Vulnerability title: Privilege Escalation Via Symlink Attacks On POSIX Shared Memory With Insecure Permissions In AMD fglrx-driver
|Reported by:||Tim Brown|
In the process of validating the fix for CVE-2015-7723, it has been identified that the userland portion of the fglrx-driver utilised by Xorg still allows privilege escalation via symlink attacks on POSIX shared memory with insecure permissions.
As with the original vulnerability, with a Linux kernel where fs.protected_symlinks is not set to 1, or where this feature is not available, the following code (present in shared libraries distributed as part of fglrx-driver) calls shm_open() with insecure flags specified which allows the Linux kernel to follow an existent symlink under /dev/shm:
mov $0x1b6,%edx ; $edx (mode) = 0666 mov $0x42,%esi ; $esi (oflag) = O_CREAT | O_RDWR - missing O_EXCL mov %r13,%rdi callq 208b68 <shm_open@plt>
The same call to shm_open() requests that the permissions of newly created file are 0666 and as a result, an arbitrary file, owned by root, with permissions of 0666 can be created anywhere on the filesystem.
Furthermore, the code then proceeds to force the permissions on the resultant file to 0666 using fchmod(). This can be useful if the symlink target already exists:
mov $0x1b6,%esi ; $esi (mode) = 0666 mov %eax,%edi callq 209058 <fchmod@plt>
An attacker could create security sensitive files (such as libraries or configuration files), which when processed by a privileged program will lead to privilege escalation in instances where the program runs with privileges that the attacker does not have. For example, if the program is SetUID root, by creating a malicious library within a trusted location, an attacker would obtain root privileges upon exploiting this vulnerability.
The proof of concept exploit is available.
The vendor has released a patch.
|07/10/2015||Cloned from CVE-2015-7723|
|07/10/2015||MITRE assigned CVE-2015-7724|
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