CVE-2026-44660: UltraJSON: Memory Leak in ujson.dump() on Write Failure

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-05-27 · updated 2026-06-17
Weakness: CWE-401

Description

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.

Affected products

ultrajson: before 5.12.1 — fixed in 5.12.1

How to fix

Upgrade to a fixed version: ultrajson 5.12.1. Check the vendor advisory for mitigations.

References

https://github.com/ultrajson/ultrajson/commit/82af1d0ac01d09aa40c887b460d44b9d9f4bccd9
https://github.com/ultrajson/ultrajson/releases/tag/5.12.1
https://github.com/ultrajson/ultrajson/security/advisories/GHSA-c38f-wx89-p2xg
https://github.com/ultrajson/ultrajson/security/advisories/GHSA-c38f-wx89-p2xg

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