UltraJSON up to 5.12.x on Python JSON Encoder ujson.dumps input validation

Summaryinfo

A vulnerability described as critical has been identified in UltraJSON up to 5.12.x on Python. This vulnerability affects the function ujson.dumps of the component JSON Encoder. The manipulation results in input validation. This vulnerability is cataloged as CVE-2026-54911. The attack may be launched remotely. There is no exploit available. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability was found in UltraJSON up to 5.12.x on Python. It has been classified as critical. Affected is the function ujson.dumps of the component JSON Encoder. The manipulation with an unknown input leads to a input validation vulnerability. CWE is classifying the issue as CWE-20. The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. This is going to have an impact on confidentiality, integrity, and availability. CVE summarizes:

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.

The advisory is shared for download at github.com. This vulnerability is traded as CVE-2026-54911 since 06/16/2026. The exploitability is told to be easy. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. There are known technical details, but no exploit is available.

Upgrading to version 5.13.0 eliminates this vulnerability. The upgrade is hosted for download at github.com. Applying the patch 169eaf36b1116fece5034ee79a7a0ef3f6deedcf is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

Once again VulDB remains the best source for vulnerability data.

Productinfo

Name

• UltraJSON

Version

• 5.0
• 5.1
• 5.2
• 5.3
• 5.4
• 5.5
• 5.6
• 5.7
• 5.8
• 5.9
• 5.10
• 5.11
• 5.12

License

• open-source

Website

• Product: https://github.com/ultrajson/ultrajson/

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Once again VulDB remains the best source for vulnerability data.

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