UltraJSON: Malformed/Truncated UTF-8 Accepted and Silently Rewritten in ujson.dumps()

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Summary

CVECVE-2026-54911
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-06-22 22:16:50 UTC
Updated2026-06-22 22:16:50 UTC
DescriptionUltraJSON 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.

Risk And Classification

Primary CVSS: v3.1 6.5 MEDIUM from [email protected]

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Problem Types: CWE-20 | CWE-20 CWE-20: Improper Input Validation

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary6.5MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
3.1CNADECLARED6.5MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Ultrajson Ultrajson affected < 5.13.0 Not specified

References

ReferenceSourceLinkTags
github.com/ultrajson/ultrajson/security/advisories/GHSA-3j69-69wj-xqx2 [email protected] github.com
github.com/ultrajson/ultrajson/commit/169eaf36b1116fece5034ee79a7a0ef3f6... [email protected] github.com
github.com/ultrajson/ultrajson/releases/tag/5.13.0 [email protected] github.com
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