Quick Analysis for the SSID Format String Bug

20 Jun 2021

Days ago a twitter post revealed a bug in iOS Wi-Fi service:

After joining my personal WiFi with the SSID “%p%s%s%s%s%n”, my iPhone permanently disabled it’s WiFi functionality. Neither rebooting nor changing SSID fixes it :~) pic.twitter.com/2eue90JFu3

— Carl Schou (@vm_call) June 18, 2021

Looks like it’s a format string bug, which is rarely seen nowadays.

Now set up a hotspot with the same SSID and use my test device to join, the wifid crashes soon.

Here’s the symbolicated crash report wifid-2021-06-20-xxxxxx.ips

Thread 2 name:  Dispatch queue: com.apple.wifid.managerQueue
Thread 2 Crashed:
0   libsystem_platform.dylib      	0x00000001ebcb9724 _platform_strlen + 4
1   CoreFoundation                	0x00000001a381d84c __CFStringAppendFormatCore + 8812
2   CoreFoundation                	0x00000001a381efa8 _CFStringCreateWithFormatAndArgumentsReturningMetadata + 160
3   WiFiPolicy                    	0x00000001d0895f8c -[WFLogger WFLog:message:] + 192
4   ???                           	0x000000010692c00c 0 + 4405248012
5   wifid                         	0x0000000100f58a74 0x100e40000 + 1149556
6   wifid                         	0x0000000100f58c74 0x100e40000 + 1150068

So it’s really a format string bug!

Decompile this function -[WFLogger WFLog:message:] in dyld_shared_cache. There are two references to CFStringCreateWithFormatAndArguments.

v7 = j__CFStringCreateWithCString_107(0LL, a4, 0x8000100u); // the format string
    if ( v7 || (v7 = j__CFStringCreateWithCString_107(0LL, a4, 0)) != 0LL )
    {
      if ( self->_destination == 2 )
      {
        v8 = j__CFStringCreateWithFormatAndArguments_26(0LL, 0LL, v7, v21);
        v18[3] = (__int64)v8;
      }

Another one

      if ( self->_destination != 2
        && (!self->_wflRunningOnWatchClassDevice || self->_wflEnableDualLoggingOnWatchClassDevice) )
      {
        *(_QWORD *)&v16.tm_sec = 0LL;
        *(_QWORD *)&v16.tm_hour = &v16;
        *(_QWORD *)&v16.tm_mon = 0x2020000000LL;
        *(_QWORD *)&v16.tm_wday = 0LL;
        v10 = j__CFStringCreateWithFormatAndArguments_26(0LL, 0LL, v7, v21); // <-- here

Debugging this issue with lldb will be painful because this method hits too often. Instead, attach frida to it: frida-trace -U wifid -m '-[WFLogger WFLog:message:]' and modify the auto-generated script a little:

  onEnter(log, args, state) {
    const msg = '' + args[3].readUtf8String();
    log(`-[WFLogger WFLog:${args[2]} message:${msg}]`);
    if (msg.indexOf('%p%s%s%s%s%n') > -1) {
      for (let i = 3; i < 10; i++) {
        log(args[i], JSON.stringify(Process.findRangeByAddress(args[i])));
      }

      log('called from:\n' +
        Thread.backtrace(this.context, Backtracer.ACCURATE)
        .map(DebugSymbol.fromAddress).join('\n') + '\n');
    }
  },

Here’s the log right before the crash.

17863 ms -[WFLogger WFLog:0x3 message:Dequeuing command type: “%@” pending commands: %ld]

17863 ms -[WFLogger WFLog:0x3 message:{ASSOC+} Attempting Apple80211AssociateAsync to %p%s%s%s%s%n]

According to the backtrace, this is the root cause:

v27 = sub_1000A25D4(v21);
v28 = objc_msgSend(
        &OBJC_CLASS___NSString,
        "stringWithFormat:",
        CFSTR("Attempting Apple80211AssociateAsync to %@"),
        v27);
v29 = objc_msgSend(&OBJC_CLASS___NSString, "stringWithFormat:", CFSTR("{ %@+} %@"), CFSTR("ASSOC"), v28);
v30 = objc_autoreleasePoolPush();
v31 = (void *)qword_100251888;
if ( qword_100251888 )
{
    v32 = objc_msgSend(v29, "UTF8String");
    objc_msgSend(v31, "WFLog:message:", 3LL, v32);
}
objc_autoreleasePoolPop(v30);

It concats the SSID to a format string and pass it to WFLog:message: method. Destination is 3 so it was the second xref of CFStringCreateWithFormatAndArguments that triggered the denial of service.

For the exploitability, it doesn’t echo and the rest of the parameters don’t seem like to be controllable. Thus I don’t think this case is exploitable. After all, to trigger this bug, you need to connect to that WiFi, where the SSID is visible to the victim. A phishing Wi-Fi portal page might as well be more effective.