Vulnerability · Published Jun 22, 2026 · Assigned by GitHub_M
CVE-2026-54911UltraJSON: Malformed/Truncated UTF-8 Accepted and Silently Rewritten in ujson.dumps()
Medium 6.5
In plain English
A medium-severity improper input validation in ultrajson from ultrajson. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 31% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | ujson | no fix listed | DEBIAN-CVE-2026-54911 |
| Debian:13 | ujson | no fix listed | DEBIAN-CVE-2026-54911 |
| Debian:14 | ujson | 5.13.0-1 | DEBIAN-CVE-2026-54911 |
| PyPI | ujson | 5.13.0 | GHSA-3j69-69wj-xqx2 |
| PyPI | ujson | 5.13.0 | PYSEC-2026-2294 |
| Ubuntu:22.04:LTS | collada2gltf | no fix listed | UBUNTU-CVE-2026-54911 |
| Ubuntu:22.04:LTS | pandas | no fix listed | UBUNTU-CVE-2026-54911 |
| Ubuntu:24.04:LTS | pandas | no fix listed | UBUNTU-CVE-2026-54911 |
| Ubuntu:25.10 | pandas | no fix listed | UBUNTU-CVE-2026-54911 |
| Ubuntu:26.04:LTS | pandas | no fix listed | UBUNTU-CVE-2026-54911 |
| Ubuntu:26.10 | pandas | no fix listed | UBUNTU-CVE-2026-54911 |
| Ubuntu:25.10 | ujson | no fix listed | UBUNTU-CVE-2026-54911 |
| Ubuntu:26.04:LTS | ujson | no fix listed | UBUNTU-CVE-2026-54911 |
| Ubuntu:26.10 | ujson | no fix listed | UBUNTU-CVE-2026-54911 |
Affected products (as reported)
Weakness: CWE-20 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N