CVE-2026-54911: Ultrajson Project Ultrajson

Medium severity, CVSS 6.5. EPSS: 0.4% chance of exploitation in the next 30 days.

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 products

Published 2026-06-22. Last modified 2026-06-26.