CVE-2021-38385: 
NixOS vulnerability analysis and mitigation

CVE-2021-38385 affects Tor versions before 0.3.5.16, 0.4.5.10, and 0.4.6.7. The vulnerability involves a mishandling of the relationship between batch-signature verification and single-signature verification, which can lead to a remote assertion failure. This vulnerability is also tracked as TROVE-2021-007 (Tor Blog).

The vulnerability stems from a behavior mismatch between batch-signature verification code and single-signature verification code in Tor's cryptographic implementation. The issue was discovered by Henry de Valence and affects the Ed25519 cryptographic functionality. The vulnerability has a CVSS v3.1 base score of 7.5 (HIGH), with a vector of CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H (NVD).

The vulnerability could allow remote attackers to trigger an assertion failure, leading to a denial of service attack. This affects all Tor installations whether running as a client, relay, or onion service (Tor Blog).

The vulnerability was fixed in Tor versions 0.3.5.16, 0.4.5.10, and 0.4.6.7. Users running earlier versions should upgrade to one of these patched versions. The fix involves disabling the unused batch verification feature of ed25519-donna (Tor Blog, Ubuntu Notice).