CVE-2026-54911
UltraJSON: Malformed/Truncated UTF-8 Accepted and Silently Rewritten in ujson.dumps()
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.
INFO
Published Date :
June 22, 2026, 8:53 p.m.
Last Modified :
June 22, 2026, 8:53 p.m.
Remotely Exploit :
Yes !
Source :
GitHub_M
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | MEDIUM | MITRE-CVE |
Solution
- Update UltraJSON to version 5.13.0 or later.
- Avoid using reject_bytes=False in older versions.
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