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🐍 PyPI
Not in CISA KEV
MEDIUM severity

UltraJSON vulnerable to Out-of-bounds WriteGHSA-fh56-85cw-5pq6

MEDIUMFix: ultrajson/ultrajson#504

GHSA-fh56-85cw-5pq6 is a medium-severity (CVSS 5.5) Out-of-bounds Write vulnerability in ujson. 3 public exploit or proof-of-concept references are indexed, so weaponization risk is real. A fix is available for ujson — see the affected versions and patch details below.

Also known asCVE-2021-45958PYSEC-2022-25
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
3 known
Exploitation data as of Nov 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.7%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs77th percentile — riskier than 77% of all scored CVEsHighest risk
0.00%0.72%1.44%2.17%0.3%1.7%May 26Aug 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected
🐍ujson

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

UltraJSON (aka ujson) through 5.1.0 has a stack-based buffer overflow in Buffer_AppendIndentUnchecked (called from encode). Exploitation can, for example, use a large amount of indentation.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPIujson≥ 1.34&&< 5.2.05.2.0pip install --upgrade 'ujson==5.2.0'

Affected Products

3 products · 5 configurations
OS
debian linuxdebian
1 version
9.0
OS
fedorafedoraproject
3 versions
353637
Application
ultrajsonultrajson_project
< 5.2.0
range
Exploits & PoCs
3

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

UltraJSON (aka ujson) through 5.1.0 has a stack-based buffer overflow in…

github.comNVDJan 2022

Report on bugs.chromium.org

bugs.chromium.orgNVDJan 2022

Report on github.com

github.comNVDMar 2022

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for ujson, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update ujson to 5.2.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-fh56-85cw-5pq6 is resolved across your whole dependency graph.

  3. Workarounds

    Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.

Frequently Asked Questions

UltraJSON (aka ujson) 1.34 through 5.1.0 has a stack-based buffer overflow in Buffer_AppendIndentUnchecked (called from encode).
O3 Security · Impact-Aware SCA

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UltraJSON vulnerable to Out-of-bounds Write (Medium 5.5)