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reversing-xamarin-maui

Security & auditing

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Updated 10-16 02:31

About this skill

Reversing Xamarin and .NET MAUI Apps

Xamarin apps hide a complete .NET application inside an Android package. Once you extract and decompress the assemblies you get near-source-quality C#, which makes this one of the highest-return reversing jobs — but only if you get past the container format, which changed twice and defeats naive unzipping.

When to Use

  • The APK contains lib/*/libmonodroid.so, libmonosgen-2.0.so, or libxamarin-app.so
  • The APK contains assemblies/assemblies.blob or assemblies/*.dll
  • jadx shows only mono.MonoPackageManager and Xamarin bootstrap classes
  • An IPA contains .dll files or a Frameworks/Mono* layout
  • The target is described as Xamarin, Xamarin.Forms, or .NET MAUI

When NOT to Use

  • Unity (global-metadata.dat, libil2cpp.so) — use reversing-unity-il2cpp; Unity is also .NET but uses a completely different packaging
  • Flutter or React Native — use reversing-flutter-apps or reversing-react-native-apps
  • TLS interception problems — use bypassing-mobile-pinning
  • The wider assessment — use testing-mobile-applications

Identify the Packaging Generation

Three formats shipped over the product's life, and the extraction differs.

unzip -l target.apk | rg 'assemblies|libmonodroid|libxamarin|libmonosgen'
What you see Generation Extraction
assemblies/*.dll, readable PE headers (MZ) Classic, uncompressed Unzip and decompile directly
assemblies/*.dll starting with XALZ Classic, LZ4-compressed Decompress the XALZ wrapper first
assemblies/assemblies.blob + .manifest AssemblyStore pyxamstore to unpack
lib/*/libxamarin-app.so with no assemblies/ .NET 6+ / MAUI, assemblies in the ELF Extract from the shared library
No managed assemblies anywhere NativeAOT Not .NET reversing — use analyzing-binaries 
# Which one is it? Check the first four bytes of an assembly
unzip -p target.apk 'assemblies/Mono.Android.dll' 2>/dev/null | xxd | head -1
# 4d5a...  → MZ, a plain .NET PE, decompile now
# 58414c5a → "XALZ", LZ4-compressed, decompress first

Extraction

# AssemblyStore (assemblies.blob)
pyxamstore unpack -d ./out target.apk
# produces the individual .dll files under ./out

# XALZ-compressed individual DLLs
# The header is: magic "XALZ" | descriptor index | uncompressed size | LZ4 block
python3 -c "
import lz4.block, struct, sys
d = open(sys.argv[1],'rb').read()
assert d[:4] == b'XALZ'
size = struct.unpack('<I', d[8:12])[0]
open(sys.argv[2],'wb').write(lz4.block.decompress(d[12:], uncompressed_size=size))
" Mono.Android.dll Mono.Android.decompressed.dll

# MAUI / .NET 6+ assemblies embedded in libxamarin-app.so
# They are stored as sections; carve by MZ header or use pyxamstore's newer modes
unzip -j target.apk 'lib/arm64-v8a/*' -d libs && binwalk -Me libs/libxamarin-app.so

Once you have plain .dll files it is ordinary .NET work:

ilspycmd ./out/YourApp.dll -o ./decompiled     # or dnSpy / dnSpyEx / dotPeek
rg -n 'https?://|ApiKey|ConnectionString|Bearer|password' ./decompiled | head -40

Start with the app's own assembly, not the framework ones. It is usually named after the product; Mono.Android.dll, System.*, and Xamarin.* are stock and waste your time.

Runtime Hooking

Frida's Java bridge does not see Mono methods — the app's logic is not in the JVM. You need Mono-aware instrumentation.

# Enumerate the Mono runtime and its loaded assemblies
frida -U -f com.target.app -l frida-mono-api.js
# Fridax targets Xamarin specifically; also useful:
#   mono_get_root_domain, mono_assembly_foreach, mono_class_get_methods,
#   mono_compile_method  → resolve a managed method to a native address, then
#   Interceptor.attach at that address

The practical route: decompile first, find the exact class and method you want, then resolve it through the Mono API and attach. Blind hooking of Mono is far slower than reading the C# you already recovered.

Certificate validation in Xamarin lives in managed code, which is why generic Android pinning bypasses miss it entirely:

// The two hook targets, both in the managed layer
ServicePointManager.ServerCertificateValidationCallback
HttpClientHandler.ServerCertificateCustomValidationCallback

Force these to return true via Mono hooking, or patch the assembly and repack. See bypassing-mobile-pinning for the surrounding diagnosis.

Where the Findings Are

Xamarin apps concentrate risk in ways that are easy to spot once decompiled:

  • Hardcoded secrets in C#. Developers treat compiled assemblies as opaque; API keys, connection strings, and storage credentials ship in plain IL.
  • Full backend logic in the client. Shared code between the mobile app and the server-side project is a Xamarin idiom, so the client often contains the authoritative business rules — and sometimes the server's own models and validation, which tells you exactly how to shape a malicious request.
  • Custom ServerCertificateValidationCallback returning true. Common in development builds that shipped.
  • Insecure local storage. Xamarin.Essentials.SecureStorage is usually fine; Preferences and raw file writes are not.
  • Embedded SQLite with credentials or full offline datasets.

Patching and Repacking

# Edit IL directly in dnSpy, save the assembly, then rebuild the container
pyxamstore pack -d ./out          # rebuild assemblies.blob
apktool b target -o patched.apk && apksigner sign --ks debug.keystore patched.apk

Repacking must reproduce the original container format exactly — an uncompressed DLL where the loader expects XALZ, or a mis-sized blob, fails at startup with an unhelpful native crash. Prefer runtime hooking when you only need to observe.

Rationalizations to Reject

  • "jadx found nothing, so it's obfuscated." There is nothing in the dex. The app is in assemblies/.
  • "Unzipping gave me DLLs but dnSpy rejects them." They are XALZ-compressed. Check the first four bytes.
  • "Frida's Java hooks aren't working." The methods are Mono, not JVM. Use the Mono API.
  • "It's compiled to a DLL, so the key is safe." IL decompiles to readable C#. Treat every assembly constant as public.
  • "The pinning bypass ran and traffic still fails." Xamarin validates in managed code; the Java-layer bypass never touched it.
  • "There are hundreds of assemblies." Most are stock framework. Read the one named after the product.

References

  • testing-mobile-applications — the wider assessment and platform storage
  • bypassing-mobile-pinning — diagnosing the interception failure around this
  • reversing-unity-il2cpp — the other .NET-based mobile framework
  • analyzing-binaries — NativeAOT builds with no managed assemblies
  • pyxamstore, dnSpy/dnSpyEx, ILSpy, Fridax, frida-mono-api