GPGTools installerHelper - setuid Local Privilege Escalation

Tracking and CVE:
Affected version(s):
  • < GPGTools 2014.12-b4
Fixed version:
  • GPGTools 2015.06
  • GPGTools 2015.08 - vulnerability disclosed (Release Notes)


GPGTools ships with a set-setuid installHelper binary:

$ ls -l /Library/Frameworks/Libmacgpg.framework/Versions/Current/Resources/

-rw-r--r--  1 root  wheel   1252 Oct 23  2013 Info.plist
-rw-r--r--  1 root  wheel    407 Oct 23  2013 Keyservers.plist
drwxr-xr-x  3 root  wheel    102 Aug 15 13:06 de.lproj
drwxr-xr-x  3 root  wheel    102 Aug 15 13:06 en.lproj
-rwsr-xr-x  1 root  wheel  68576 Oct 23  2013 installerHelper
-rw-r--r--  1 root  wheel    550 Oct 23  2013 org.gpgtools.Libmacgpg.xpc.plist
drwxr-xr-x  3 root  wheel    102 Aug 15 13:06

installHelper is used to install signed pkg-files as a normal user without root privileges:

$ /Library/Frameworks/Libmacgpg.framework/Versions/Current/Resources/installerHelper

Usage: installerHelper pkg-file [xml-file]
This tool checks the signature of an pkg file and installs it.
You can specify a xml-file to override the standard choices.

installHelper installs pkg-files by passing the pkg-file path and the xml-file path into the installPackage function:

BOOL installPackage(NSString *pkgPath, NSString *xmlPath) {
    // Run the installer command.
    NSString *commandString;
    if (xmlPath) {
        commandString = [NSString stringWithFormat:@"/usr/sbin/installer -applyChoiceChangesXML \"%@\" -pkg \"%@\" -target /", xmlPath, pkgPath];
    else {
        commandString = [NSString stringWithFormat:@"/usr/sbin/installer -pkg \"%@\" -target /", pkgPath];
    const char *command = [commandString UTF8String];
    uid_t uid = getuid();
    int result = setuid(0);
    if (result == 0) {
        //Run only this command with root privileges.
        result = system(command);
    } else {
        printf("This tool needs the setuid-bit to be set and the owner must be root!\nStart a normal installation using the GUI.\n");
        commandString = [NSString stringWithFormat:@"/usr/bin/open -Wnb \"%@\"", pkgPath];
        command = [commandString UTF8String];
        result = system(command);
    return !!result;

installPackage directly passed the two paths into the system function and therefore is vulnerable to command injection.

Exploitation / Proof of Concept:

The PoC makes use of the command substitution (`command`) applied by system to injection user-defined commands.

Before installPackage is called several requirements need to be fulfilled (see here):

  • the pkg-file must exist
  • the pkg-file must be signed with GPGTools’s key
  • if passed the xml-file must exist

A signed pkg-file (Install.pkg) can be obtained by downloading an older release of GPGTools.
Versions after 2014.12-b4 won’t work, because the signing key has been replaced to prevent downgrading of fixed installHelper versions to vulnerable ones.

After fulfilling the pkg-file requirements we want to inject commands via the xml-file path.
Therefore we create a file called `dummy` and pass it as xml-file to installHelper.
installHelper will accept it as a valid path and the command dummy will get executed with elevated rights.
So if we put an executable file named dummy into $PATH it will get executed.

For further details check out the full PoC: