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One of the servers I look after is failing a PCI Compliance ASV scan.

The warning being picked up is:

The SSH server running on the remote host is affected by an information disclosure vulnerability.
According to its banner, the version of OpenSSH running on the remote host is prior to 7.5. It is, therefore, affected by an information disclosure vulnerability :

 - An unspecified timing flaw exists in the CBC padding oracle countermeasures, within the ssh and sshd functions, that allows an unauthenticated, remote attacker to disclose potentially sensitive information.
   Note that the OpenSSH client disables CBC ciphers by default. However, sshd offers them as lowest-preference options, which will be removed by default in a future release. (VulnDB 144000)

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

Open SSH 7.4 is the current version in stable debian packages.

I understand that Debian often backport the security fixes, but to get a 'false positive' approved I have to provide some evidence of this to the scanning company.

Can anyone suggest where/how to confirm this?

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Right in the text it warns you that all this does is check the reported version.

You could remove CBC ciphers from the server configuration, then share that configuration with them. There's a few guides on this out there, pick one you think is trustworthy.

I would stay with Debian's builds for stability. Default behavior with up to date servers and clients has a high chance of being secure, but the maintainers are going to remove backwards comparability with older ciphers slowly and carefully.

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