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High severityNVD Advisory· Published Oct 7, 2020· Updated Sep 17, 2024

Prototype Pollution

CVE-2020-7742

Description

Prototype Pollution in simpl-schema <1.10.2 allows attackers to inject properties into Object.prototype via unsafe recursive merge.

AI Insight

LLM-synthesized narrative grounded in this CVE's description and references.

Prototype Pollution in simpl-schema <1.10.2 allows attackers to inject properties into Object.prototype via unsafe recursive merge.

Overview

CVE-2020-7742 is a prototype pollution vulnerability in the simpl-schema npm package before version 1.10.2 [1][2]. The root cause lies in an unsafe recursive merge operation within the package's utility functions, specifically in merge.js [1][2]. When processing objects, it fails to properly prevent the pollution of Object.prototype via the __proto__ property [2].

Exploitation

An attacker can exploit this flaw by providing a specially crafted JSON object to any API endpoint or method that accepts nested objects and processes them using the vulnerable merge logic [2]. No authentication is required if the application exposes such functionality to unauthenticated users. The attack works by defining a property named __proto__ with malicious properties, which during the recursive merge pollutes the global Object.prototype [2].

Impact

Successful exploitation allows an attacker to inject arbitrary properties into the prototype chain of all JavaScript objects [2]. This can lead to denial of service via JavaScript exceptions, tampering with application logic to force unintended code paths, or in severe cases, remote code execution if the polluted properties affect security-sensitive checks [2]. The widespread inheritance of polluted properties can subvert the entire application's behavior.

Mitigation

Users should upgrade to simpl-schema version 1.10.2 or later, which includes a fix that protects the object merge from prototype pollution [3][4]. The commit adds a test to verify that __proto__ pollution is blocked [4]. No workarounds are documented; upgrading is the recommended action.

References
  1. NVD - CVE-2020-7742
  2. Snyk Vulnerability Database | Snyk
  3. Release 1.10.2 · longshotlabs/simpl-schema
  4. fix: protect obj merge · longshotlabs/simpl-schema@5012884

AI Insight generated on May 21, 2026. Synthesized from this CVE's description and the cited reference URLs; citations are validated against the source bundle.

Affected packages

Versions sourced from the GitHub Security Advisory.

PackageAffected versionsPatched versions
simpl-schemanpm
< 1.10.21.10.2

Affected products

2

Patches

1
50128841fa7f

fix: protect obj merge

https://github.com/longshotlabs/simpl-schemaEric DobbertinOct 6, 2020via ghsa
commit
2 files changed · +9 −0
  • package/lib/SimpleSchema.tests.js+8 −0 modified
    @@ -49,6 +49,14 @@ describe('SimpleSchema', function () {
     }).toThrow('"someArray" is Array type but the schema does not include a "someArray.$" definition for the array items');
   });
 
+  it('does not allow prototype pollution', function () {
+    const obj = {};
+    expect(obj.polluted).toBe(undefined);
+    const badObj = JSON.parse('{"__proto__":{"polluted":"yes"}}');
+    SimpleSchema.setDefaultMessages(badObj);
+    expect(obj.polluted).toBe(undefined);
+  });
+
   describe('nesting', function () {
     it('throws an error if a nested schema defines a field that its parent also defines', function () {
       expect(function () {
  • package/lib/utility/merge.js+1 −0 modified
    @@ -11,6 +11,7 @@
 export default function merge(destination, ...sources) {
   sources.forEach((source) => {
     Object.keys(source).forEach((prop) => {
+      if (prop === '__proto__') return; // protect against prototype pollution
       if (
         source[prop]
         && source[prop].constructor

Vulnerability mechanics

Generated on May 9, 2026. Inputs: CWE entries + fix-commit diffs from this CVE's patches. Citations validated against bundle.

References

6

News mentions

0

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