ecdsa Denial of Service vulnerability in signature verification and signature malleability

possible DoS in signature verification and signature malleability

Impact

Code using VerifyingKey.verify() and VerifyingKey.verify_digest() may receive exceptions other than the documented BadSignatureError when signatures are malformed. If those other exceptions are not caught, they may lead to program termination and thus Denial of Service

Code using VerifyingKey.verify() and VerifyingKey.verify_digest() with sigdecode option using ecdsa.util.sigdecode_der will accept signatures even if they are not properly formatted DER. This makes the signatures malleable. It impacts only applications that later sign the signatures or verify signatures of signatures, e.g. Bitcoin.

All versions between 0.5 and 0.13.2 (inclusive) are thought to be vulnerable. Code before 0.5 may be vulnerable but didn’t receive extended analysis to rule this issue out.

Patches

The patches have been merged to master branch in https://github.com/warner/python-ecdsa/pull/115.
The backported patches for a release in the 0.13 branch are in https://github.com/warner/python-ecdsa/pull/124

They are part of the 0.13.3 release.

There are no plans to backport them to earlier releases.

Workarounds

It may be possible to prevent the Denial of Service by catching also UnexpectedDER, IndexError and AssertionError exceptions. That list hasn’t been verified to be complete though. If those exceptions are raised, the signature verification process should consider the signature to be invalid.

To remediate signature malleability and the Denial of Service vulnerability, it may be possible to first verify that the signature is properly DER formatted ECDSA-Sig-Value, as defined in RFC3279, before passing it to verify() or verify_digest() methods. If the signature is determined to not follow the DER or encode a different structure, the signature verification process should consider the signature to be invalid.

References

https://en.bitcoinwiki.org/wiki/Transaction_Malleability

For more information

If you have any questions or comments about this advisory please open an issue in python-ecdsa project.