| Description: | In PuTTY 0.68 through 0.80 before 0.81, biased ECDSA nonce generation
allows an attacker to recover a user's NIST P-521 secret key via a
quick attack in approximately 60 signatures. This is especially
important in a scenario where an adversary is able to read messages
signed by PuTTY or Pageant. The required set of signed messages may be
publicly readable because they are stored in a public Git service that
supports use of SSH for commit signing, and the signatures were made
by Pageant through an agent-forwarding mechanism. In other words, an
adversary may already have enough signature information to compromise
a victim's private key, even if there is no further use of vulnerable
PuTTY versions. After a key compromise, an adversary may be able to
conduct supply-chain attacks on software maintained in Git. 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), even though this server is not fully trusted by the victim, 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.
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