Tracking Android/Joker payloads with Medusa, static analysis
(and patience)
By @cryptax
Published: 2022-06-20 · Archived: 2026-04-05 14:44:10 UTC
I am looking into a new sample of Android/Joker, reported on June 19, 2022 by @ReBensk:
afeb6efad25ed7bf1bc183c19ab5b59ccf799d46e620a5d1257d32669bedff6f
Android/Joker is known for using many payloads: a first payload loads another payload, which loads another
one etc. Matryoshka dolls-style 😁. See an analysis of a previous Joker sample. This sample uses many payloads
too, but the implementation to load the payloads is a bit different. I’ll detail.
Medusa
I recently discovered Medusa and like it very much… for dynamic analysis (I still prefer static analysis,
everybody knows that by now?). Medusa is easy to use and comes with a collection of ready-to-use Frida
hooks. Launch an Android emulator, a Frida server, install the sample, then launch Medusa python3 medusa.py .
Select the hooks you want to use (search through hooks with the search command, then use to use a given
hook, finally compile the list of hooks). Those are the hooks you need (I recently contributed to the last two
hooks):
use http_communications/uri_logger
use encryption/cipher_1
use code_loading/dump_dyndex
use code_loading/load_class
Finally, start the malware ( run -f package_name , or run -n 0 if you have a single sample installed on your
emulator).
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https://cryptax.medium.com/tracking-android-joker-payloads-with-medusa-static-analysis-and-patience-672348b81ac2
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I use URI hooks (http_communications/uri_logger) in Medusa and see the malware calls those
URLs. Android/Joker is known to use URLs such as xxx[.]aliyuncs.com.
As Android/Joker samples usually don’t make things simple for malware analysts, I somewhat expected those
URLs to be encrypted. Medusa has decryption hooks too.
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Bingo! The look4.oss-ap[..]aliyuncs.com URL is encrypted. The decryption hooks,
encryption/cipher_1, with shows the decrypted value.
My dynamic DEX dumper + the convenient loadClass hooks show several files are dynamically loaded:
DexClassLoader called: /data/user/0/com.designemoji.keyboard/files/audience_network.dex
[+] Dumped /data/user/0/com.designemoji.keyboard/files/audience_network.dex to dump_1
loadClass: com.designemoji.keyboard.EnableActivity
loadClass: com.facebook.ads.internal.dynamicloading.DynamicLoaderImpl
...
PathClassLoader(f,p) called: /data/user/0/com.designemoji.keyboard/cache/nuff
[+] Dumped /data/user/0/com.designemoji.keyboard/cache/nuff to dump_2
loadClass: seek...
DexClassLoader called: /data/user/0/com.designemoji.keyboard/files/seek
[+] Dumped /data/user/0/com.designemoji.keyboard/files/seek to dump_3
DexClassLoader called: /data/user/0/com.designemoji.keyboard/files/Yang
[+] Dumped /data/user/0/com.designemoji.keyboard/files/Yang to dump_4
loadClass: com.xjuys
loadClass: com.android.installreferrer.api.InstallReferrerClient
https://cryptax.medium.com/tracking-android-joker-payloads-with-medusa-static-analysis-and-patience-672348b81ac2
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The first DEX ( audience_network.dex ) belongs to Facebook. I am not after this. The 3 other DEXes ( nuff ,
seek and Yang ) are far more promising. Note they are loaded by PathClassLoader for nuff , and
DexClassLoader for the other 2.
Loading nuff (payload 1)
DroidLysis doesn’t detect any use of DexClassloader , PathClassLoader or InMemoryDexClassLoader . So,
how is the first payload loaded? Let’s locate the URL (look4[…]aliyuncs.com). It is encrypted, so I search where
encrypted is used in DroidLysis’ detailed report.
## Cipher
- file=./emojikeyboard.apk-afeb6efad25ed7bf1bc183c19ab5b59ccf799d46e620a5d1257d32669bedff6f/smali/f/a
- file=./emojikeyboard.apk-afeb6efad25ed7bf1bc183c19ab5b59ccf799d46e620a5d1257d32669bedff6f/smali/f/a
Fortunately, there are not many different locations, and I directly head to the good one: f.a.a.a . Encrypted
strings are decrypted using PBEWithMD5AndDES . I write a static decryptor.
Decrypted=https://look4[.]oss-ap-southeast-5[.]aliyuncs.com/designemoji
Decrypted=getClassLoader
Decrypted=loadClass
Decrypted=seek
Decrypted=melody
The URL gets a JAR, stores it in a cache directory of the application, and then loads it via … getClassLoader !
That’s why DroidLysis didn’t see it! (to be fixed).
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Code loading the JAR with getClassLoader, then invokes a method named melody()
Static analysis of nuff (payload 1)
The JAR contains a classes.dex with a single class named seek , and a method named melody. It is simple to
understand:
https://cryptax.medium.com/tracking-android-joker-payloads-with-medusa-static-analysis-and-patience-672348b81ac2
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1. It downloads DEX file from https://look4.oss-ap-southeast-5[.]aliyuncs[.]com/nunber
2. It stores that DEX in the application’s file directory, with filename seek
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3. It loads the DEX using DexClassLoader
4. It invokes cantus.bustle() in that DEX
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Code of payload 1. Download URL for payload 2 — we also see that class cantus, method bustle is
called.
Static analysis of payload 2
Just guess what cantus.bustle() does? It downloads yet another DEX from https://xjuys.oss-accelerate[.]aliyuncs.com/xjuys !
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Payload 2 is loading … Payload 3
This time, the payload will be stored in a file named Yang , and it will search for class com.xjuys and method
xjuys .
Static analysis of payload 3
This com.xjuys JAR had been already used in several other samples of Joker (sha256:
2edaf2a2d8fd09a254ea41afa4d32b145dcec1ab431a127b2462b5ea58e2903d ).
It loads dynamically 2 other ZIPs:
1. https://xjuys.oss-accelerate[.]aliyuncs.com/fbhx1. We have already seen this payload. It is the same as in
this article and contains facebook hooks.
2. https://beside.oss-eu-west-1[.]aliyuncs.com/af2. It stores the file in the app’s file directory, with filename
KBNViao . Then, it loads com.appsflyer.AppsFlyerLib and methods init() then startTracking()
[love the name of the method, don’t we? 😏]. This is Apps Flyer SDK, a mobile analytics library.
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Connect to remote URL and download payload 4.
Summary
The initial DEX is quite heavily obfuscated
Payload 1 ( designmoji / nuff ) has no other use than loading Payload 2
Payload 2 ( nunber / seek ) enables notification listeners (we haven’t detailed this in this article) and loads
Payload 3
Payload 3 ( xjuys / Yang ) has yet more malicious code (not detailed here) and loads 2 additional DEX:
one for Facebook, the other one contains Apps Flyer SDK.
Payload 4a and 4b: Facebook hooks + Apps Flyer SDK.
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https://cryptax.medium.com/tracking-android-joker-payloads-with-medusa-static-analysis-and-patience-672348b81ac2
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— Cryptax
Source: https://cryptax.medium.com/tracking-android-joker-payloads-with-medusa-static-analysis-and-patience-672348b81ac2
https://cryptax.medium.com/tracking-android-joker-payloads-with-medusa-static-analysis-and-patience-672348b81ac2
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