Oj (Optimized JSON) is a JSON parser and Object marshaller packaged as a Ruby gem. In versions prior to 3.17.2,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. This issue has been fixed in version 3.17.2.

Affected: RubyGems: oj

### 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`:

```c 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 } ```

```c // 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 0x51900008…

Affected: Ubuntu:14.04:LTS: ruby-oj, Ubuntu:16.04:LTS: ruby-oj, Ubuntu:18.04:LTS: ruby-oj, Ubuntu:20.04:LTS: ruby-oj, Ubuntu:22.04:LTS: ruby-oj, Ubuntu:24.04:LTS: ruby-oj, Ubuntu:25.10: ruby-oj, Ubuntu:26.04:LTS: ruby-oj

Oj (Optimized JSON) is a JSON parser and Object marshaller packaged as a Ruby gem. In versions prior to 3.17.2,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. This issue has been fixed in version 3.17.2.