EUVD-2026-40423Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Network-reachable and unauthenticated, but exploitation depends on the app reusing an Unpacker after a caught error, so AC:H; impact is crash-only, hence C:N/I:N/A:H.
Primary rating from Vendor (github).
CVSS VectorVendor: github
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Lifecycle Timeline
2Blast Radius
ecosystem impact- 14,864 pypi packages depend on msgpack (1,638 direct, 13,325 indirect)
Ecosystem-wide dependent count for version 1.2.1.
DescriptionCVE.org
MessagePack is the serializer implementation for Python msgpack.org. Prior to 1.2.1, there is an Out-of-bounds read/crash on Unpacker reuse after a caught error, potentially leading to a DoS attack. If the Unpacker is used repeatedly after an error occurs, the process may crash with a SEGV. This issue has been fixed in version 1.2.1.
Articles & Coverage 1
AnalysisAI
Denial of service in the msgpack-python serialization library (versions prior to 1.2.1) lets remote attackers crash a process via an out-of-bounds read when an application reuses an Unpacker instance after a previously raised error was caught. Sending malformed MessagePack data that triggers an error, then feeding further data to the same Unpacker, can drive the process to a SEGV. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | The specific precondition is that the application must reuse the same Unpacker object after an unpacking error was raised and caught - i.e., catch-and-continue decoding on a shared or long-lived Unpacker (typical of streaming/persistent-connection designs). … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 3.1 vector AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H yields 7.5 and isolates the impact to availability - no confidentiality or integrity loss - consistent with a crash/DoS rather than RCE. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | An attacker connected to a network service that streams MessagePack over a persistent connection sends a deliberately malformed payload to trigger a caught decode error, then sends additional bytes to the same long-lived Unpacker. The subsequent out-of-bounds read crashes the worker process with a SEGV, denying service; repeating this against a shared/reused Unpacker keeps the service down. … |
| Remediation | Upgrade msgpack to the fixed release - Vendor-released patch: 1.2.1 - by pinning msgpack>=1.2.1 in requirements and rebuilding/redeploying, per advisory GHSA-6v7p-g79w-8964 (https://github.com/msgpack/msgpack-python/security/advisories/GHSA-6v7p-g79w-8964). … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
24 hours: Audit all applications and dependencies for msgpack-python < 1.2.1; determine if affected systems process external MessagePack data. …
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External POC / Exploit Code
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