CVE-2026-44660 is a high-severity security vulnerability in ujson (pip), affecting versions <= 5.12.0. It is fixed in 5.12.1.
Summary 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. Code that uses ujson.dumps() rather than ujson.dump() or only JSON load/decode methods is unaffected. Details Vulnerability Location: src/ujson/python/objToJSON.c:913 - objToJSONFile() function start src/ujson/python/objToJSON.c:931 - Error return on write failure src/ujson/python/objToJSON.c:942 - Early return without cleanup Root Cause: The objToJSONFile() function allocates a Python string object via ujsondumpsinternal(), calls the file's write() method, and returns early if write() raises an exception, but never calls Py_DECREF(string) on the early exit path. PoC Impact Any application that serializes data through ujson.dump() to an attacker-influenced file-like object that can fail can be driven into linear memory growth. An attacker can quickly use up all the memory of say a web server that sends JSON responses using ujson.dump() by repeatedly making requests then closing the connection mid response. Remediation The missing dec-refs were added in 82af1d0ac01d09aa40c887b460d44b9d9f4bccd9. We recommend upgrading to UltraJSON 5.12.1. Workarounds Replacing ujson.dump(obj, file) with file.write(ujson.dumps(obj)) is equivalent (contrary to popular misconception, there are no streaming benefits to using ujson.dump()) and will avoid the memory leak.
CVE-2026-44660 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment.
A fixed version is available (5.12.1). Upgrading removes the vulnerable code path.
pip
ujson (<= 5.12.0)ujson → 5.12.1 (pip)Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter instead of chasing every advisory.
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Upgrade ujson to 5.12.1 or later to resolve this vulnerability.
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
CVE-2026-44660 is a high-severity security vulnerability in ujson (pip), affecting versions <= 5.12.0. It is fixed in 5.12.1.
CVE-2026-44660 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
ujson (pip) versions <= 5.12.0 is affected.
Yes. CVE-2026-44660 is fixed in 5.12.1. Upgrade to this version or later.
Whether CVE-2026-44660 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
Upgrade ujson to 5.12.1 or later.