cbor2 has a Denial of Service via Uncontrolled Recursion in cbor2.loads

Exploitation

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

CVSS

7.5 · CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-26209: 0.40% estimated exploitation activity in the next 30 days; percentile 33.97 (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.

Summary - The cbor2 library is vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. - This vulnerability affects both the pure Python implementation and the C extension (cbor2). The C extension correctly uses Python's C-API for recursion protection (PyEnterRecursiveCall), but this mechanism is designed to prevent a stack overflow by raising a RecursionError. In some environments, this exception is not caught, thus causing the service process to terminate. - While the library handles moderate nesting, it lacks a configurable, data-driven depth limit independent of Python's global recursion setting. An attacker can supply a crafted CBOR payload containing thousands of nested arrays (e.g., 0x81). When cbor2.loads() attempts to parse this, it hits the interpreter's recursion limit, causing the call to raise a RecursionError. - By sending a stream of small (<100KB) malicious packets, an attacker can repeatedly crash…

Published by the source

23 Mar 2026, 20:23 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:37 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-3c37-wwvx-h642

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-3c37-wwvx-h642

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# Patch Next advisory brief: CVE-2026-26209

Advisory: cbor2 has a Denial of Service via Uncontrolled Recursion in cbor2.loads
IDs: CVE-2026-26209, GHSA-3c37-wwvx-h642
Record: https://patchnext.com/advisories/cve-2026-26209 (revision 2, source checked 26 Oct 2026, 13:37 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 5.9.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI cbor2: affected <= 5.8.0; fixed in 5.9.0

## 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-3c37-wwvx-h642 (OSV record GHSA-3c37-wwvx-h642 lists affected ranges: PyPI cbor2 >=0 <5.9.0 (fixed))
- Verified: https://github.com/advisories/GHSA-3c37-wwvx-h642 (GitHub-reviewed advisory GHSA-3c37-wwvx-h642 lists affected ranges: PyPI cbor2 >=0 <=5.8.0)

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