Description of problem:
Filed bug also to CentOS 7 ticket system (https://bugs.centos.org/view.php?id=13935) but here's the description also.
There is a bug in the current OpenSSH version in Centos7 (openssh-server-7.4p1-12.el7_4.x86_64) which prevents WinSCP >= 5.10 clients for connecting. This can be reproduced in our environment all the time.
The bug is described here: https://bugzilla.mindrot.org/show_bug.cgi?id=2748
Basically the issue is that OpenSSH has some compatibility things for older WinSCP versions but unfortunately 5.1* matches also for 5.10 and 5.11.
Here's the debug log from the server side:
"sshd: debug1: match: WinSCP_release_5.11.1 pat WinSCP_release_4*,WinSCP_release_5.0*,WinSCP_release_5.1*,WinSCP_release_5.5*,WinSCP_release_5.6*,WinSCP_release_5.7,WinSCP_release_5.7.1,WinSCP_release_5.7.2,WinSCP_release_5.7.3,WinSCP_release_5.7.4 compat 0x00004000"
Version-Release number of selected component (if applicable): OpenSSH 7.4, WinSCP 5.10 or newer
How reproducible: always
Steps to Reproduce:
1) Install openssh-server-7.4 to the server side and WinSCP 5.10 or newer to the client
2) Try to connect and it fails
Connection is successful.
This sounds like a good candidate to be fixed in the next update. It is a long-existing bug in OpenSSH, which was recently fixed upstream (after revealing itself).
As I described here, it doesn't appear with the default config, but still appears with the OpenSSH 7.4 + WinSCP 5.11 (probably also with 5.10) when sshd config is altered regarding KexAlgorithms.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.