CVE-2019-14853: High severity Python-ecdsa Project Python-ecdsa vulnerability
possible DoS in signature verification and signature malleability
Impact Code using VerifyingKey.verify() and VerifyingKey.verifydigest() 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.verifydigest() with sigdecode option using ecdsa.util.sigdecodeder 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 verifydigest() 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/TransactionMalleability
For more information If you have any questions or comments about this advisory please open an issue in python-ecdsa project.
Other sources
A flaw was found in python-ecdsa. Unexpected and undocumented exceptions can be raised during signature decoding may lead to denial of service in some cases. All the versions between at least 0.5 and 0.13.2 are thought to be vulnerable.
Upstream issue:
https://github.com/warner/python-ecdsa/issues/114
Upstream patch:
https://github.com/warner/python-ecdsa/pull/115
References:
https://github.com/warner/python-ecdsa/blob/bb359d32e93acc3eb4d216aff4ba0e7531599cfb/ecdsa/keys.py#L98-L113
— Red Hat
An error-handling flaw was found in python-ecdsa before version 0.13.3. During signature decoding, malformed DER signatures could raise unexpected exceptions (or no exceptions at all), which could lead to a denial of service.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
debian/python-ecdsato a version that resolves this vulnerability.Fixed in 0.13-3+deb10u1Fixed in 0.16.1-1Fixed in 0.18.0-3 - Upgrade
Upgrade
redhat/python-ecdsato a version that resolves this vulnerability.Fixed in 0.13.3 - Upgrade
Upgrade
pip/ecdsato a version that resolves this vulnerability.Fixed in 0.13.3 - Upgrade
Upgrade
debian/python-ecdsato a version that resolves this vulnerability.Fixed in 0.13-3+deb10u1 - Upgrade
Upgrade
debian/python-ecdsato a version that resolves this vulnerability.Fixed in 0.16.1-1 - Upgrade
Upgrade
debian/python-ecdsato a version that resolves this vulnerability.Fixed in 0.18.0-3 - Configuration
Before calling VerifyingKey.verify() or VerifyingKey.verify_digest() (including when using ecdsa.util.sigdecode_der), validate that the signature is properly DER-formatted ECDSA-Sig-Value as defined in RFC3279. Additionally, wrap calls to verify() and verify_digest() in try/except blocks that catch UnexpectedDER, IndexError, and AssertionError and treat those exceptions as signature verification failures to avoid program termination (Denial of Service).
Applications using python-ecdsa VerifyingKey.verify()/verify_digest() signature_validation = validate DER ECDSA-Sig-Value per RFC3279 before verification; catch UnexpectedDER, IndexError, AssertionError and treat signature as invalid
Event History
Frequently Asked Questions
What is the vulnerability ID of this security flaw?
The vulnerability ID of this security flaw is CVE-2019-14853.
What is the severity rating of CVE-2019-14853?
CVE-2019-14853 has a severity rating of 7.5 (high).
How does this vulnerability impact code using VerifyingKey.verify() and VerifyingKey.verify_digest()?
Code using VerifyingKey.verify() and VerifyingKey.verify_digest() may receive exceptions other than the documented BadSignatureError when signatures are malformed.
How can the vulnerability be fixed?
To fix this vulnerability, update to python-ecdsa version 0.13.3 or later.
Where can I find more information about CVE-2019-14853?
You can find more information about CVE-2019-14853 on the GitHub security advisory page (https://github.com/warner/python-ecdsa/security/advisories/GHSA-pwfw-mgfj-7g3g) and the NIST National Vulnerability Database (https://nvd.nist.gov/vuln/detail/CVE-2019-14853).