CVE-2026-54911
Medium
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.
SeverityMedium
CVSS6.5
Exploit probability—
ProductUltrajson
PublishedJun 22, 2026
First seenJun 22, 2026
Weakness typeCWE-20Improper input validation
How to fix
Update
- ultrajson_project ultrajson5.13.0 or later
- ujson5.13.0 or later
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| ultrajson_project ultrajson | v < 5.13.0 | 5.13.0 |
| ujsonPyPI | v < 5.13.0 | 5.13.0 |
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