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CVE-2026-44660 — UltraJSON: Memory Leak in ujson.dump() on Write Failure

2026-10-02T15:39:00.490893+00:00

Affected: ultrajson

UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+. Prior to 5.12.1, when ujson.dump() writes to a file-like object and the write operation raises an exception, the serialized JSON string object is not decremented, leaking memory. Each failed write operation leaks the full size of the serialized payload. This vulnerability is fixed in 5.12.1.

GHSA-c38f-wx89-p2xg — UltraJSON has a Memory Leak in ujson.dump() on Write Failure

2026-10-02T15:39:00.490992+00:00

Affected: PyPI: ujson

### Summary

When `ujson.dump()` writes to a file-like object and the write operation raises an exception, the serialized JSON string object is not decremented, leaking memory. Each failed write operation leaks the full size of the serialized payload.

Code that uses `ujson.dumps()` rather than `ujson.dump()` or only JSON load/decode methods is unaffected.

### Details

**Vulnerability Location:** - `src/ujson/python/objToJSON.c:913` - `objToJSONFile()` function start - `src/ujson/python/objToJSON.c:931` - Error return on write failure - `src/ujson/python/objToJSON.c:942` - Early return without cleanup **Root Cause:**

The `objToJSONFile()` function allocates a Python string object via `ujson_dumps_internal()`, calls the file's `write()` method, and returns early if `write()` raises an exception—but never calls `Py_DECREF(string)` on the early exit path.

### PoC ```python import gc, tracemalloc, ujson

class BadFile: def write(self, s): raise RuntimeError("boom")

obj = {"x": "A" * 200000}

def run(): try: ujson.dump(obj, BadFile()) except RuntimeError: pass

run() tracemalloc.start() gc.collect() base = tracemalloc.get_traced_memory()[0]

for i in range(5): run() gc.collect() cur = tracemalloc.get_traced_memory()[0] print(i, cur - base) ```

### Impact

Any application that serializes data through `ujson.dump()` to an attacker-influenced file-like object that can fail can be driven into linear memory growth. An attacker…

PYSEC-2026-2293

2026-10-02T15:39:00.491132+00:00

Affected: PyPI: ujson