CVE-2021-3521: 
NixOS vulnerability analysis and mitigation

CVE-2021-3521 is a security vulnerability in RPM (Red Hat Package Manager) where the package manager does not require subkeys to have a valid binding signature. The vulnerability was discovered in 2021 and affects RPM versions prior to 4.18.0 (Gentoo Security).

The vulnerability stems from RPM's failure to validate and require binding signatures for subkeys in PGP public keys as mandated by the OpenPGP RFC. This could potentially result in a signature being wrongly trusted in scenarios where a malicious subkey (to which an attacker has the secret key) is added to a legitimate public key through a process that rejects main keys but not subkeys and does not verify binding signatures (Red Hat Bugzilla).

The greatest impact of this vulnerability is to data integrity. If successfully exploited, an attacker could potentially have their malicious subkey wrongly trusted by the system, leading to potential package verification bypasses (NVD).

The primary mitigation is to upgrade to RPM version 4.18.0 or later which includes the fix. The fix implements proper validation of subkey binding signatures on PGP public keys. It is strongly recommended to only use RPMs and public keys from trusted sources (GitHub PR).