python-ecdsa: Denial of Service via improper DER length validation in crafted private keys

Exploitation

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

CVSS

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

EPSS

• CVE-2026-33936: 0.48% estimated exploitation activity in the next 30 days; percentile 41.79 (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 An issue in the low-level DER parsing functions can cause unexpected exceptions to be raised from the public API functions. 1. ecdsa.der.removeoctetstring() accepts truncated DER where the encoded length exceeds the available buffer. For example, an OCTET STRING that declares a length of 4096 bytes but provides only 3 bytes is parsed successfully instead of being rejected. 2. Because of that, a crafted DER input can cause SigningKey.fromder() to raise an internal exception (IndexError: index out of bounds on dimension 1) rather than cleanly rejecting malformed DER (e.g., raising UnexpectedDER or ValueError). Applications that parse untrusted DER private keys may crash if they do not handle unexpected exceptions, resulting in a denial of service. Impact Potential denial-of-service when parsing untrusted DER private keys due to unexpected internal exceptions, and malformed DER acceptance due to missing bounds checks in DER helper functions. Reproduction Attach and run the…

Published by the source

27 Mar 2026, 15:56 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 13:40 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-9f5j-8jwj-x28g

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-9f5j-8jwj-x28g

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# Patch Next advisory brief: CVE-2026-33936

Advisory: python-ecdsa: Denial of Service via improper DER length validation in crafted private keys
IDs: CVE-2026-33936, GHSA-9f5j-8jwj-x28g
Record: https://patchnext.com/advisories/cve-2026-33936 (revision 2, source checked 26 Oct 2026, 13:40 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: ecdsa 0.19.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI ecdsa: affected < 0.19.2; fixed in 0.19.2

## Suggested inspection (read-only)
- `python -m pip show ecdsa`: 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 ecdsa.
- 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-9f5j-8jwj-x28g (OSV record GHSA-9f5j-8jwj-x28g lists affected ranges: PyPI ecdsa >=0 <0.19.2 (fixed))
- Verified: https://github.com/advisories/GHSA-9f5j-8jwj-x28g (GitHub-reviewed advisory GHSA-9f5j-8jwj-x28g lists affected ranges: PyPI ecdsa >=0 <0.19.2)

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