CVE-2026-44660: Ultrajson Project Ultrajson

High severity, CVSS 7.5. EPSS: 0.6% chance of exploitation in the next 30 days.

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 Project Ultrajson: before 5.12.1 (fixed in 5.12.1)

Published 2026-05-27. Last modified 2026-06-17.