GCVE-110-MAGEIA-2024-140

The PuTTY client and all related components generate heavily biased ECDSA nonces in the case of NIST P-521. To be more precise, the first 9 bits of each ECDSA nonce are zero. This allows for full secret key recovery in roughly 60 signatures by using state-of-the-art techniques. These signatures can either be harvested by a malicious server (man-in-the-middle attacks are not possible given that clients do not transmit their signature in the clear) or from any other source, e.g. signed git commits through forwarded agents. The nonce generation for other curves is slightly biased as well. However, the bias is negligible and far from enough to perform lattice-based key recovery attacks (not considering cryptanalytical advancements).