apk-reverse/SKILL.md

name, description

name

description

apk-reverse

Reverse engineer, debloat, de-ad, patch, or re-sign Android APKs, and analyze their runtime and server-side behavior. Use when a task involves an .apk/.aab/.dex/.so sample, smali or dex patching, Frida/objection runtime hooking, repacking and re-signing, removing ads or SDK trackers, probing a mobile app's HTTP API, or deciding whether a client-side patch is even capable of achieving the goal. Covers recon, anti-tamper, ad removal, membership/paywall limits, dex-level surgical patching, repack pitfalls, device and emulator setup, and a hard-won failure catalogue.

APK Reverse Engineering & Patching

Goal of this skill: get to a verified, installable, still-working artifact fast, and avoid the whole class of mistakes that silently destroy an APK.

Two rules override everything else in this skill:

1. Never ship an unverified APK. "It assembles" is not "it works". Install it, launch it, exercise the feature you changed, on a real device or a close emulator.
2. Know which layer owns the behavior before you patch. Ads, paywalls, and feature gates live in different places. Patching the wrong layer either does nothing or breaks the app. Classify first (see step 2 below), patch second.

Start here: classify the target in seven questions

Answer these before touching a tool. Every one of them changes the whole plan.

1. Is the app packed/hardened? → references/recon.md Read the manifest's application android:name. If it is a third-party shell class rather than the app's own Application, you have a packer and must handle it first.

2. Where does the behavior you want to change actually live?

   • Ad SDK (Pangle/GDT/AnyThink/Kuaishou/Baidu/Sigmob…) → usually client-side and removable → references/ad-removal.md
   • Server-issued ad config / sponsored cards → client renders server data → references/ad-removal.md §server-driven
   • Membership / VIP / paid content → usually server-authorized, client patch is cosmetic → references/membership-and-limits.md (read this before spending hours)
   • Feature flag, UI gate, debug switch → usually client-side
   • Anything decided by an API response → server-side → references/server-api.md

3. Is the app's own code in plain dex, or moved to native/Flutter/Unity? Plain dex → you can patch. Flutter (libflutter.so + libapp.so) / Unity (libil2cpp.so) / pure native → different toolchain entirely. See references/recon.md §code-location and references/framework-runtimes.md. Runtime check (cheap -- do it before committing to a layer): hook the obvious Java classes for the UI you care about, then reproduce that UI. If those hooks fire, the behavior is Java-owned. If they fire zero times while the UI is plainly on screen, the behavior is drawn by the runtime or by native code, and a dex-only plan will stall. Do not keep hunting in dex after a zero-hit probe -- that is the most expensive wrong turn in this skill's history.

4. Does the app verify its own signature, or does the server? App-side → you must bypass it. Server-side → re-signing silently breaks the app later. See references/repack-and-sign.md and references/server-api.md.

5. What is your device situation? → references/environment.md Rooted real device (best), emulator with root, or no device (static only). Also: this determines whether Frida is usable.

6. Is one specific feature failing at runtime — login, registration, payment, an API-backed screen — while the rest of the app works? → references/tls-and-cert.md. A feature-scoped network failure is very often a TLS/certificate problem on one code path, not a consequence of your patch. The app can even carry two independent trust chains, so "other requests work" proves nothing. Rule this out in minutes before hunting for a signature check.

7. Was the input a build you did not produce (a "cracked"/"modded" APK circulating online)? → references/third-party-builds.md. Audit it before adopting it: such builds are frequently re-protected (sometimes with more layers than the original) and may carry injected components or endpoints. Never use one as a patching workbench.

   Long-task rule: if this is likely to run long, open references/long-task-discipline.md now and keep its record updated as you go. Re-read the refuted-conclusions and dead-routes sections before starting any new experiment. Losing earlier findings is the most expensive failure in this skill, and it is entirely preventable.

The workflow, end to end

1. Recon — references/recon.md. Manifest, package name, version, ABI, dex count, packer, embedded SDKs, where the app's own code lives. Ten minutes here saves hours. If it is packed, unpack before anything else (references/recon.md §unpacking): you cannot patch code you cannot read, the encrypted payload lengths tell you which dumped dex is the original, and a memory dump must be de-duplicated by hash and structurally validated before any of it is trusted.
2. Extract strings and endpoints — build a picture of the app's API surface and SDK inventory from the dex string tables. No decompiler needed for this, and it is fast. Scripts: scripts/dex_strings.py.
3. Trace to the owning class — find the class that wraps the behavior (the app almost always wraps third-party SDKs in one helper). Reverse-lookup instructions: references/dex-patching.md §finding-the-call-site.
4. Decide the patch layer — client SDK call / client rendering / client data consumption / server contract. See the table in references/ad-removal.md.
5. Patch surgically — references/dex-patching.md. Prefer dexlib2 method-level rewriting over whole-tree smali round-trip. Whole-tree round-trip damages R8-optimized dex in ways that only show up at runtime.
6. Repack and sign — references/repack-and-sign.md. Do not strip the whole META-INF/. This single mistake destroys otherwise-correct builds.
7. Verify on device — references/environment.md + references/verification.md. Check: launches, the changed behavior actually changed, nothing unrelated broke, and the app reaches its normal UI with no blocking dialog. First prove the artifact actually changed on the device -- a package manager reporting success does not prove an interposed confirmation was accepted (P18). Capture continuously for the first ~20 seconds after launch; sampling gaps are how a blocking modal goes unseen (P20).
8. Log what you learned — if a failure cost you more than thirty minutes, add it to references/pitfalls.md. That file is the most valuable artifact in this skill.

Non-negotiable constraints

• Read-only inputs. Keep the original APK/dex untouched; work on copies. Always keep a known-good baseline to diff against.
• One variable at a time. If you change two things and it breaks, you learn nothing. Build a control (same pipeline, zero patches) and compare.
• Verify structure after every dex edit. scripts/dex_classdiff.py must report zero differences in class set and access flags for classes you did not intend to change.
• Do not patch a method that is widely shared. Before patching any helper, count its callers (scripts/find_refs.py). A Long.valueOf wrapper with 30 callers is not an ad-specific hook.
• Do not make an API fail to suppress a UI element. A 404/400 on an endpoint that other features depend on takes the whole screen down with it. Suppress at the data-consumption or render layer instead.
• Every claim needs evidence. "Probably", "should be", "in theory" are not findings. Either you observed it, or you label it unverified.
  • "Done" means the user-visible outcome, not an internal signal. A blocking dialog still on screen means the task is not done, however many errors disappeared from the log. Absence of a log line is absence of evidence, never evidence of success.
  • Do not discard a route on compound evidence. If a failure followed two simultaneous changes, the attribution is a hypothesis, not a finding. Re-run it single-variable before writing the route off -- mis-attributed failures have removed viable approaches for a long time.
  • Prove the device changed before measuring. Install success describes the request, not the app on disk. Confirm the artifact actually advanced, or every following observation describes the previous build.
• Attribute a failure to the right layer before patching again. When something stops working after a rebuild, first check whether the unmodified original fails the same way on the same device and network. Feature-scoped network failures in particular are frequently the app's own TLS/certificate problem (references/tls-and-cert.md); chasing a signature check that does not exist burns hours.

Reference index

Load only what the current step needs.

File	Load when
references/recon.md	Starting any new sample; identifying packer, SDKs, code location, ABI
references/packers.md	The app is packed/hardened, or an edit makes it die before your code runs. Also load before discarding any route as "blocked by the shell"
references/framework-runtimes.md	The UI is not native (Flutter / React Native / Unity / Cordova), or Java-layer hooks fire zero times while the UI clearly works
references/native-and-so.md	Patching in a .so, needing code to run before the app's own code, or hand-built native payloads that crash inside the linker
references/ad-removal.md	Task involves ads, trackers, sponsored cards, splash/interstitial/reward
references/membership-and-limits.md	Task involves VIP, subscription, paid content, unlock, "fully cracked"
references/server-api.md	The behavior is decided by a response, or you need to know if a patch can even matter
references/dex-patching.md	Any actual editing of dex/smali, choosing a patch layer, choosing a tool
references/repack-and-sign.md	Rebuilding, signing, installing, or a repacked app misbehaves
references/runtime-data.md	Local state matters: DataStore, SharedPreferences, SQLite, protobuf caches, tokens
references/dynamic-frida.md	Frida setup, hooking strategy, tracing caller chains, finding the real call site
references/environment.md	Device/emulator setup, root, ADB, networking, offline devices
references/verification.md	Defining what "done" means; building the evidence chain
references/tls-and-cert.md	One feature fails at runtime (login, registration, payment, an API-backed screen) while the rest of the app works
references/third-party-builds.md	The input is a "cracked"/"modded" build you did not produce — audit it before trusting it
references/long-task-discipline.md	The task will run long, or you are resuming one. Live record, conclusion grading, drift checkpoints, handover
references/pitfalls.md	Always worth a skim before building. This is the failure catalogue.

Script index

All scripts are parameterized and path-agnostic; pass paths explicitly. Run --help or read the header of each.

Script	Purpose
scripts/smtool.py	baksmali/smali wrapper with a bundled classpath (assemble/disassemble dex)
scripts/patch_smali.py	Method-body replacement in a smali tree, matched by signature
scripts/dex_strpatch.py	Byte-level string constant patch with string_ids ordering guard
scripts/dex_classdiff.py	Compare two dex class tables (set + access flags) to prove a patch was surgical
scripts/dex_strings.py	Dump/extract strings and endpoints from dex without a decompiler
scripts/find_refs.py	Count and list callers of a method/field (blast-radius check)
scripts/repack.py	Rebuild APK with replaced dex, strip only signatures, sign
scripts/dexpatch/	dexlib2 method-level surgical rewriter (the preferred patch tool) + build notes
scripts/devsh.py	Quoting-safe ADB shell helper for rooted devices
scripts/usb_net_proxy.py	Give an offline device network over USB (adb reverse + local proxy)
scripts/datastore_inject.py	Encode/inject AndroidX DataStore preferences (protobuf) safely
scripts/probe_api.py	Probe an app's HTTP API with correct headers, report status/shape
scripts/install_test.py	Install a build and run a launch/health check with logcat signal extraction
scripts/frida_probe.js	Four-layer runtime probe: app network layer + OkHttp + java.net + swallowed exception messages
scripts/run_probe.py	Inject a probe, stream it to a timestamped log file, stay resident while you operate the app
scripts/tls_check.py	Strict certificate check for one or more hosts (expired / wrong host / untrusted CA)