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Date: Mon, 13 Aug 2012 14:34:17 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: oss-security@...ts.openwall.com
Subject: Re: Tunnel Blick: Multiple Vulnerabilities to Local
 Root and DoS (OS X)

Hi Kurt,

Sure, I'll trace each one, and include line numbers with the code.
This code comes from:
http://code.google.com/p/tunnelblick/source/browse/trunk/tunnelblick/openvpnstart.m?r=2095


1. A race condition in file permissions checking can lead to local root.
PoC: http://git.zx2c4.com/Pwnnel-Blicker/tree/pwnnel-blicker.c

   927	int runScript(NSString * scriptName,
   928	               int        argc,
   929	               char     * cfgName,
   930	               char     * cfgLoc)
   931	{
 * 964	                if (  checkOwnerAndPermissions(scriptPath, 0,
0, @"744")  ) {
   965	
   966	                    fprintf(stderr, "'%s' executing...\n",
[scriptName UTF8String]);
 * 967	                    returnValue = runAsRoot(scriptPath, [NSArray array]);
   968	                    fprintf(stderr, "'%s' returned with status
%d\n", [scriptName UTF8String], returnValue);
   969	                }

Here, there's a race condition between the two stared lines.


2. Insufficient checking of merely 0:0 744 can lead to local root on
systems with particular configurations.

   964	                if (  checkOwnerAndPermissions(scriptPath, 0,
0, @"744")  ) {
and
   801	            if (  ! checkOwnerAndPermissions(preConnectPath, 0,
0, @"744")  ) {
and
   847	            if (  ! checkOwnerAndPermissions(postTunTapPath, 0,
0, @"744")  ) {
and
  1675	                if (  ! checkOwnerAndPermissions(filePath, 0,
0, @"744")  ) {       // shell scripts are 744

Testing a file for whether or not it's 744 and owned by root:root is
not sufficient for deciding whether a unprivileged should be able to
run it as root. This not only makes every 744 root:root file on the
file system a potential vector, but destroys the nosuid mount flag OS
X uses for all user mountable images and network shares.


3. Insufficient validation of path names can allow for arbitrary
kernel module loading, which can lead to local root.

     86	    execPath = [[NSString stringWithUTF8String:argv[0]]
stringByDeletingLastPathComponent];
  1355	void loadKexts(unsigned int bitMask)
  1356	{
  1357	    if (  ( bitMask & (OPENVPNSTART_OUR_TAP_KEXT |
OPENVPNSTART_OUR_TUN_KEXT) ) == 0  ) {
  1358	        return;
  1359	    }
  1360	
  1361	    NSMutableArray*	arguments = [NSMutableArray arrayWithCapacity: 2];
  1362	    if (  (bitMask & OPENVPNSTART_OUR_TAP_KEXT) != 0  ) {
  1363	        NSString * tapkext = [@"tap" stringByAppendingString:
TunTapSuffixToUse([execPath stringByAppendingPathComponent: @"tap"])];
  1364	        [arguments addObject: [execPath
stringByAppendingPathComponent: tapkext]];
  1365	        fprintf(stderr, "Loading %s\n", [tapkext UTF8String]);
  1366	    }
  1367	    if (  (bitMask & OPENVPNSTART_OUR_TUN_KEXT) != 0  ) {
  1368	        NSString * tunkext = [@"tun" stringByAppendingString:
TunTapSuffixToUse([execPath stringByAppendingPathComponent: @"tun"])];
  1369	        [arguments addObject: [execPath
stringByAppendingPathComponent: tunkext]];
  1370	        fprintf(stderr, "Loading %s\n", [tunkext UTF8String]);
  1371	    }
  1372	
  1373	    becomeRoot();

  1374	    int status;
  1375	    int i;
  1376	    for (i=0; i < 5; i++) {
  1377	        NSTask * task = [[[NSTask alloc] init] autorelease];
  1378	
  1379	        [task setLaunchPath:@"/sbin/kextload"];
  1380	
  1381	        [task setArguments:arguments];
  1382	
  1383	        [task launch];

As you can see, the file name of the kernel extension being loaded is
derived from argv[0], which can be trivially bypassed ( execl(...,
"/attacker/controlled/argv/zero", ...) ).


4. Insufficient validation of path names can allow execution of
arbitrary scripts as root, leading to local root.
PoC: http://git.zx2c4.com/Pwnnel-Blicker/tree/pwnnel-blicker-for-kids.sh

     86	    execPath = [[NSString stringWithUTF8String:argv[0]]
stringByDeletingLastPathComponent];
   919	void runOpenVpnToGetVersion(NSString * openvpnVersion)
   920	{
   921	    NSString * openvpnPath = openvpnToUsePath([execPath
stringByAppendingPathComponent: @"openvpn"], openvpnVersion);
   922	    runAsRoot(openvpnPath, [NSArray arrayWithObject: @"--version"]);
   923	}

This basically amounts to this being run as root:  $(dirname
argv[0])/openvpn --version.



5. Insufficient path validation in errorExitIfAttackViaString can lead
to deletion of files as root, leading to DoS.
   164	        } else if( strcmp(command, "deleteLogs") == 0 ) {
   165				if (argc == 4) {
   166					NSString* configFile = [NSString stringWithUTF8String:argv[2]];
   167	                errorExitIfAttackViaString(configFile);
   168	                unsigned cfgLocCode = atoi(argv[3]);
   169	                deleteLogFiles(configFile, cfgLocCode);
   170	                syntaxError = FALSE;
   171	            }
  1735	void errorExitIfAttackViaString(NSString * string)
  1736	{
  1737	    BOOL startsWithDot = [string hasPrefix: @"."];
  1738	    NSRange r = [string rangeOfString: @"/.."];
  1739	    if (   startsWithDot
  1740	        || (r.length != 0)  ) {
  1741	        fprintf(stderr, "Tunnelblick openvpnstart: Apparent
attack detected; string being tested is %s\n", [string UTF8String]);
  1742	        [pool drain];
  1743	        exit(EXIT_FAILURE);
  1744	    }
  1745	}

The "exit if attack string" function doesn't check for links of any
kind, symbolic or hard, so this validation is not sufficient.


6. Allowing OpenVPN to run with user given configurations can lead to
local root.
   939	        if (  [configFile hasSuffix: @"tblk"]  ) {
   940	            unsigned  cfgLocCode = atoi(cfgLoc);
   941	            switch (cfgLocCode) {
   942	                case 0:
   943	                    configPrefix = [NSHomeDirectory()
stringByAppendingPathComponent:@"/Library/Application
Support/Tunnelblick/Configurations"];
   944	                    break;
   945	                case 1:
   946	                    configPrefix = [NSString
stringWithFormat:@"/Library/Application Support/Tunnelblick/Users/%@",
NSUserName()];
   947	                    break;
   948	                case 2:
   949	                    configPrefix = [execPath
stringByAppendingPathComponent: @"Deploy"];
   950	                    break;
   951	                case 3:
   952	                    configPrefix = [NSString stringWithString:
@"/Library/Application Support/Tunnelblick/Shared"];
   953	                    break;
   954	                default:
   955	                    break;
   956	            }
   957	        }
The purpose of this SUID helper is so that users can run OpenVPN with
their own provided configuration files as root. OpenVPN configuration
files can run scripts based on various OpenVPN events.


7. Race condition in process killing.
  1496	BOOL isOpenvpn(pid_t pid)
  1497	{
  1498		BOOL				is_openvpn	= FALSE;
  1499		int					count		= 0,
  1500		i			= 0;
  1501		struct kinfo_proc*	info		= NULL;
  1502		
  1503		getProcesses(&info, &count);
  1504	    for (i = 0; i < count; i++) {
  1505	        char* process_name = info[i].kp_proc.p_comm;
  1506	        pid_t thisPid = info[i].kp_proc.p_pid;
  1507	        if (pid == thisPid) {
  1508				if (strcmp(process_name, "openvpn")==0) {
  1509					is_openvpn = TRUE;
  1510				} else {
  1511					is_openvpn = FALSE;
  1512				}
  1513				break;
  1514			}
  1515	    }
  1516	    free(info);
  1517		return is_openvpn;
  1518	}

   991		if(isOpenvpn(pid)) {
   992			becomeRoot();
   993			didnotKill = kill(pid, SIGTERM);
and
  1022			if(strcmp(process_name, "openvpn") == 0) {
  1023				becomeRoot();
  1024				didnotKill = kill(pid, SIGTERM);

There's a race between checking the name of the process and killing
that PID. Since PIDs are cycled, eventually that PID could point to a
different process the user shouldn't have permission to kill.


Hope this clarifies things. Let me know if you have more questions.

Jason

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