    });
+  });
+
+  describe('Unicode and special characters', () => {
+    it('handles unicode characters in paths', () => {
+      const allowed = ['/home/user/café'];
+
+      expect(isPathWithinAllowedDirectories('/home/user/café', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/café/file', allowed)).toBe(true);
+
+      // Different unicode representation won't match (not normalized)
+      const decomposed = '/home/user/cafe\u0301'; // e + combining accent
+      expect(isPathWithinAllowedDirectories(decomposed, allowed)).toBe(false);
+    });
+
+    it('handles paths with spaces correctly', () => {
+      const allowed = ['/home/user/my project'];
+
+      expect(isPathWithinAllowedDirectories('/home/user/my project', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/my project/file', allowed)).toBe(true);
+
+      // Partial matches should fail
+      expect(isPathWithinAllowedDirectories('/home/user/my', allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories('/home/user/my proj', allowed)).toBe(false);
+    });
+  });
+
+  describe('Overlapping allowed directories', () => {
+    it('handles nested allowed directories correctly', () => {
+      const allowed = ['/home', '/home/user', '/home/user/project'];
+
+      // All paths under /home are allowed
+      expect(isPathWithinAllowedDirectories('/home/anything', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/anything', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/project/anything', allowed)).toBe(true);
+
+      // First match wins (most permissive)
+      expect(isPathWithinAllowedDirectories('/home/other/deep/path', allowed)).toBe(true);
+    });
+
+    it('handles root directory as allowed', () => {
+      const allowed = ['/'];
+
+      // Everything is allowed under root (dangerous configuration)
+      expect(isPathWithinAllowedDirectories('/', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/any/path', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/etc/passwd', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/secret', allowed)).toBe(true);
+
+      // But only on the same filesystem root
+      if (path.sep === '\\') {
+        expect(isPathWithinAllowedDirectories('D:\\other', ['/'])).toBe(false);
+      }
+    });
+  });
+
+  describe('Cross-platform behavior', () => {
+    it('handles Windows-style paths on Windows', () => {
+      if (path.sep === '\\') {
+        const allowed = ['C:\\Users\\project'];
+        expect(isPathWithinAllowedDirectories('C:\\Users\\project', allowed)).toBe(true);
+        expect(isPathWithinAllowedDirectories('C:\\Users\\project\\src', allowed)).toBe(true);
+        expect(isPathWithinAllowedDirectories('C:\\Users\\project2', allowed)).toBe(false);
+        expect(isPathWithinAllowedDirectories('C:\\Users\\project_backup', allowed)).toBe(false);
+      }
+    });
+
+    it('handles Unix-style paths on Unix', () => {
+      if (path.sep === '/') {
+        const allowed = ['/home/user/project'];
+        expect(isPathWithinAllowedDirectories('/home/user/project', allowed)).toBe(true);
+        expect(isPathWithinAllowedDirectories('/home/user/project/src', allowed)).toBe(true);
+        expect(isPathWithinAllowedDirectories('/home/user/project2', allowed)).toBe(false);
+      }
+    });
+  });
+
+  describe('Validation Tests - Path Traversal', () => {
+    it('blocks path traversal attempts', () => {
+      const allowed = ['/home/user/project'];
+
+      // Basic traversal attempts
+      expect(isPathWithinAllowedDirectories('/home/user/project/../../../etc/passwd', allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories('/home/user/project/../../other', allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories('/home/user/project/../project2', allowed)).toBe(false);
+
+      // Mixed traversal with valid segments
+      expect(isPathWithinAllowedDirectories('/home/user/project/src/../../project2', allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories('/home/user/project/./../../other', allowed)).toBe(false);
+
+      // Multiple traversal sequences
+      expect(isPathWithinAllowedDirectories('/home/user/project/../project/../../../etc', allowed)).toBe(false);
+    });
+
+    it('blocks traversal in allowed directories', () => {
+      const allowed = ['/home/user/project/../safe'];
+
+      // The allowed directory itself should be normalized and safe
+      expect(isPathWithinAllowedDirectories('/home/user/safe/file', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/project/file', allowed)).toBe(false);
+    });
+
+    it('handles complex traversal patterns', () => {
+      const allowed = ['/home/user/project'];
+
+      // Double dots in filenames (not traversal) - these normalize to paths within allowed dir
+      expect(isPathWithinAllowedDirectories('/home/user/project/..test', allowed)).toBe(true); // Not traversal
+      expect(isPathWithinAllowedDirectories('/home/user/project/test..', allowed)).toBe(true); // Not traversal
+      expect(isPathWithinAllowedDirectories('/home/user/project/te..st', allowed)).toBe(true); // Not traversal
+
+      // Actual traversal
+      expect(isPathWithinAllowedDirectories('/home/user/project/../test', allowed)).toBe(false); // Is traversal - goes to /home/user/test
+
+      // Edge case: /home/user/project/.. normalizes to /home/user (parent dir)
+      expect(isPathWithinAllowedDirectories('/home/user/project/..', allowed)).toBe(false); // Goes to parent
+    });
+  });
+
+  describe('Validation Tests - Null Bytes', () => {
+    it('rejects paths with null bytes', () => {
+      const allowed = ['/home/user/project'];
+
+      expect(isPathWithinAllowedDirectories('/home/user/project\x00/etc/passwd', allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories('/home/user/project/test\x00.txt', allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories('\x00/home/user/project', allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories('/home/user/project/\x00', allowed)).toBe(false);
+    });
+
+    it('rejects allowed directories with null bytes', () => {
+      const allowed = ['/home/user/project\x00'];
+
+      expect(isPathWithinAllowedDirectories('/home/user/project', allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories('/home/user/project/file', allowed)).toBe(false);
+    });
+  });
+
+  describe('Validation Tests - Special Characters', () => {
+    it('allows percent signs in filenames', () => {
+      const allowed = ['/home/user/project'];
+
+      // Percent is a valid filename character
+      expect(isPathWithinAllowedDirectories('/home/user/project/report_50%.pdf', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/project/Q1_25%_growth', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/project/%41', allowed)).toBe(true); // File named %41
+
+      // URL encoding is NOT decoded by path.normalize, so these are just odd filenames
+      expect(isPathWithinAllowedDirectories('/home/user/project/%2e%2e', allowed)).toBe(true); // File named "%2e%2e"
+      expect(isPathWithinAllowedDirectories('/home/user/project/file%20name', allowed)).toBe(true); // File with %20 in name
+    });
+
+    it('handles percent signs in allowed directories', () => {
+      const allowed = ['/home/user/project%20files'];
+
+      // This is a directory literally named "project%20files"
+      expect(isPathWithinAllowedDirectories('/home/user/project%20files/test', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/project files/test', allowed)).toBe(false); // Different dir
+    });
+  });
+
+  describe('Path Normalization', () => {
+    it('normalizes paths before comparison', () => {
+      const allowed = ['/home/user/project'];
+
+      // Trailing slashes
+      expect(isPathWithinAllowedDirectories('/home/user/project/', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/project//', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/project///', allowed)).toBe(true);
+
+      // Current directory references
+      expect(isPathWithinAllowedDirectories('/home/user/project/./src', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/./project/src', allowed)).toBe(true);
+
+      // Multiple slashes
+      expect(isPathWithinAllowedDirectories('/home/user/project//src//file', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home//user//project//src', allowed)).toBe(true);
+
+      // Should still block outside paths
+      expect(isPathWithinAllowedDirectories('/home/user//project2', allowed)).toBe(false);
+    });
+
+    it('handles mixed separators correctly', () => {
+      if (path.sep === '\\') {
+        const allowed = ['C:\\Users\\project'];
+
+        // Mixed separators should be normalized
+        expect(isPathWithinAllowedDirectories('C:/Users/project', allowed)).toBe(true);
+        expect(isPathWithinAllowedDirectories('C:\\Users/project\\src', allowed)).toBe(true);
+        expect(isPathWithinAllowedDirectories('C:/Users\\project/src', allowed)).toBe(true);
+      }
+    });
+  });
+
+  describe('Edge Cases', () => {
+    it('rejects non-string inputs safely', () => {
+      const allowed = ['/home/user/project'];
+
+      expect(isPathWithinAllowedDirectories(123 as any, allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories({} as any, allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories([] as any, allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories(null as any, allowed)).toBe(false);
+      expect(isPathWithinAllowedDirectories(undefined as any, allowed)).toBe(false);
+
+      // Non-string in allowed directories
+      expect(isPathWithinAllowedDirectories('/home/user/project', [123 as any])).toBe(false);
+      expect(isPathWithinAllowedDirectories('/home/user/project', [{} as any])).toBe(false);
+    });
+
+    it('handles very long paths', () => {
+      const allowed = ['/home/user/project'];
+
+      // Create a very long path that's still valid
+      const longSubPath = 'a/'.repeat(1000) + 'file.txt';
+      expect(isPathWithinAllowedDirectories(`/home/user/project/${longSubPath}`, allowed)).toBe(true);
+
+      // Very long path that escapes
+      const escapePath = 'a/'.repeat(1000) + '../'.repeat(1001) + 'etc/passwd';
+      expect(isPathWithinAllowedDirectories(`/home/user/project/${escapePath}`, allowed)).toBe(false);
+    });
+  });
+
+  describe('Additional Coverage', () => {
+    it('handles allowed directories with traversal that normalizes safely', () => {
+      // These allowed dirs contain traversal but normalize to valid paths
+      const allowed = ['/home/user/../user/project'];
+
+      // Should normalize to /home/user/project and work correctly
+      expect(isPathWithinAllowedDirectories('/home/user/project/file', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/other', allowed)).toBe(false);
+    });
+
+    it('handles symbolic dots in filenames', () => {
+      const allowed = ['/home/user/project'];
+
+      // Single and double dots as actual filenames (not traversal)
+      expect(isPathWithinAllowedDirectories('/home/user/project/.', allowed)).toBe(true);
+      expect(isPathWithinAllowedDirectories('/home/user/project/..', allowed)).toBe(false); // This normalizes to parent
+      expect(isPathWithinAllowedDirectories('/home/user/project/...', allowed)).toBe(true); // Three dots is a valid filename
+      expect(isPathWithinAllowedDirectories('/home/user/project/....', allowed)).toBe(true); // Four dots is a valid filename
+    });
+
+    it('handles UNC paths on Windows', () => {
+      if (path.sep === '\\') {
+        const allowed = ['\\\\server\\share\\project'];
+
+        expect(isPathWithinAllowedDirectories('\\\\server\\share\\project', allowed)).toBe(true);
+        expect(isPathWithinAllowedDirectories('\\\\server\\share\\project\\file', allowed)).toBe(true);
+        expect(isPathWithinAllowedDirectories('\\\\server\\share\\other', allowed)).toBe(false);
+        expect(isPathWithinAllowedDirectories('\\\\other\\share\\project', allowed)).toBe(false);
+      }
+    });
+  });
+
+  describe('Symlink Tests', () => {
+    let testDir: string;
+    let allowedDir: string;
+    let forbiddenDir: string;
+
+    beforeEach(async () => {
+      testDir = await fs.mkdtemp(path.join(os.tmpdir(), 'fs-error-test-'));
+      allowedDir = path.join(testDir, 'allowed');
+      forbiddenDir = path.join(testDir, 'forbidden');
+
+      await fs.mkdir(allowedDir, { recursive: true });
+      await fs.mkdir(forbiddenDir, { recursive: true });
+    });
+
+    afterEach(async () => {
+      await fs.rm(testDir, { recursive: true, force: true });
+    });
+
+    it('validates symlink handling', async () => {
+      // Test with symlinks
+      try {
+        const linkPath = path.join(allowedDir, 'bad-link');
+        const targetPath = path.join(forbiddenDir, 'target.txt');
+
+        await fs.writeFile(targetPath, 'content');
+        await fs.symlink(targetPath, linkPath);
+
+        // In real implementation, this would throw with the resolved path
+        const realPath = await fs.realpath(linkPath);
+        const allowed = [allowedDir];
+
+        // Symlink target should be outside allowed directory
+        expect(isPathWithinAllowedDirectories(realPath, allowed)).toBe(false);
+      } catch (error) {
+        // Skip if no symlink permissions
+      }
+    });
+
+    it('handles non-existent paths correctly', async () => {
+      const newFilePath = path.join(allowedDir, 'subdir', 'newfile.txt');
+
+      // Parent directory doesn't exist
+      try {
+        await fs.access(newFilePath);
+      } catch (error) {
+        expect((error as NodeJS.ErrnoException).code).toBe('ENOENT');
+      }
+
+      // After creating parent, validation should work
+      await fs.mkdir(path.dirname(newFilePath), { recursive: true });
+      const allowed = [allowedDir];
+      expect(isPathWithinAllowedDirectories(newFilePath, allowed)).toBe(true);
+    });
+
+    // Test path resolution consistency for symlinked files
+    it('validates symlinked files consistently between path and resolved forms', async () => {
+      try {
+        // Setup: Create target file in forbidden area
+        const targetFile = path.join(forbiddenDir, 'target.txt');
+        await fs.writeFile(targetFile, 'TARGET_CONTENT');
+
+        // Create symlink inside allowed directory pointing to forbidden file
+        const symlinkPath = path.join(allowedDir, 'link-to-target.txt');
+        await fs.symlink(targetFile, symlinkPath);
+
+        // The symlink path itself passes validation (looks like it's in allowed dir)
+        expect(isPathWithinAllowedDirectories(symlinkPath, [allowedDir])).toBe(true);
+
+        // But the resolved path should fail validation
+        const resolvedPath = await fs.realpath(symlinkPath);
+        expect(isPathWithinAllowedDirectories(resolvedPath, [allowedDir])).toBe(false);
+
+        // Verify the resolved path goes to the forbidden location (normalize both paths for macOS temp dirs)
+        expect(await fs.realpath(resolvedPath)).toBe(await fs.realpath(targetFile));
+      } catch (error) {
+        // Skip if no symlink permissions on the system
+        if ((error as NodeJS.ErrnoException).code !== 'EPERM') {
+          throw error;
+        }
+      }
+    });
+
+    // Test allowed directory resolution behavior
+    it('validates paths correctly when allowed directory is resolved from symlink', async () => {
+      try {
+        // Setup: Create the actual target directory with content
+        const actualTargetDir = path.join(testDir, 'actual-target');
+        await fs.mkdir(actualTargetDir, { recursive: true });
+        const targetFile = path.join(actualTargetDir, 'file.txt');
+        await fs.writeFile(targetFile, 'FILE_CONTENT');
+
+        // Setup: Create symlink directory that points to target
+        const symlinkDir = path.join(testDir, 'symlink-dir');
+        await fs.symlink(actualTargetDir, symlinkDir);
+
+        // Simulate resolved allowed directory (what the server startup should do)
+        const resolvedAllowedDir = await fs.realpath(symlinkDir);
+        const resolvedTargetDir = await fs.realpath(actualTargetDir);
+        expect(resolvedAllowedDir).toBe(resolvedTargetDir);
+
+        // Test 1: File access through original symlink path should pass validation with resolved allowed dir
+        const fileViaSymlink = path.join(symlinkDir, 'file.txt');
+        const resolvedFile = await fs.realpath(fileViaSymlink);
+        expect(isPathWithinAllowedDirectories(resolvedFile, [resolvedAllowedDir])).toBe(true);
+
+        // Test 2: File access through resolved path should also pass validation
+        const fileViaResolved = path.join(resolvedTargetDir, 'file.txt');
+        expect(isPathWithinAllowedDirectories(fileViaResolved, [resolvedAllowedDir])).toBe(true);
+
+        // Test 3: Demonstrate inconsistent behavior with unresolved allowed directories
+        // If allowed dirs were not resolved (storing symlink paths instead):
+        const unresolvedAllowedDirs = [symlinkDir];
+        // This validation would incorrectly fail for the same content:
+        expect(isPathWithinAllowedDirectories(resolvedFile, unresolvedAllowedDirs)).toBe(false);
+
+      } catch (error) {
+        // Skip if no symlink permissions on the system
+        if ((error as NodeJS.ErrnoException).code !== 'EPERM') {
+          throw error;
+        }
+      }
+    });
+
+    it('resolves nested symlink chains completely', async () => {
+      try {
+        // Setup: Create target file in forbidden area
+        const actualTarget = path.join(forbiddenDir, 'target-file.txt');
+        await fs.writeFile(actualTarget, 'FINAL_CONTENT');
+
+        // Create chain of symlinks: allowedFile -> link2 -> link1 -> actualTarget
+        const link1 = path.join(testDir, 'intermediate-link1');
+        const link2 = path.join(testDir, 'intermediate-link2');
+        const allowedFile = path.join(allowedDir, 'seemingly-safe-file');
+
+        await fs.symlink(actualTarget, link1);
+        await fs.symlink(link1, link2);
+        await fs.symlink(link2, allowedFile);
+
+        // The allowed file path passes basic validation
+        expect(isPathWithinAllowedDirectories(allowedFile, [allowedDir])).toBe(true);
+
+        // But complete resolution reveals the forbidden target
+        const fullyResolvedPath = await fs.realpath(allowedFile);
+        expect(isPathWithinAllowedDirectories(fullyResolvedPath, [allowedDir])).toBe(false);
+        expect(await fs.realpath(fullyResolvedPath)).toBe(await fs.realpath(actualTarget));
+
+      } catch (error) {
+        // Skip if no symlink permissions on the system
+        if ((error as NodeJS.ErrnoException).code !== 'EPERM') {
+          throw error;
+        }
+      }
+    });
+  });
+
+  describe('Path Validation Race Condition Tests', () => {
+    let testDir: string;
+    let allowedDir: string;
+    let forbiddenDir: string;
+    let targetFile: string;
+    let testPath: string;
+
+    beforeEach(async () => {
+      testDir = await fs.mkdtemp(path.join(os.tmpdir(), 'race-test-'));
+      allowedDir = path.join(testDir, 'allowed');
+      forbiddenDir = path.join(testDir, 'outside');
+      targetFile = path.join(forbiddenDir, 'target.txt');
+      testPath = path.join(allowedDir, 'test.txt');
+
+      await fs.mkdir(allowedDir, { recursive: true });
+      await fs.mkdir(forbiddenDir, { recursive: true });
+      await fs.writeFile(targetFile, 'ORIGINAL CONTENT', 'utf-8');
+    });
+
+    afterEach(async () => {
+      await fs.rm(testDir, { recursive: true, force: true });
+    });
+
+    it('validates non-existent file paths based on parent directory', async () => {
+      const allowed = [allowedDir];
+
+      expect(isPathWithinAllowedDirectories(testPath, allowed)).toBe(true);
+      await expect(fs.access(testPath)).rejects.toThrow();
+
+      const parentDir = path.dirname(testPath);
+      expect(isPathWithinAllowedDirectories(parentDir, allowed)).toBe(true);
+    });
+
+    it('demonstrates symlink race condition allows writing outside allowed directories', async () => {
+      const allowed = [allowedDir];
+
+      await expect(fs.access(testPath)).rejects.toThrow();
+      expect(isPathWithinAllowedDirectories(testPath, allowed)).toBe(true);
+
+      await fs.symlink(targetFile, testPath);
+      await fs.writeFile(testPath, 'MODIFIED CONTENT', 'utf-8');
+
+      const targetContent = await fs.readFile(targetFile, 'utf-8');
+      expect(targetContent).toBe('MODIFIED CONTENT');
+
+      const resolvedPath = await fs.realpath(testPath);
+      expect(isPathWithinAllowedDirectories(resolvedPath, allowed)).toBe(false);
+    });
+
+    it('shows timing differences between validation approaches', async () => {
+      const allowed = [allowedDir];
+
+      const validation1 = isPathWithinAllowedDirectories(testPath, allowed);
+      expect(validation1).toBe(true);
+
+      await fs.symlink(targetFile, testPath);
+
+      const resolvedPath = await fs.realpath(testPath);
+      const validation2 = isPathWithinAllowedDirectories(resolvedPath, allowed);
+      expect(validation2).toBe(false);
+
+      expect(validation1).not.toBe(validation2);
+    });
+
+    it('validates directory creation timing', async () => {
+      const allowed = [allowedDir];
+      const testDir = path.join(allowedDir, 'newdir');
+
+      expect(isPathWithinAllowedDirectories(testDir, allowed)).toBe(true);
+
+      await fs.symlink(forbiddenDir, testDir);
+
+      expect(isPathWithinAllowedDirectories(testDir, allowed)).toBe(true);
+
+      const resolved = await fs.realpath(testDir);
+      expect(isPathWithinAllowedDirectories(resolved, allowed)).toBe(false);
+    });
+
+    it('demonstrates exclusive file creation behavior', async () => {
+      const allowed = [allowedDir];
+
+      await fs.symlink(targetFile, testPath);
+
+      await expect(fs.open(testPath, 'wx')).rejects.toThrow(/EEXIST/);
+
+      await fs.writeFile(testPath, 'NEW CONTENT', 'utf-8');
+      const targetContent = await fs.readFile(targetFile, 'utf-8');
+      expect(targetContent).toBe('NEW CONTENT');
+    });
+
+    it('should use resolved parent paths for non-existent files', async () => {
+      const allowed = [allowedDir];
+
+      const symlinkDir = path.join(allowedDir, 'link');
+      await fs.symlink(forbiddenDir, symlinkDir);
+
+      const fileThroughSymlink = path.join(symlinkDir, 'newfile.txt');
+
+      expect(fileThroughSymlink.startsWith(allowedDir)).toBe(true);
+
+      const parentDir = path.dirname(fileThroughSymlink);
+      const resolvedParent = await fs.realpath(parentDir);
+      expect(isPathWithinAllowedDirectories(resolvedParent, allowed)).toBe(false);
+
+      const expectedSafePath = path.join(resolvedParent, path.basename(fileThroughSymlink));
+      expect(isPathWithinAllowedDirectories(expectedSafePath, allowed)).toBe(false);
+    });
+
+    it('demonstrates parent directory symlink traversal', async () => {
+      const allowed = [allowedDir];
+      const deepPath = path.join(allowedDir, 'sub1', 'sub2', 'file.txt');
+
+      expect(isPathWithinAllowedDirectories(deepPath, allowed)).toBe(true);
+
+      const sub1Path = path.join(allowedDir, 'sub1');
+      await fs.symlink(forbiddenDir, sub1Path);
+
+      await fs.mkdir(path.join(sub1Path, 'sub2'), { recursive: true });
+      await fs.writeFile(deepPath, 'CONTENT', 'utf-8');
+
+      const realPath = await fs.realpath(deepPath);
+      const realAllowedDir = await fs.realpath(allowedDir);
+      const realForbiddenDir = await fs.realpath(forbiddenDir);
+
+      expect(realPath.startsWith(realAllowedDir)).toBe(false);
+      expect(realPath.startsWith(realForbiddenDir)).toBe(true);
+    });
+
+    it('should prevent race condition between validatePath and file operation', async () => {
+      const allowed = [allowedDir];
+      const racePath = path.join(allowedDir, 'race-file.txt');
+      const targetFile = path.join(forbiddenDir, 'target.txt');
+
+      await fs.writeFile(targetFile, 'ORIGINAL CONTENT', 'utf-8');
+
+      // Path validation would pass (file doesn't exist, parent is in allowed dir)
+      expect(await fs.access(racePath).then(() => false).catch(() => true)).toBe(true);
+      expect(isPathWithinAllowedDirectories(racePath, allowed)).toBe(true);
+
+      // Race condition: symlink created after validation but before write
+      await fs.symlink(targetFile, racePath);
+
+      // With exclusive write flag, write should fail on symlink
+      await expect(
+        fs.writeFile(racePath, 'NEW CONTENT', { encoding: 'utf-8', flag: 'wx' })
+      ).rejects.toThrow(/EEXIST/);
+
+      // Verify content unchanged
+      const targetContent = await fs.readFile(targetFile, 'utf-8');
+      expect(targetContent).toBe('ORIGINAL CONTENT');
+
+      // The symlink exists but write was blocked
+      const actualWritePath = await fs.realpath(racePath);
+      expect(actualWritePath).toBe(await fs.realpath(targetFile));
+      expect(isPathWithinAllowedDirectories(actualWritePath, allowed)).toBe(false);
+    });
+
+    it('should allow overwrites to legitimate files within allowed directories', async () => {
+      const allowed = [allowedDir];
+      const legitFile = path.join(allowedDir, 'legit-file.txt');
+
+      // Create a legitimate file
+      await fs.writeFile(legitFile, 'ORIGINAL', 'utf-8');
+
+      // Opening with w should work for legitimate files
+      const fd = await fs.open(legitFile, 'w');
+      try {
+        await fd.write('UPDATED', 0, 'utf-8');
+      } finally {
+        await fd.close();
+      }
+
+      const content = await fs.readFile(legitFile, 'utf-8');
+      expect(content).toBe('UPDATED');
+    });
+
+    it('should handle symlinks that point within allowed directories', async () => {
+      const allowed = [allowedDir];
+      const targetFile = path.join(allowedDir, 'target.txt');
+      const symlinkPath = path.join(allowedDir, 'symlink.txt');
+
+      // Create target file within allowed directory
+      await fs.writeFile(targetFile, 'TARGET CONTENT', 'utf-8');
+
+      // Create symlink pointing to allowed file
+      await fs.symlink(targetFile, symlinkPath);
+
+      // Opening symlink with w follows it to the target
+      const fd = await fs.open(symlinkPath, 'w');
+      try {
+        await fd.write('UPDATED VIA SYMLINK', 0, 'utf-8');
+      } finally {
+        await fd.close();
+      }
+
+      // Both symlink and target should show updated content
+      const symlinkContent = await fs.readFile(symlinkPath, 'utf-8');
+      const targetContent = await fs.readFile(targetFile, 'utf-8');
+      expect(symlinkContent).toBe('UPDATED VIA SYMLINK');
+      expect(targetContent).toBe('UPDATED VIA SYMLINK');
+    });
+
+    it('should prevent overwriting files through symlinks pointing outside allowed directories', async () => {
+      const allowed = [allowedDir];
+      const legitFile = path.join(allowedDir, 'existing.txt');
+      const targetFile = path.join(forbiddenDir, 'target.txt');
+
+      // Create a legitimate file first
+      await fs.writeFile(legitFile, 'LEGIT CONTENT', 'utf-8');
+
+      // Create target file in forbidden directory
+      await fs.writeFile(targetFile, 'FORBIDDEN CONTENT', 'utf-8');
+
+      // Now replace the legitimate file with a symlink to forbidden location
+      await fs.unlink(legitFile);
+      await fs.symlink(targetFile, legitFile);
+
+      // Simulate the server's validation logic
+      const stats = await fs.lstat(legitFile);
+      expect(stats.isSymbolicLink()).toBe(true);
+
+      const realPath = await fs.realpath(legitFile);
+      expect(isPathWithinAllowedDirectories(realPath, allowed)).toBe(false);
+
+      // With atomic rename, symlinks are replaced not followed
+      // So this test now demonstrates the protection
+
+      // Verify content remains unchanged
+      const targetContent = await fs.readFile(targetFile, 'utf-8');
+      expect(targetContent).toBe('FORBIDDEN CONTENT');
+    });
+
+    it('demonstrates race condition in read operations', async () => {
+      const allowed = [allowedDir];
+      const legitFile = path.join(allowedDir, 'readable.txt');
+      const secretFile = path.join(forbiddenDir, 'secret.txt');
+
+      // Create legitimate file
+      await fs.writeFile(legitFile, 'PUBLIC CONTENT', 'utf-8');
+
+      // Create secret file in forbidden directory
+      await fs.writeFile(secretFile, 'SECRET CONTENT', 'utf-8');
+
+      // Step 1: validatePath would pass for legitimate file
+      expect(isPathWithinAllowedDirectories(legitFile, allowed)).toBe(true);
+
+      // Step 2: Race condition - replace file with symlink after validation
+      await fs.unlink(legitFile);
+      await fs.symlink(secretFile, legitFile);
+
+      // Step 3: Read operation follows symlink to forbidden location
+      const content = await fs.readFile(legitFile, 'utf-8');
+
+      // This shows the vulnerability - we read forbidden content
+      expect(content).toBe('SECRET CONTENT');
+      expect(isPathWithinAllowedDirectories(await fs.realpath(legitFile), allowed)).toBe(false);
+    });
+
+    it('verifies rename does not follow symlinks', async () => {
+      const allowed = [allowedDir];
+      const tempFile = path.join(allowedDir, 'temp.txt');
+      const targetSymlink = path.join(allowedDir, 'target-symlink.txt');
+      const forbiddenTarget = path.join(forbiddenDir, 'forbidden-target.txt');
+
+      // Create forbidden target
+      await fs.writeFile(forbiddenTarget, 'ORIGINAL CONTENT', 'utf-8');
+
+      // Create symlink pointing to forbidden location
+      await fs.symlink(forbiddenTarget, targetSymlink);
+
+      // Write temp file
+      await fs.writeFile(tempFile, 'NEW CONTENT', 'utf-8');
+
+      // Rename temp file to symlink path
+      await fs.rename(tempFile, targetSymlink);
+
+      // Check what happened
+      const symlinkExists = await fs.lstat(targetSymlink).then(() => true).catch(() => false);
+      const isSymlink = symlinkExists && (await fs.lstat(targetSymlink)).isSymbolicLink();
+      const targetContent = await fs.readFile(targetSymlink, 'utf-8');
+      const forbiddenContent = await fs.readFile(forbiddenTarget, 'utf-8');
+
+      // Rename should replace the symlink with a regular file
+      expect(isSymlink).toBe(false);
+      expect(targetContent).toBe('NEW CONTENT');
+      expect(forbiddenContent).toBe('ORIGINAL CONTENT'); // Unchanged
+    });
+  });
+});