PatchTheCVECVE-2026-54911: UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+
CVE-2026-54911 is a medium-severity flaw (CVSS 6.5) affecting ultrajson. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
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 5.1
Why this priority
- Remotely exploitable, no login, no user action
A fixed release or distro patch exists.
Risk facts
| CVSS score | 6.5 (Medium) |
| Attack vector | Network (remote) |
| Attack complexity | Low |
| Privileges needed | None |
| User interaction | None |
| EPSS exploit chance | 0.4% (top 69%) |
| CISA exploited list | No |
| Weakness | CWE-20 |
| Published | 2026-06-22 |
| Last modified | 2026-06-26 |
Last updated 2026-06-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Updated Record last modified
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Quick answers
How do I fix CVE-2026-54911?
ultrajson: upgrade to 5.13.0. Exact commands for each OS are on this page.
Is CVE-2026-54911 being exploited?
CVE-2026-54911 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.
What does CVE-2026-54911 affect?
CVE-2026-54911 affects ultrajson. Severity is medium (CVSS 6.5). 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 trunca
How urgent is CVE-2026-54911?
Priority 35/100. Remotely exploitable, no login, no user action