Summary
MessagePack: Unbounded memory leak in msgpack C extension on strictmapkey validation failure
Additional impact: stale parser-stack references
The reference-ownership audit performed while fixing this leak found an
additional memory-safety issue in the streaming Unpacker.
After unpack() completed a nested container, an inactive parser-stack slot
could retain a non-owning pointer to the decoded object. A subsequent skip()
could reactivate that slot. If parsing then failed, context cleanup treated the
stale pointer as an owned reference and decremented it.
This corrupts the object's reference count and can result in a use-after-free
or double Py_DECREF when its actual owners later release it.
Although few users use this API, but those who do may experience crashes or unexpected errors due to a use-after-free.
Details
In msgpack/unpack.h, unpack_callback_map_item() rejects keys that are neither str nor bytes. At this point the map value has already been constructed. The early error return does not release the value reference.
In msgpack/unpack_template.h, the value is held in the local obj in unpack_execute(). The CT_MAP_VALUE error path jumps to _failed without decrementing that reference. Cleanup of the parser stack does not recover the value because it is not stored in the stack. The fallback checks the key before decoding the value.
Affected versions
The report reproduces the issue on 1.2.2 and reports it on development commit 475c1b7. The affected-product field lists the reproduced release only; it is not a claim that earlier releases are unaffected.
The earliest affected release needs confirmation: the report's summary says 1.1.x through 1.2.2, while its root-cause analysis attributes introduction to commit cd81370 (PR #673). Resolve this discrepancy before publishing a definitive affected-version range.
Reproduction
Run in a disposable local process with the C extension installed:
import msgpack
payload = msgpack.packb({42: b"A" * (4 * 1024 * 1024)}, use_bin_type=True)
msgpack.unpackb(payload) # Raises ValueError; the report observes a 4 MiB leak.
Streaming API trigger:
import msgpack
payload = msgpack.packb({42: b"A" * (4 * 1024 * 1024)}, use_bin_type=True)
u = msgpack.Unpacker()
u.feed(payload)
list(u) # Same strict_map_key error path.
Workarounds
The pure-Python fallback is reported not to leak on this path and may be used as a temporary workaround after application compatibility and performance checks. Request-size and rate limits may reduce the impact but do not eliminate cumulative leakage.
Impact
The msgpack-python C extension (msgpack._cmsgpack) leaks a decoded map value when deserialization rejects a key with strict_map_key=True (the default). Repeated malformed inputs can cause unbounded process memory growth and denial of service in applications that deserialize attacker-controlled MessagePack data. Both unpackb() and the streaming Unpacker API are affected. The pure-Python fallback does not exhibit this leak in the reported tests.
The report measured approximately 4 MiB retained per failed parse and 100 MiB after 25 failed parses on Windows with Python 3.14 and msgpack 1.2.2. These are reporter-provided results.
Memory is accessed after it has been freed, leading to undefined behavior in native code. Typical impact: memory corruption, crash, or potential code execution.
CVE-2026-114613 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 (1.2.3); 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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Frequently Asked Questions
- What is CVE-2026-114613? CVE-2026-114613 is a high-severity use after free vulnerability in msgpack (pip), affecting versions < 1.2.3. It is fixed in 1.2.3. Memory is accessed after it has been freed, leading to undefined behavior in native code.
- How severe is CVE-2026-114613? CVE-2026-114613 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 msgpack are affected by CVE-2026-114613? msgpack (pip) versions < 1.2.3 is affected.
- Is there a fix for CVE-2026-114613? Yes. CVE-2026-114613 is fixed in 1.2.3. Upgrade to this version or later.
- Is CVE-2026-114613 exploitable, and should I be worried? Whether CVE-2026-114613 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-2026-114613 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-2026-114613? Upgrade
msgpackto 1.2.3 or later.