CVE-2026-44660: UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+

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HIGH 7.5REMOTEUPGRADE

CVE-2026-44660 is a high-severity flaw (CVSS 7.5) 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 5.12.1, when ujson.dump() writes to a file-like object and the write operation raises an exception, the serialized JSON string object is not decremented, leaking memory. Each failed write operation leaks the full size of the serialized payload. This vulnerability is fixed in 5.12.1.

Why this priority

High severity
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 64%)
CISA exploited list	No
Weakness	CWE-401
Published	2026-05-27
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: ultrajson project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-05-27 21:16 UTCPublished CVE record published
2026-06-17 10:51 UTCUpdated Record last modified

ultrajson project ultrajson UPGRADE

Affected: < 5.12.1

Fixed in: 5.12.1

Safe to downgrade to: no unaffected older release

1. Check installed version

pip show ultrajson

2. UPGRADE to 5.12.1

pip install --upgrade "ultrajson>=5.12.1"

Verify at the official source NVD · CVE Program · Report a problem with this page
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-2026-44660?

ultrajson: upgrade to 5.12.1. Exact commands for each OS are on this page.

Is CVE-2026-44660 being exploited?

CVE-2026-44660 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-2026-44660 affect?

CVE-2026-44660 affects ultrajson. Severity is high (CVSS 7.5). UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+. Prior to 5.12.1, when ujson.dump() writes to a file-like object and the write operation raises an exception, the serialized JSON string object is

How urgent is CVE-2026-44660?

Priority 45/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

Software pages

Python vulnerabilities

Related CVEs

CVE-2026-54911 MEDIUM 6.5 UPGRADE UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+

References

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