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.

No CVSS v4.0

Attack Vector Network

Attack Complexity Low

Privileges Required None

Scope Unchanged

Confidentiality Impact Low

Integrity Impact Low

Availability Impact None

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

No EPSS score available.

Key SSVC decision points have not yet been added.

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions
ultrajson ultrajson affected
Version Status Constraints
< 5.13.0 affected —

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Project Subscriptions

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Advisories
Source ID Title
Github GHSA Github GHSA GHSA-3j69-69wj-xqx2 UltraJSON: Malformed/Truncated UTF-8 Accepted and Silently Rewritten in ujson.dumps()
Fixes

Solution

No solution given by the vendor.

Workaround

No workaround given by the vendor.

References
Link Providers
https://github.com/ultrajson/ultrajson/commit/169eaf36b1116fece5034ee79a7a0ef3f6deedcf cve-icon
https://github.com/ultrajson/ultrajson/releases/tag/5.13.0 cve-icon
https://github.com/ultrajson/ultrajson/security/advisories/GHSA-3j69-69wj-xqx2 cve-icon
History

Mon, 22 Jun 2026 22:00:00 +0000

Type Values Removed Values Added
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.
Title UltraJSON: Malformed/Truncated UTF-8 Accepted and Silently Rewritten in ujson.dumps()
Weaknesses CWE-20
References
Metrics cvssV3_1

{'score': 6.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N'}

Projects

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cve-icon MITRE

Status: PUBLISHED

Assigner: GitHub_M

Published:

Updated: 2026-06-22T20:53:07.019Z

Reserved: 2026-06-16T13:49:33.556Z

Link: CVE-2026-54911

cve-icon Vulnrichment

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cve-icon NVD

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cve-icon Redhat

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cve-icon OpenCVE Enrichment

Updated: 2026-06-22T23:45:04Z

Weaknesses