How severe is CVE-2026-54911?

CVE-2026-54911 has a CVSS score of 6.5/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2026-54911?

CVE-2026-54911 affects the following packages: ujson (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2026-54911?

Update ujson to 5.13.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-54911 is resolved across your whole dependency graph.

How do I detect CVE-2026-54911 in my PyPI dependencies?

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.

How do I mitigate CVE-2026-54911 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2026-54911 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-54911 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-54911?

CVE-2026-54911 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 31th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-54911?

CVE-2026-54911 is classified as Improper Input Validation (CWE-20). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-54911 published, and has it been updated?

CVE-2026-54911 was published on June 22, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

MEDIUM severity

CVE-2026-54911 — ujson

MEDIUMFix: ultrajson/ultrajson@169eaf3

CVE-2026-54911 is a medium-severity (CVSS 6.5) Improper Input Validation vulnerability in ujson. A fix is available for ujson — see the affected versions and patch details below.

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

Also known asGHSA-3j69-69wj-xqx2PYSEC-2026-2294

Published

Jun 22, 2026

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 28, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-54911.

EPSS Exploitation Probability

via FIRST.org ↗

0.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs31th percentile — riskier than 31% of all scored CVEsHighest risk

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

Summary

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.

Details

The expected behavior is that for x being any bytes string, x == ujson.loads(ujson.dumps(x, reject_bytes=False)).encode(errors="surrogatepass") should always either be true or ujson.dumps() will throw an exception. In reality, some strings which should've been errors are silently rewritten as other strings:

Invalid continuation bytes are replaced with valid ones: b'\xcf\x13' -> b'\xcf\x93'
Unterminated sequence completes the sequence: b'\xc3' -> b'\xc3\x80'
... or leads to reading past the end of string: b'\xf0\x90\x94' -> b"\xf0\x90\x94\x80inxcontrib'"

Impact

An application relying on reject_bytes=False for UTF-8 handling may experience:

Data integrity issues
Experience validation bypass if said validation occurs before serialisation

Remediation

The missing/broken UTF-8 validation checks were added/fixed in https://github.com/ultrajson/ultrajson/commit/169eaf36b1116fece5034ee79a7a0ef3f6deedcf. We recommend upgrading to UltraJSON 5.13.0.

Workarounds

Decoding bytes to strings in Python before passing them to ujson.dumps() avoids this issue.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`ujson`	all versions	5.13.0`pip install --upgrade 'ujson==5.13.0'`

Affected Products

1 product · 1 configurations

Application

ultrajsonultrajson_project

< 5.13.0

range

Detection & mitigation playbook

Open-source dependency

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.
Fix
Update ujson to 5.13.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-54911 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

ujson.dumps()/dump()/encode() prior to 5.13.0 have a reject_bytes=False option; when a caller explicitly sets it and passes raw bytes (not decoded str) input, malformed or truncated UTF-8 sequences are silently rewritten into different Unicode characters instead of being rejected, causing data integrity issues. This…

Workaround published by Red Hat

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.

Source: Red Hat security advisory for CVE-2026-54911 (CC BY 4.0)

Frequently Asked Questions

### Summary `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. ### Details The expected behavior is that for `x` being any bytes string, `x == ujson.loads(ujson.dumps(x, reject_bytes=False)).encode(errors="surrogatepass")` should always either be true or `ujson.dumps()` will throw an exception. In reality, some strings which should've