Hidden demons? MailDemon Patch Analysis: iOS 13.4.5 Beta vs. iOS 13.5

Summary and TL;DR

Further to Apple’s patch of the MailDemon vulnerability (see our blog here), ZecOps Research Team has analyzed and compared the MailDemon patches of iOS 13.4.5 beta and iOS 13.5. 

Our analysis concluded  that the patches are different, and that iOS 13.4.5 beta patch was incomplete and could be still vulnerable under certain circumstances. 

Since the 13.4.5 beta patch was insufficient, Apple issued a complete patch utilising a different approach which fixed this issue completely on both iOS 13.5 and iOS 12.4.7 as a special security update for older devices. 

This may explain why it took about one month for a full patch to be released. 

iOS 13.4.5 beta patch

The following is the heap-overflow vulnerability patch on iOS 13.4.5 beta.  

The function  -[MFMutableData appendBytes:length:] raises an exception if  -[MFMutableData _mapMutableData] returns false.

In order to see when -[MFMutableData _mapMutableData] returns false, let’s take a look at how it is implemented:

When mmap fails it returns False, but still allocates a 8-bytes chunk and stores the pointer in self->bytes. This patch raises an exception before copying data into self->bytes, which solves the heap overflow issue partially.

 -[MFMutableData appendData:]
 |
 +--  -[MFMutableData appendBytes:length:] **<-patch**
    |
    +-- -[MFMutableData _mapMutableData]

The patch makes sure an exception will be raised inside -[MFMutableData appendBytes:length:]. However, there are other functions that call -[MFMutableData _mapMutableData] and interact with self->bytes which will be an 8-bytes chunk if mmap fails, these functions do not check if mmap fails or not since the patch only affects -[MFMutableData appendBytes:length:].

Following is an actual backtrace taken from MobileMail:

 * frame #0: 0x000000022a0fd018 MIME`-[MFMutableData _mapMutableData:]
    frame #1: 0x000000022a0fc2cc MIME`-[MFMutableData bytes] + 108
    frame #2: 0x000000022a0fc314 MIME`-[MFMutableData mutableBytes] + 52
    frame #3: 0x000000022a0f091c MIME`_MappedAllocatorAllocate + 76
    frame #4: 0x0000000218cd4e9c CoreFoundation`_CFRuntimeCreateInstance + 324
    frame #5: 0x0000000218cee5c4 CoreFoundation`__CFStringCreateImmutableFunnel3 + 1908
    frame #6: 0x0000000218ceeb04 CoreFoundation`CFStringCreateWithBytes + 44
    frame #7: 0x000000022a0eab94 MIME`_MFCreateStringWithBytes + 80
    frame #8: 0x000000022a0eb3a8 MIME`_filter_checkASCII + 84
    frame #9: 0x000000022a0ea7b4 MIME`MFCreateStringWithBytes + 136

-[MFMutableData mutableBytes]
|
+--  -[MFMutableData bytes]
   |
   +--  -[MFMutableData _mapMutableData:]

Since the bytes returned by mutableBytes is usually considered to be modifiable given following from Apple’s documentation:

This property is similar to, but different than the bytes property. The bytes property contains a pointer to a constant. You can use The bytes pointer to read the data managed by the data object, but you cannot modify that data. However, if the mutableBytes property contains a non-null pointer, this pointer points to mutable data. You can use the mutableBytes pointer to modify the data managed by the data object.

Apple’s documentation

Both -[MFMutableData mutableBytes] and -[MFMutableData bytes] returns self->bytes points to the 8-bytes chunk if mmap fails, which might lead to heap overflow under some circumstances.

The following is an example of how things could go wrong, the heap overflow still would happen even if it checks length before memcpy:

size_t length = 0x30000;
MFMutableData* mdata = [MFMutableData alloc];
data = malloc(length);
[mdata initWithBytesNoCopy:data length:length];    
size_t mdata_len = [mdata length];
char* mbytes = [mdata mutableBytes];//mbytes could be a 8-bytes chunk
size_t new_data_len = 90;
char* new_data = malloc(new_data_len);
if (new_data_len <= mdata_len) {
    memcpy(mbytes, new_data, new_data_len);//heap overflow if mmap fails
}

iOS 13.5 Patch

Following the iOS 13.5 patch, an exception is raised in “-[MFMutableData _mapMutableData] ”, right after mmap fails and it doesn’t return the 8-bytes chunk anymore. This approach fixes the issue completely.

Summary

iOS 13.5 patch is the correct way to patch the heap overflow vulnerability. It is important to double check security patches and verify that the patch is complete. 

At ZecOps we help developers to find security weaknesses, and validate if the issue was correctly solved automatically. If you would like to find similar vulnerabilities in your applications/programs, we are now adding additional users to our CrashOps SDK beta program. 
If you do not own an app, and would like to inspect your phone for suspicious activity – check out ZecOps iOS DFIR solution – Gluon.