Improper input validation in Ultrajson
CVE-2026-54911
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.
Vulnerability class: Drupalgeddon 2 (CVE-2018-7600)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N.
EPSS exploit prediction
EPSS: 0.004 (28.5th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 29th percentile — 28.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.004 · change: 0.000
Affected products
- Ultrajson — versions < 5.13.0
- Ultrajson Project
Weakness classification (CWE)
References
- github.com/ultrajson/ultrajson/security/advisories/GHSA-3j69-69wj-xqx2 (Patch, Mitigation, Vendor Advisory)
- github.com/ultrajson/ultrajson/commit/169eaf36b1116fece5034ee79a7a0ef3f6deedcf (Patch)
- github.com/ultrajson/ultrajson/releases/tag/5.13.0 (Product, Release Notes)
Frequently asked questions
- What is CVE-2026-54911?
- CVE-2026-54911 is a medium-severity vulnerability in Ultrajson, classified under Improper Input Validation. CVSS score: 6.5/10. Published 2026-06-22.
- How severe is CVE-2026-54911?
- Medium severity. CVSS v3 base score is 6.5 out of 10.