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Vulnerability record · CVE-2026-44660 · published 27 May 2026

CVE-2026-44660: Ultrajson project ultrajson memory leak vulnerability

UUltrajson Project · Ultrajson

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.

8.7 CVSS 4.0 High EPSS 0.64% · top 51.2% CWE-401 · Memory leak
8.7CVSS 4.0 base score
0.64%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
4References, 2 tagged exploit
17 Jun 2026Last modified by NVD

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.

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/ultrajson/ultrajson/commit/82af1d0ac01d09aa40c887b460d44b9d9f4bccd9 Patch
https://github.com/ultrajson/ultrajson/releases/tag/5.12.1 ProductRelease Notes
https://github.com/ultrajson/ultrajson/security/advisories/GHSA-c38f-wx89-p2xg ExploitMitigationPatchVendor Advisory
https://github.com/ultrajson/ultrajson/security/advisories/GHSA-c38f-wx89-p2xg ExploitMitigationPatchVendor Advisory

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Source: NIST National Vulnerability Database (record CVE-2026-44660), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.