Is CVE-2026-54911 being exploited?

No public exploit is currently tracked.

How severe is CVE-2026-54911?

CVE-2026-54911 has a CVSS v3 severity of MEDIUM (6.5).

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CVE-2026-54911

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.