# CVE-2026-54911: ujson: UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+

CVE-2026-54911 is fixed in ujson on 1 platform by DEBIAN-CVE-2026-54911 (version 5.13.0-1), released July 9, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-54911
- Priority: Routine
- CVSS: 6.5 (medium)
- EPSS: 0.0037
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jun 22, 2026

## Description

UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+. Prior to 5.13.0, ujson.dumps() (or ujson.dump() or ujson.encode()) have a reject_bytes=False option. When set, they may accept malformed or truncated UTF-8 byte sequences, silently rewriting them into different Unicode characters instead of rejecting them. This leads to input validation bypass and data integrity issues. This vulnerability is fixed in 5.13.0.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | ujson Debian 14 | 5.13.0-1 | [DEBIAN-CVE-2026-54911](https://patchblox.com/security/data/fix/linux/debian-cve-2026-54911.md) | Jul 9, 2026 |

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Data as of 2026-11-08T19:12:23Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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