CVE-2021-3634

A flaw has been found in libssh in versions prior to 0.9.6. The SSH
protocol keeps track of two shared secrets during the lifetime of the
session. One of them is called secret_hash and the other session_id.
Initially, both of them are the same, but after key re-exchange, previous
session_id is kept and used as an input to new secret_hash. Historically,
both of these buffers had shared length variable, which worked as long as
these buffers were same. But the key re-exchange operation can also change
the key exchange method, which can be based on hash of different size,
eventually creating “secret_hash” of different size than the session_id
has. This becomes an issue when the session_id memory is zeroed or when it
is used again during second key re-exchange.

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