Summary
UltraJSON has a Memory Leak in ujson.dump() on Write Failure
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 startsrc/ujson/python/objToJSON.c:931- Error return on write failuresrc/ujson/python/objToJSON.c:942- Early return without cleanup
Root Cause:
The objToJSONFile() function allocates a Python string object via ujson_dumps_internal(), 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
import gc, tracemalloc, ujson
class BadFile:
def write(self, s):
raise RuntimeError("boom")
obj = {"x": "A" * 200000}
def run():
try:
ujson.dump(obj, BadFile())
except RuntimeError:
pass
run()
tracemalloc.start()
gc.collect()
base = tracemalloc.get_traced_memory()[0]
for i in range(5):
run()
gc.collect()
cur = tracemalloc.get_traced_memory()[0]
print(i, cur - base)
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.
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.
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.
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 missing dec-refs were added in 82af1d0ac01d09aa40c887b460d44b9d9f4bccd9. We recommend upgrading to UltraJSON 5.12.1.
Frequently Asked Questions
- What is CVE-2026-44660? CVE-2026-44660 is a high-severity security vulnerability in ujson (pip), affecting versions <= 5.12.0. It is fixed in 5.12.1.
- How severe is CVE-2026-44660? 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.
- Which versions of ujson are affected by CVE-2026-44660? ujson (pip) versions <= 5.12.0 is affected.
- Is there a fix for CVE-2026-44660? Yes. CVE-2026-44660 is fixed in 5.12.1. Upgrade to this version or later.
- Is CVE-2026-44660 exploitable, and should I be worried? 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
- What actually determines whether CVE-2026-44660 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-2026-44660? Upgrade
ujsonto 5.12.1 or later.