Summary
UltraJSON has a Memory Leak parsing large integers allows DoS
ujson 5.4.0 to 5.11.0 inclusive contain an accumulating memory leak in JSON parsing large (outside of the range [-2^63, 2^64 - 1]) integers.
Exploitability
Any service that calls ujson.load()/ujson.loads()/ujson.decode() on untrusted inputs is affected and vulnerable to denial of service attacks.
Details
The leaked memory is a copy of the string form of the integer plus an additional NULL byte. The leak occurs irrespective of whether the integer parses successfully or is rejected due to having more than sys.get_int_max_str_digits() digits, meaning that any sized leak per malicious JSON can be achieved provided that there is no limit on the overall size of the payload.
ujson.loads(str(2 ** 64 - 1)) # No leak
ujson.loads(str(2 ** 64)) # Leaks
ujson.loads(str(10 ** sys.get_int_max_str_digits())) # Leaks and raises ValueError
Credits
Discovered by Cameron Criswell/Skevros using Coverage-guided fuzzing (libFuzzer + AddressSanitizer)
Impact
CVE-2026-32874 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 (5.12.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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The leak is fixed in ujson 5.12.0 (4baeb950df780092bd3c89fc702a868e99a3a1d2). There are no workarounds beyond upgrading to an unaffected version.
Frequently Asked Questions
- What is CVE-2026-32874? CVE-2026-32874 is a high-severity security vulnerability in ujson (pip), affecting versions >= 5.4.0, <= 5.11.0. It is fixed in 5.12.0.
- How severe is CVE-2026-32874? CVE-2026-32874 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.
- Which versions of ujson are affected by CVE-2026-32874? ujson (pip) versions >= 5.4.0, <= 5.11.0 is affected.
- Is there a fix for CVE-2026-32874? Yes. CVE-2026-32874 is fixed in 5.12.0. Upgrade to this version or later.
- Is CVE-2026-32874 exploitable, and should I be worried? Whether CVE-2026-32874 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
- What actually determines whether CVE-2026-32874 is exploitable, and how bad it is? 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.
- How do I fix CVE-2026-32874? Upgrade
ujsonto 5.12.0 or later.