CVE-2021-20305: 
NixOS vulnerability analysis and mitigation

A vulnerability was discovered in Nettle versions before 3.7.2, where several Nettle signature verification functions (GOST DSA, EDDSA & ECDSA) result in the Elliptic Curve Cryptography point (ECC) multiply function being called with out-of-range scalers. The vulnerability was disclosed in April 2021 and affects the cryptographic library's signature verification functionality (MITRE CVE, NVD).

The vulnerability stems from the Elliptic Curve Cryptography point (ECC) multiply function being called with out-of-range scalars, which may result in incorrect results or assertion failures. This issue affects all NIST secp* curves as well as the GOST curves. The vulnerability has been assigned a CVSS v3.1 base score of 8.1 (High) with the vector CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H (Ubuntu Security, NetApp Security).

The vulnerability can lead to disclosure of sensitive information, addition or modification of data, or Denial of Service (DoS). An attacker could force an invalid signature, causing an assertion failure or possible validation issues, affecting the system's confidentiality, integrity, and availability (Debian Security, Gentoo Security).

The vulnerability can be exploited remotely by sending specially crafted valid-looking input signatures. The attack complexity is considered high, requires no privileges or user interaction, and the scope is unchanged. The vulnerability affects the network attack vector (NetApp Security, Red Hat Bugzilla).

The vulnerability has been fixed in Nettle version 3.7.2. Multiple patches were released to address the issue, including new functions ecc_mod_mul_canonical and ecc_mod_sqr_canonical, fixes for ecc_ecdsa_verify, ecdsa_sign output canonical reduction, and similar fixes for eddsa and gostdsa_vko. Users are recommended to upgrade to the latest version of Nettle (Red Hat Bugzilla).