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Background

Late Saturday (April 15^the), @MalwareHunterTeam tweeted about a new LockBit ransomware variant targeting macOS:

As shown in the tweet, the ransomware binary initially was undetected by any of the anti-virus engines on VirusTotal:

Shortly after @MalwareHunterTeam’s tweet, the fine folks of @vxunderground added their thoughts and shared samples:

The relevance of this macOS specimen is well articulated in their tweet:

“Lockbit ransomware group has created their first MacOS-based payload. We believe this is the first time a large ransomware threat group has developed a payload for Apple products.” vx-underground

Ok, so even though it’s the weekend, we have what appears to be a new macOS malware specimen from one of the more notorious ransomware gangs! Coupled with the fact that this may be, (as noted by @VXUnderground), “the first time a large ransomware threat group has developed a payload for Apple products” …I was intrigued to decided to dig right in!

In this blog post we’ll tear apart the sample, showing that ultimately, while yes it can indeed run on Apple Silicon, that is basically the extent of it’s impact. Thus macOS users have nothing to worry about …for now!

Triage

The (SHA-1) hash for the binary (aptly named locker_Apple_M1_64) is 2D15286D25F0E0938823DCD742BC928E78199B3D

While we're only focusing on the macOS sample, it appears the LockBit group has compiled this sample for a host of other OS's:

Using macOS’s file utility, we see the locker_Apple_M1_64 binary is a 64-bit arm64 Mach-O:

% file LockBit/locker_Apple_M1_64 
LockBit/locker_Apple_M1_64: Mach-O 64-bit executable arm64

And what about it’s code signing information? Let’s take a peek via macOS’s codesign and spctl utilities:

% codesign -dvv LockBit/locker_Apple_M1_64           
Executable=LockBit/locker_Apple_M1_64
Identifier=locker
Format=Mach-O thin (arm64)
CodeDirectory v=20400 size=3295 flags=0x20002(adhoc,linker-signed) hashes=100+0 location=embedded
Signature=adhoc
Info.plist=not bound
TeamIdentifier=not set
Sealed Resources=none
Internal requirements=none

% spctl -a -vvv -t install LockBit/locker_Apple_M1_64
LockBit/locker_Apple_M1_64: invalid signature (code or signature have been modified)

The codesign utility shows that though it’s signed, it’s signed “ad-hoc” (say vs. an Apple Developer ID). This means if downloaded to a macOS system (i.e. deployed by the attackers) macOS won’t let it run. This is confirmed by the spctl utility which shows “invalid signature” …or if you’re brave enough to try run it:

Let’s now run the strings command (with the "-" option which instructs it to scan the whole file), we find a few strings that appear to be related to standard ransomware activity including:

• Encryption
• File Extensions
• A file path, perhaps containing ransom instructions

% strings - -n 3 locker_Apple_M1_64 

curve25519xsalsa20poly1305
blake2b
blake2b_final
blake2b-ref.c
sodium_crit_enter
_sodium_malloc
/dev/urandom
/dev/random

...
cmd
ani
adv
msi
msp
com
nls
ocx
mpa
cpl
mod
hta
prf
rtp
rdp
bin
shs
wpx
bat
rom
msc
spl
ics
key
exe
dll
...

restore-my-files.txt
...

winmd
ntldr
ntuser.dat.log
bootsect.bak
autorun.inf
thumbs.db
iconcache.db

Also interesting are the strings related to Windows artifacts (e.g. autorun.inf, ntuser.dat.log, etc…). This may indicate the ransomware was originally written to target Windows platforms.

This wraps up our triage of the locker_Apple_M1_64 binary. Time to dive in deeper with our trusty friends: the disassembler and debugger!

Analysis of locker_Apple_M1_64

In this section we’ll more deeply analyze the malicious logic of the locker_Apple_M1_64 binary …which is simplified by the fact that the binary’s symbols aren’t stripped. (So function and method names are left intact ….yay!)

First, we pointed out that the locker_Apple_M1_64 is an arm64 binary (Mach-O 64-bit executable arm64). This means we better have at least a cursory understanding of AArch64. If you’re not familiar with this instruction set, here’s two of my favorite resources to get you up to speed:

• “Arm’d & Dangerous: Analyzing arm64 Malware Targeting macOS” (P. Wardle)

• “Blue Fox: Arm Assembly Internals and Reverse Engineering” (M. Markstedter)

Opening the malware in a disassembler, (starting at its main) we spot some (fairly common) anti-debugging logic:

main
...

000000010000b108         bl         imp___stubs__getppid                        
000000010000b10c         mov        x1, x0                                      
000000010000b110         mov        w0, #0x1f                                   
000000010000b114         mov        x2, #0x0                                    
000000010000b118         mov        w3, #0x0                                    
000000010000b11c         bl         imp___stubs__ptrace
000000010000b120         cmn        w0, #0x1

In short, this snippet of code invokes ptrace with PT_DENY_ATTACH (0x1f) which will kill the process if a debugger is currently attached, and also prevent future attachments.

As we’ll want to debug the malware to gain a deeper understanding of its functionality, we’ll have to bypass this. Good news, this is trivial! Just set a breakpoint on the address of the call to ptrace (b 0x000000010000b11c), and once this is hit, simply skip over the call by modifying the instruction pointer to point it to the next instruction (reg write $pc 0x000000010000b120). Since the call to ptrace will now be skipped, we’re free to debug to our hearts content.

% lldb locker_Apple_M1_64 
(lldb) target create "locker_Apple_M1_64"
Current executable set to 'locker_Apple_M1_64' (arm64).

(lldb) b 0x10000b11c
(lldb) r

* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
locker_Apple_M1_64`main:
->  0x10000b11c <+72>: bl     0x1000429cc               ; symbol stub for: ptrace
    
(lldb) reg write $pc 0x10000b120

Next the malware copies a chunk of encrypted data (size: 0x2468 bytes) from a variable named apple_config to a variable named g_Config.

000000010000b128         adr        x21, #0x10005908c ;g_Config
000000010000b12c         nop
000000010000b130         adr        x1, #0x100058008  ;apple_config                        
000000010000b134         nop
000000010000b138         mov        x0, x21                                     
000000010000b13c         mov        w2, #0x2468                                 
000000010000b140         bl         imp___stubs__memcpy

The apple_config variable is noteworthy as this is the only instance (I found) of any macOS specific references / customizations. (The rest of the malware’s binary simple looks like Linux code, compiled for macOS). Still as we’ll see this config doesn’t appear to contain anything specific to macOS.

Next, we find an XOR loop that decrypts this configuration.

000000010000b144         mov        x8, #0x0
000000010000b148         mov        w9, #0x2458

                     loc_10000b14c:
000000010000b14c         and        x10, x8, #0xf                               
000000010000b150         ldrb       w10, [x21, x10]
000000010000b154         add        x11, x21, x8
000000010000b158         ldrb       w12, [x11, #0x10]
000000010000b15c         eor        w10, w12, w10
000000010000b160         strb       w10, [x11, #0x10]
000000010000b164         add        x8, x8, #0x1
000000010000b168         cmp        x8, x9
000000010000b16c         b.ne       loc_10000b14c

Once de-XOR’d we find a few strings as "!!!-Restore-My-Files-!!!", "VMware vCenter Server", and XP_SP;Win10. Clearly, nothing relating to macOS specifically …in fact more indications this code is simply a recompile of (pieces of?) LockBit’s ransomware that targets Windows/Linux, and also VMware ESXi.

The malware then initializes various global variables, either with hard-coded values, or from the decrypted configuration (now held in g_Config). For example, here we see it set the bdaemon variable to 0x1, or ’true’ …even though its already set to that value.

_bdaemon:
0000000100059088         dd         0x00000001

...
000000010000b174         adrp       x9, #0x100059000
...
000000010000b17c         mov        w8, #0x1
000000010000b180         nop
000000010000b184         str        w8, [x9, #0x88] ; _bdaemon

Here’s the list of global variables that are referenced. Based on their names these provide insight into the malware configuration options:

• iMinfilesize
• iSpotMaximum
• bdaemon
• bSelfRemove
• publickey
• idelayinmin
• wholefile
• bfullog
• bnostop
• noext
• no_log
• bwipe
• bVMDKmode

…also, by manually manipulating these values in a debugger (or as we’ll see via the command line) we can simplify analysis. For example, setting bdaemon to false will prevent the binary from daemonizing (making it easier to debug), while setting bfullog to true to coerce the malware to log its actions!

The malware then de-XORs other strings, with the hard-coded key of 0x39. In the following disassembly, note the value of 0x39 is loaded into the w8 register. As w8 is the lower 32-bit of the 64-bit x8 register, only the 0x39 (of the 0x1964126200000039) will be loaded into w8.

Subsequent XOR instructions (eor) then make use of the w8 register, which holds the XOR key (0x39):

000000010000b260         ldr        w8, =0x1964126200000039
...

000000010000b270         eor        w10, w10, w8
...

000000010000b28c         eor        w10, w10, w8

Most of the strings are de-XOR’d in a function aptly named de_xor_all. We could step through this and dump each string manually, or, as a hard-coded XOR key is used (0x39), we can simply de-XOR the entire binary and see what falls out. This can be implemented in a few lines of Python:

with open("locker_Apple_M1_64", "rb") as f_in, open("strings", "wb") as f_out:
    while True:
        chunk = f_in.read(1024)
        if not chunk:
            break
        
        encrypted_chunk = bytes(byte ^ 0x39 for byte in chunk)
        f_out.write(encrypted_chunk)

This produces strings, including the ransomware’s command-line usage:

Usage: %s [OPTION]... -i '/path/to/crypt'
Recursively crypts files in a path or by extention.
Mandatory arguments to long options are mandatory for short options too.
  -i, --indir        path to crypt
  -m, --minfile      minimal size of a crypted file, no less than 4096
  -r, --remove       self remove this file after work
  -l, --log          prints the log to the console
  -n, --nolog        do not print the log to the file /tmp/locker.log
  -d, --daemonize    runs a program as Unix daemon
  -w, --wholefile    encrypts whole file
  -b, --beginfile    encrypts first N bytes
  -e, --extentions   encrypts files by extentions
  -o, --nostop       prevent to stop working VM
  -t, --wipe         wipe free space
  -s, --spot         upper bound limitation value of spot in Mb
  -p, --pass         password
  -f, --full         full log
  -a, --delay        start delay in minutes
  -y, --noexts       do not search for extentions
  -v, --vmdk         search for extentions inside VMDK files

Clearly these match to the global variables (that apparently can be set via the command-line as well). As we’ll set this makes analysis and trigger the malware’s core logic, even easier!

Also amongst the de-XOR’d strings, we find the full ransom note (here, truncated):

~~~ LockBit 3.0 the world's fastest and most stable ransomware from 2019~~~
>>>>> Your data is stolen and encrypted.
If you don't pay the ransom, the data will be published on our TOR darknet sites. Keep in mind that once your data appears on our leak site, it could be bought by your competitors at any second, so don't hesitate for a long time. The sooner you pay the ransom, the sooner your company will be safe.

Tor Browser Links:
http://lockbitapt2d73krlbewgv27tquljgxr33xbwwsp6rkyieto7u4ncead.onion
http://lockbitapt2yfbt7lchxejug47kmqvqqxvvjpqkmevv4l3azl3gy6pyd.onion
http://lockbitapt34kvrip6xojylohhxrwsvpzdffgs5z4pbbsywnzsbdguqd.onion
http://lockbitapt5x4zkjbcqmz6frdhecqqgadevyiwqxukksspnlidyvd7qd.onion
http://lockbitapt6vx57t3eeqjofwgcglmutr3a35nygvokja5uuccip4ykyd.onion
http://lockbitapt72iw55njgnqpymggskg5yp75ry7rirtdg4m7i42artsbqd.onion
http://lockbitaptawjl6udhpd323uehekiyatj6ftcxmkwe5sezs4fqgpjpid.onion
http://lockbitaptbdiajqtplcrigzgdjprwugkkut63nbvy2d5r4w2agyekqd.onion
http://lockbitaptc2iq4atewz2ise62q63wfktyrl4qtwuk5qax262kgtzjqd.onion
Links for normal browser:
http://lockbitapt2d73krlbewgv27tquljgxr33xbwwsp6rkyieto7u4ncead.onion.ly
http://lockbitapt2yfbt7lchxejug47kmqvqqxvvjpqkmevv4l3azl3gy6pyd.onion.ly
http://lockbitapt34kvrip6xojylohhxrwsvpzdffgs5z4pbbsywnzsbdguqd.onion.ly
http://lockbitapt5x4zkjbcqmz6frdhecqqgadevyiwqxukksspnlidyvd7qd.onion.ly
http://lockbitapt6vx57t3eeqjofwgcglmutr3a35nygvokja5uuccip4ykyd.onion.ly
http://lockbitapt72iw55njgnqpymggskg5yp75ry7rirtdg4m7i42artsbqd.onion.ly
http://lockbitaptawjl6udhpd323uehekiyatj6ftcxmkwe5sezs4fqgpjpid.onion.ly
http://lockbitaptbdiajqtplcrigzgdjprwugkkut63nbvy2d5r4w2agyekqd.onion.ly
http://lockbitaptc2iq4atewz2ise62q63wfktyrl4qtwuk5qax262kgtzjqd.onion.ly
...
>>>> Very important! For those who have cyber insurance against ransomware attacks.
Insurance companies require you to keep your insurance information secret, this is to never pay the maximum amount specified in the contract or to pay nothing at all, disrupting negotiations. The insurance company will try to derail negotiations in any way they can so that they can later argue that you will be denied coverage because your insurance does not cover the ransom amount. For example your company is insured for 10 million dollars, while negotiating with your insurance agent about the ransom he will offer us the lowest possible amount, for example 100 thousand dollars, we will refuse the paltry amount and ask for example the amount of 15 million dollars, the insurance agent will never offer us the top threshold of your insurance of 10 million dollars. He will do anything to derail negotiations and refuse to pay us out completely and leave you alone with your problem. If you told us anonymously that your company was insured for $10 million and other important details regarding insurance coverage, we
>>>>> If you do not pay the ransom, we will attack your company again in the future.

Once the malware has de-XOR’d all its strings, it invokes a method named go. This parses command-line options (updating global variables if relevant), and performs various other initializations, such as invoking a function named sodium_init (likely to initialize the commonly used Sodium crypto library). Interestingly, the malware invokes a function named get_password to read from stdin …it checks against the value of test

000000010000a618         bl         imp___stubs__strcmp

(lldb) x/s $x0
0x16fdfeb10: "hunter2"
(lldb) x/s $x1
0x10005a0d2: "test

At this point, armed with an decent understanding of the malware (yah, its ransomware) plus understanding of command-line options (that also allow us to enable logging), coupled with the fact that it’s a Sunday (aka the day of rest, not reversing malware) I decided to switch to passive dynamic analysis. Also, maybe you’re tired of reading arm64!? 😅

My goal was now to trigger the malware to encrypt (ransom) files, while passively observing this in action.

On my dedicated analysis machine, I first created a directory for it to encrypt (~/Downloads/lock_me_up) which I filled with files to encrypt (for some reason, I thought it was be funny to put other malware samples in there for the ransomware to encrypt).

Then I kicked off a file monitor, filtering on the locker_Apple_M1_64 process:

# FileMonitor.app/Contents/MacOS/FileMonitor -pretty -json -filter locker_Apple_M1_64
...

Next, I ran the malware with the following command line:

• -f : full log
• -p pass : password
• -i ~/Downloads/lock_me_up : directory to encrypt / ransom

First, the file monitoring detected the ransomware opening its log file (tmp/locklog). We’ll take a peek at the log file’s contents shortly.

# FileMonitor.app/Contents/MacOS/FileMonitor -pretty -json -filter locker_Apple_M1_64
...
{
  "event" : "ES_EVENT_TYPE_NOTIFY_OPEN",
  "file" : {
    "destination" : "/private/tmp/locklog",
    "process" : {
        ...
        "pid" : 8231
        "name" : "locker_Apple_M1_64",
        "path" : "/Users/user/Downloads/locker_Apple_M1_64",
        ...      
    }
  }
}

Then, we can see the malware accessing (opening) files in the specified directory, to encrypt them, and renaming the encrypted files with the .lockbit extension:

# FileMonitor.app/Contents/MacOS/FileMonitor -pretty -json -filter locker_Apple_M1_64
...
{
  "event" : "ES_EVENT_TYPE_NOTIFY_OPEN",
  "file" : {
    "destination" : "/Users/user/Downloads/lock_me_up/DazzleSpy.zip",
    "process" : {
        ...
        "pid" : 8231
        "name" : "locker_Apple_M1_64",
        "path" : "/Users/user/Downloads/locker_Apple_M1_64",
        ...      
    }
  }
},
{
  "event" : "ES_EVENT_TYPE_NOTIFY_RENAME",
  "file" : {
    "destination" : "/Users/user/Downloads/lock_me_up/DazzleSpy.zip.lockbit",
    "process" : {
        ...
        "pid" : 8231
        "name" : "locker_Apple_M1_64",
        "path" : "/Users/user/Downloads/locker_Apple_M1_64",
        ...      
    }
  }
}

Finally, the ransomware creates a file named "!!!-Restore-My-Files-!!!" which is ransom note with the decryption instructions:

# FileMonitor.app/Contents/MacOS/FileMonitor -pretty -json -filter locker_Apple_M1_64
...
{
  "event" : "ES_EVENT_TYPE_NOTIFY_CREATE",
  "file" : {
    "destination" : "/Users/user/Downloads/lock_me_up/!!!-Restore-My-Files-!!!",
    "process" : {
        ...
        "pid" : 8231
        "name" : "locker_Apple_M1_64",
        "path" : "/Users/user/Downloads/locker_Apple_M1_64",
        ...      
    }
  }
}

Let’s take a peek at the log (/tmp/locker.log), which show us details of the ransomware’s actions:

[19:16:45][74073472l][+] Launch parameters: ./locker_Apple_M1_64 -i '/Users/user/Downloads/lock_me_up' -m 16 -w 0 -b 0 -r 0 -l 1 -n 0 -d 1 -e '' -s 10 -p test -o 0 -t 0 -f 1 -a 0 -z 0 -y 0

~~~~~~~~~~~~~~~Hardware~~~~~~~~~~~~~~~~~~
[19:32:07][4329948544][+] Add directory to encrypt: /Users/user/Downloads/lock_me_up
[19:32:07][6140964864][+] Start encrypting file /Users/user/Downloads/lock_me_up/DazzleSpy.zip
[19:32:07][6140964864][+] Start encrypting file /Users/user/Downloads/lock_me_up/DazzleSpy.zip spot 0 from 1. Original checksum 2846984875
...
[19:32:07][6140964864][+] End file /Users/user/Downloads/lock_me_up/DazzleSpy.zip size 223464 time 1672883981 is encrypted. Checksum after encryption 3269819564
...

…easy enough to see it is, as expected, encrypting the files in the specified directory.

At this point, we’ve performed a fairly comprehensive analysis of the malware, and have confirmed it is (as wholly expected) ransomware. And, being able to trigger its ransomware logic is helpful to testing detection tools (discussed next).

Detection

Of course it goes without saying, having your files ransomed sucks! But good news, in this case the average macOS user is unlikely to be impacted by this LockBit macOS sample. Still the fact that a large ransomware gang has apparently set its sights on macOS, should give us pause for concern and also catalyze conversions about detecting and preventing this (and future) samples in the first place!

First, Apple has been fairly proactive about mitigating ransomware attacks on macOS (and maybe this is why we’ve yet seen a major ransomware outbreak on macOS). So, kudos to Cupertino. Specifically Apple has implemented SIP (and now read-only system volumes) to protect OS-level files. This means even if ransomware finds its way onto a macOS system it won’t (easily) be able to mess with core OS files. Apple has also added (TCC) protections to user files founds in (now) protected directories such as ~/Desktop, ~/Documents, etc. etc. This means, that without an exploit or explicit user-approval users files will remain protected.

Still an additional layer or detection/protection may be warranted, especially as we’ve all probably inadvertently clicked “Allow” on access prompts, while even Apple has been known to notarize malware.

If we stop to think specifically about ransomware, in theory, it should be trivial to detect (at least in most cases). Why? Simply put, ransomware’s actions provide us with a powerful detection heuristic.

In April 2016 (yes, wayyyy back then!) I posted a blog titled, “Towards Generic Ransomware Detection”. In this post, I mused,

“If we can monitor file I/O events and detect the rapid creation of encrypted files by untrusted processes, then ransomware may be generically detected” -(a younger) Patrick Wardle

To back this up, I released a free (and now open-source) tool called “RansomWhere?” that implemented this heuristic-based approach. Specifically it monitors file I/O events and for newly created files asks:

1. Is the process creating file untrusted (e.g. not an Apple platform binary)?
2. Is created file encrypted?
3. Is the (untrusted) process rapidly creating several encrypted files?

If these are all true, “RansomWhere?” will suspend the process as its likely ransomware and alert the user and handle the response (resume/terminate).

And though “RansomWhere?” is a bit dated, it appears to be able to generically detect this LockBit sample …even though it had no a priori knowledge of this malware:

Conclusion

Today we dove into a macOS ransomware sample created by the infamous LockBit ransomware gang. And while this may be the first time a large ransomware group created ransomware capable of running on macOS, it worth nothing that this sample is far from ready for prime time. From it’s lack of a valid code-signing signature to its ignorance of TCC and other macOS file-system protections as it stands it poses no threat to macOS users.

Moreover, the variant is rather buggy …containing flaws such as buffer overflows that will cause it to prematurely exit, when run on macOS:

The likely explanation to all this, is that the specimen we looked at today is test (or beta):

“Cisco Talos researcher Azim Khodjibaev told BleepingComputer that based on their research, the encryptors were meant as a test and were never intended for deployment in live cyberattacks.”

Still, as we stated earlier, the fact that a large ransomware gang (LockBit) has apparently set its sights on macOS, should give us all pause for concern. And, if nothing else, make sure we’re adequately prepared for future attacks that likely will be more polished and thus pose a greater risk …especially as (as noted in an excellent BleepingComputer writeup LockBit is actively developing this ransomware:

In response to questions from BleepingComputer, the public-facing representative of LockBit, known as LockBitSupp, said that the Mac encryptor is “actively being developed.” -BleepingComputer

Interested in learning more Mac Malware Analysis Techniques?

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