cbor2 C extension decoder flaws can cause denial of service

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2025-64076: 0.49% estimated exploitation activity in the next 30 days; percentile 42.13 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Multiple vulnerabilities exist in cbor2 through version 5.7.0 in the decodedefinitelongstring() function of the C extension decoder (source/decoder.c): 1. Integer Underflow Leading to Out-of-Bounds Read (CWE-191, CWE-125): An incorrect variable reference and missing state reset in the chunk processing loop causes bufferlength to not be reset to zero after UTF-8 character consumption. This results in subsequent chunklength calculations producing negative values (e.g., chunklength = 65536 - bufferlength), which are passed as signed integers to the read() method, potentially triggering unlimited read operations and resource exhaustion. 2. Memory Leak via Missing Reference Count Release (CWE-401): The main processing loop fails to release Python object references (PyDECREF) for chunk objects allocated in each iteration. For CBOR strings longer than 65536 bytes, this causes cumulative memory leaks proportional to the payload size, enabling memory exhaustion attacks through repeated…

Published by the source

18 Nov 2025, 18:32 UTC

Last modified by the source

1 Sep 2026, 16:55 UTC

Source last checked

26 Oct 2026, 13:50 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-9q9c-vjxh-p795

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-9q9c-vjxh-p795

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# Patch Next advisory brief: CVE-2025-64076

Advisory: cbor2 C extension decoder flaws can cause denial of service
IDs: CVE-2025-64076, GHSA-9q9c-vjxh-p795
Record: https://patchnext.com/advisories/cve-2025-64076 (revision 2, source checked 26 Oct 2026, 13:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: cbor2 5.7.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI cbor2: affected < 5.7.1; fixed in 5.7.1

## Suggested inspection (read-only)
- `python -m pip show cbor2`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on cbor2.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-9q9c-vjxh-p795 (OSV record GHSA-9q9c-vjxh-p795 lists affected ranges: PyPI cbor2 >=0 <5.7.1 (fixed))
- Verified: https://github.com/advisories/GHSA-9q9c-vjxh-p795 (GitHub-reviewed advisory GHSA-9q9c-vjxh-p795 lists affected ranges: PyPI cbor2 >=0 <5.7.1)

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