PuTTY’s versions from 0.68 to 0.80 inclusive have a critical vulnerability involving NIST P521 private keys used for SSH authentication, which can compromise the private key, allowing attackers to forge signatures as if they were you. It was discovered by Fabian Bäumer and Marcus Brinkmann of the Ruhr University Bochum. To fix this, PuTTY has switched to the RFC 6979 technique for generating keys. The vulnerability affects only 521-bit ECDSA keys, so other key types are unaffected. Recommendation: revoke affected keys, remove them from authorized_keys files, generate new keys. The fix has been implemented in PuTTY version 0.81.
https://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-p521-bias.html