---
id: CVE-2026-32874
aliases:
  - GHSA-wgvc-ghv9-3pmm
  - PYSEC-2026-2291
title: 'UltraJSON has a Memory Leak parsing large integers allows DoS '
summary: 'UltraJSON has a Memory Leak parsing large integers allows DoS '
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
vendor: ujson
product: ujson
ecosystem: pip
affected:
  - 'ujson >= 5.4.0, < 5.12.0'
patched:
  - ujson 5.12.0
published: '2026-03-18'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:50:41.558110605Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-wgvc-ghv9-3pmm'
references:
  - url: >-
      https://github.com/ultrajson/ultrajson/security/advisories/GHSA-wgvc-ghv9-3pmm
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-32874'
  - url: >-
      https://github.com/ultrajson/ultrajson/commit/4baeb950df780092bd3c89fc702a868e99a3a1d2
  - url: 'https://github.com/ultrajson/ultrajson'
  - url: 'https://github.com/ultrajson/ultrajson/releases/tag/5.12.0'
tags:
  - osv
  - pip
epss: 0.00684
epssPercentile: 0.50448
ingestedAt: '2026-07-13T18:58:04.522Z'
---

## Overview

#### 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)

## Affected packages

- `ujson >= 5.4.0, < 5.12.0`

## Remediation

Upgrade to a patched release:

- `ujson 5.12.0`