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CVE-2026-32874 — UltraJSON has a Memory Leak parsing large integers allows DoS

Affected: ultrajson, Red Hat OpenStack Platform 16.2, Red Hat OpenStack Platform 17.1, Red Hat OpenStack Platform 18.0

UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+. Versions 5.4.0 through 5.11.0 contain an accumulating memory leak in JSON parsing large (outside of the range [-2^63, 2^64 - 1]) integers. The leaked memory is a copy of the string form of the integer plus an additional NULL byte. The leak occurs irrespective of whether the integer parses successfully or is rejected due to having more than sys.get_int_max_str_digits() digits, meaning that any sized leak per malicious JSON can be achieved provided that there is no limit on the overall size of the payload. Any service that calls ujson.load()/ujson.loads()/ujson.decode() on untrusted inputs is affected and vulnerable to denial of service attacks. This issue has been fixed in version 5.12.0.

GHSA-wgvc-ghv9-3pmm — UltraJSON has a Memory Leak parsing large integers allows DoS

Affected: PyPI: ujson

#### Summary

ujson 5.4.0 to 5.11.0 inclusive contain an accumulating memory leak in JSON parsing _large_ (outside of the range [-2^63, 2^64 - 1]) integers.

#### Exploitability

Any service that calls `ujson.load()`/`ujson.loads()`/`ujson.decode()` on untrusted inputs is affected and vulnerable to denial of service attacks.

#### Details

The leaked memory is a copy of the string form of the integer plus an additional NULL byte. The leak occurs irrespective of whether the integer parses successfully or is rejected due to having more than `sys.get_int_max_str_digits()` digits, meaning that any sized leak per malicious JSON can be achieved provided that there is no limit on the overall size of the payload.

```python ujson.loads(str(2 ** 64 - 1)) # No leak ujson.loads(str(2 ** 64)) # Leaks ujson.loads(str(10 ** sys.get_int_max_str_digits())) # Leaks and raises ValueError ```

#### Fix

The leak is fixed in `ujson 5.12.0` (4baeb950df780092bd3c89fc702a868e99a3a1d2). There are no workarounds beyond upgrading to an unaffected version.

#### Credits

Discovered by Cameron Criswell/Skevros using Coverage-guided fuzzing (libFuzzer + AddressSanitizer)

PYSEC-2026-2291

Affected: PyPI: ujson