Simon Tatham reports:
ECDSA signatures using 521-bit keys (the NIST P521 curve,
otherwise known as ecdsa-sha2-nistp521) were generated with biased
random numbers. This permits an attacker in possession of a few
dozen signatures to RECOVER THE PRIVATE KEY.
Any 521-bit ECDSA private key that PuTTY or Pageant has used to
sign anything should be considered compromised.
Additionally, if you have any 521-bit ECDSA private keys that
you've used with PuTTY, you should consider them to be
compromised: generate new keys, and remove the old public keys
from any authorized_keys files.
A second, independent scenario is that the adversary is an operator
of an SSH server to which the victim authenticates (for remote login
or file copy), [...] and the victim uses the same private key for
SSH connections to other services operated by other entities. Here,
the rogue server operator (who would otherwise have no way to
determine the victim's private key) can derive the victim's private
key, and then use it for unauthorized access to those other
services. If the other services include Git services, then again it
may be possible to conduct supply-chain attacks on software
maintained in Git. This also affects, for example, FileZilla before
3.67.0, WinSCP before 6.3.3, TortoiseGit before 2.15.0.1, and
TortoiseSVN through 1.14.6.