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Vulnerability record · CVE-2026-54911 · published 22 June 2026

CVE-2026-54911: Ultrajson project ultrajson improper input validation vulnerability

UUltrajson Project · Ultrajson

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.

6.5 CVSS 3.1 Medium EPSS 0.37% · top 71.5% CWE-20 · Improper input validation
6.5CVSS 3.1 base score
0.37%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
3References
26 Jun 2026Last modified by NVD

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.

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

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/ultrajson/ultrajson/commit/169eaf36b1116fece5034ee79a7a0ef3f6deedcf Patch
https://github.com/ultrajson/ultrajson/releases/tag/5.13.0 ProductRelease Notes
https://github.com/ultrajson/ultrajson/security/advisories/GHSA-3j69-69wj-xqx2 MitigationPatchVendor Advisory

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Source: NIST National Vulnerability Database (record CVE-2026-54911), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.