Application implements anti-debug techniques

Description

Debugging and dynamically exploring the application is a common technique during reverse engineering. For example, reverse engineers can trace the execution of the application, snoop on sensitive data, and modify memory to alter its behavior.

To complicate the task of analyzing the application dynamically, the application should implement anti-debugging techniques.

Recommendation

To implement anti-debugging techniques, use several of these techniques:

ptrace: Use the ptrace system call with the PT_DENY_ATTACH flag to prevent debuggers from attaching to the process. The system call is not part of iOS public API and requires using the dlsym function to obtain a function pointer to call it.

C

#import <dlfcn.h>
#import <sys/types.h>
#import <stdio.h>
typedef int (*ptrace_ptr_t)(int _request, pid_t _pid, caddr_t _addr, int _data);
void anti_debug() {
  ptrace_ptr_t ptrace_ptr = (ptrace_ptr_t)dlsym(RTLD_SELF, "ptrace");
  ptrace_ptr(31, 0, 0, 0); // PTRACE_DENY_ATTACH = 31
}

sysctl: The function can be used to retrieve information about the current process, including determining if the application is being debugged.

C

#include <assert.h>
#include <stdbool.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/sysctl.h>

static bool AmIBeingDebugged(void)
    // Returns true if the current process is being debugged (either
    // running under the debugger or has a debugger attached post facto).
{
    int                 junk;
    int                 mib[4];
    struct kinfo_proc   info;
    size_t              size;

    // Initialize the flags so that, if sysctl fails for some bizarre
    // reason, we get a predictable result.

    info.kp_proc.p_flag = 0;

    // Initialize mib, which tells sysctl the info we want, in this case
    // we're looking for information about a specific process ID.

    mib[0] = CTL_KERN;
    mib[1] = KERN_PROC;
    mib[2] = KERN_PROC_PID;
    mib[3] = getpid();

    // Call sysctl.

    size = sizeof(info);
    junk = sysctl(mib, sizeof(mib) / sizeof(*mib), &info, &size, NULL, 0);
    assert(junk == 0);

    // We're being debugged if the P_TRACED flag is set.

    return ( (info.kp_proc.p_flag & P_TRACED) != 0 );
}

getppid: iOS application can check the parent PID to detect if the application has been started with a debugger.

Application implements anti-debug techniques