Skip to content
Fencer Vulns
medium 6.5

CVE-2026-54911 json

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

published 2026-06-19 4 mo ago modified 2026-09-10 1 mo ago also EUVD-2026-38362 , GHSA-3j69-69wj-xqx2 , PYSEC-2026-2294

cvss
6.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 31%
public exploits
0
none known

Summary

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.

Patch / fix

  • pypi ujson 5.13.0
  • ultrajson_project ultrajson 5.13.0
  • ujson Debian 14 5.13.0-1

debian advisories DEBIAN-CVE-2026-54911

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-54911 updated cvss 5.3

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

  • Red Hat Exploit Intelligence exploit-intelligence-tech-preview/vulnerability-analysis-rhel9 Red Hat Enterprise Linux AI (RHEL AI) 3 rhelai3/bootc-cuda-rhel9, rhelai3/bootc-rocm-rhel9, rhelai3/bootc-gaudi-rhel9, rhelai3/disk-image-cuda-rhel9 Red Hat OpenShift AI (RHOAI) rhoai/odh-feature-server-rhel9, rhoai/odh-llama-stack-core-rhel9, rhoai/odh-workbench-jupyter-pytorch-cuda-py312-rhel9, rhoai/odh-workbench-jupyter-pytorch-rocm-py312-rhel9, rhoai/odh-workbench-jupyter-trustyai-cpu-py312-rhel9, rhoai/odh-workbench-jupyter-datascience-cpu-py312-rhel9, +3 more
    affected: workaround, no fix available “Upgrade ujson to 5.13.0 or later. Applications should avoid passing raw bytes to ujson.dumps()/dump()/encode() with reject_bytes=False; decoding bytes to str before encoding avoids the issue entirely.”
  • Red Hat Migration Toolkit for Applications 8 mta/mta-generic-external-provider-rhel9 Red Hat OpenShift AI (RHOAI) rhoai/odh-caikit-nlp-rhel9, rhoai/odh-caikit-tgis-serving-rhel9 Red Hat Satellite 6 satellite/iop-host-inventory-rhel9
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
4 mo ago

Exploit prediction

0.37% chance of exploitation in the next 30 days, higher than 31% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
GitHub 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
OSV 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Affected packages

ecosystem package affected fixed in
pypi ujson
<5.13.0
5.13.0

Affected products

  • ultrajson ultrajson

Classification

Weakness
  • CWE-20 Improper Input Validation

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.