CVE-2020-12607: 
Python vulnerability analysis and mitigation

An issue was discovered in fastecdsa before 2.1.2 affecting the ECDSA implementation when using the NIST P-256 curve. The vulnerability involves improper handling of the point at infinity, which causes signature verification to fail even when the signature is correct. This occurs specifically for extreme values in k and s^-1. The issue was discovered in April 2020 and was assigned CVE-2020-12607 (NVD).

The vulnerability stems from a mishandling of the point at infinity in the ECDSA implementation. When extreme values are used for k and s^-1 parameters during signature verification on the NIST P-256 curve, the verification process incorrectly fails for valid signatures. This was confirmed through Google's Wycheproof test vectors, specifically test case 345. The vulnerability has a CVSS v3.1 base score of 7.5 (HIGH) with vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N (NVD).

The vulnerability affects signature verification functionality, causing valid signatures to be incorrectly rejected. This behavior is not solely a usability problem, as there are specific threat models where an attacker could benefit by successfully identifying users for whom signature verification will fail (NVD).

The vulnerability can be triggered by using specific extreme values for k and s^-1 parameters in ECDSA signatures. A proof of concept exists demonstrating the issue using test vectors from Google's Wycheproof project (Github Issue).

The vulnerability was fixed in fastecdsa version 2.1.2 by properly handling the point at infinity in the ECDSA implementation. Users should upgrade to version 2.1.2 or later to address this issue (Github Commit).