Oj (Optimized JSON) is a JSON parser and Object marshaller packaged as a Ruby gem. In versions prior to 3.17.2, when in usual mode with create_id enabled, Oj::Parser#parse is vulnerable to heap corruption via a negative-size memcpy. When a JSON object key is exactly 65,535 bytes long, an integer truncation in form_attr (usual.c:63) converts the length to -1 before passing it to memcpy. This causes memcpy to copy SIZE_MAX bytes (interpreted as a huge size_t), corrupting heap memory and crashing the process. The issue has been fixed in version 3.17.2.

Affected: RubyGems: oj

### Summary

`Oj::Parser#parse` in usual mode with `create_id` enabled is vulnerable to heap corruption via a negative-size `memcpy`. When a JSON object key is exactly 65,535 bytes long, an integer truncation in `form_attr` (`usual.c:63`) converts the length to `-1` before passing it to `memcpy`. This causes `memcpy` to copy `SIZE_MAX` bytes (interpreted as a huge `size_t`), corrupting heap memory and crashing the process.

### Version

- **Software**: oj gem - **Affected**: all versions with `ext/oj/usual.c` - **Latest tested**: 3.17.1 (confirmed present)

### Details

`ext/oj/usual.c`, `form_attr`:

```c // usual.c:55–64 static ID form_attr(const char *str, size_t slen) { char buf[4096]; // ... int blen = (int)slen + 1; // ← truncates: 65535 + 1 = 65536 → wraps to 0 // or: 65535 cast to int = 65535 (fits), // but blen = 65536 → INT overflow on +1 if slen=INT_MAX // ... memcpy(buf, "@", 1); memcpy(buf + 1, str, (size_t)blen); // ← size_t(-1) = SIZE_MAX } ```

The cache (`cache_intern`) uses a fixed 65,536-byte slab. When `slen = 65535`, the arithmetic wraps and `memcpy` is called with `(size_t)-1`.

ASAN report: ``` ==80452==ERROR: AddressSanitizer: negative-size-param: (size=-1) #0 memcpy #1 form_attr /ext/oj/usual.c:63 #2 cache_intern /ext/oj/cache.c:326 #3 get_attr_id /ext/oj/usual.c:186 #4 close_object_create /ext/oj/usua…

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, when in usual mode with create_id enabled, Oj::Parser#parse is vulnerable to heap corruption via a negative-size memcpy. When a JSON object key is exactly 65,535 bytes long, an integer truncation in form_attr (usual.c:63) converts the length to -1 before passing it to memcpy. This causes memcpy to copy SIZE_MAX bytes (interpreted as a huge size_t), corrupting heap memory and crashing the process. The issue has been fixed in version 3.17.2.