# apk-reverse An Agent Skill for Android APK reverse engineering, debloating, ad removal, surgical dex patching, repacking, and runtime/server analysis. It is a **skill**, not a tutorial: it is written to be loaded by an agent (Claude Code, Codex, or any harness that supports the Agent Skills format) while it works, so it is organized for progressive disclosure — a short decision-oriented `SKILL.md`, detailed references loaded only when a step needs them, and parameterized scripts you can run directly. ## How an agent is expected to consume this `SKILL.md` is deliberately written as a **procedure with gates** rather than as advice, because the observed failure mode is not ignorance — it is a model reading the whole thing, agreeing with it, and then reasoning from first principles anyway. So there are four things in the body that are meant to be *acted on*, not read: - **Four override rules (R1–R4).** Where they conflict with the current plan, they win until evidence overrides them. - **A symptom index.** Each row is a failure that has already been paid for. **A matching row is a stop signal**: load that file before running another command, rather than after a few more attempts. Reasoning past a known symptom is how the same hours get spent twice. - **Four gates (G1–G4),** each an action with a pass criterion. "I understand the idea" does not clear a gate. They exist so that classification, environment truth and a control build happen *before* the first patch, not after the third failure. - **A two-strike rule and stop conditions.** Two failures of the same shape mean the model is wrong, not the parameters. The third variant of a hypothesis that already failed twice is where rounds go to die. And one thing at the end that is meant to be *withheld*: **"done"** has a definition (six items). A clean log is not one of them. Anything short of all six is a checkpoint, and should be reported as a checkpoint with what remains. If you are an agent reading this: the cheapest possible first command is `python skills/apk-reverse/scripts/doctor.py`. It tells you which of these tools exist here, which scripts can actually run, and whether something in the environment is already poisoning your measurements. ## What it is good at - Deciding **fast** whether a request is even achievable client-side, instead of burning hours on a paywall that is enforced by a server. - Deciding **what form the deliverable must take** before any work starts — an unrooted, self-contained artifact is a different problem from "make it work on this machine", and confusing the two is the most expensive drift in this domain. - Choosing the **safest patch layer** for a given change, and avoiding the layers that break the app. - Catching the repack failure that looks like success: an app that installs, launches and renders perfectly while **every signed request is rejected**, because the client derives its request-signing key from its own signing certificate. - Separating **your own mistakes from the app's or the server's problems** — a feature-scoped failure (login, registration, payment) is often a TLS/certificate issue on one code path, not a consequence of the patch you just built. Device state, a dead device server and clock drift masquerade the same way. - Establishing **which architecture and which library are actually executing**, rather than trusting what the manifest ships or what the device claims. - Working through **packed/hardened targets**: identifying the packer, unpacking, and turning a memory dump back into a patched, installable APK. - Handling a **hardened library that terminates the process on purpose** — including the deliberate-crash shape (`fault addr 0x4`) that looks exactly like an ordinary null-dereference bug, and the "neutralise it, but never by making it *not return*" rule that decides whether the fix works or freezes the whole app in a way that looks nothing like the cause. - Knowing **which tools to reach for and where each one lies** — including the ones that only exist as a GUI, so you ask for a human instead of silently substituting a weaker method. - Making a patched build **stay** patched: neutralising version checks, forced-upgrade dialogs and self-update installers so the work cannot be switched off remotely — and recognising the hot-update/remote-config channel that can quietly undo it without any version change. - Separating a **client-side sign-in gate** (patchable) from an **account-scoped resource** (empty because the server has nothing to answer with), and knowing that forging a session produces a state worse than being signed out. - Keeping a **long task honest**: a live record, graded conclusions, calibrated timeouts, and bounded waits, so progress is not lost and the same mistake is not made twice. - Avoiding the specific mistakes that produce an APK that builds perfectly and dies at runtime. ## Structure `SKILL.md`, `references/` and `scripts/` are all inside the skill directory, `skills/apk-reverse/`. Everything at the repository root is maintenance tooling shared across skills, not part of an installed skill. ``` SKILL.md a procedure with gates, not background reading: how-to-use -> four override rules (R1-R4) symptom index (a matching row is a stop signal) four gates (G1-G4, actions with pass criteria) thirteen classification questions the workflow, with a per-step skip condition and a two-strike rule what "done" means -> stop conditions -> constraints -> indexes references/ loaded on demand, one topic each recon.md identify packer, SDKs, code location, tamper checks; unpacking server-config-and-updates.md the most common shape of "ad" and the one usually mis-diagnosed: the server supplies UI the client renders (launch screen, popup, announcement, tab set). The two-layer fetch that proves it, how to find the config DTOs by the field names data classes keep, why you patch the decision and not the data, deciding the scope of "remove", and remote re-enable / cached config durability byte-level-patching.md equal-length byte edits: why they beat method rebuilding (measured), locating an instruction's exact offset without scraping listings, the instruction width traps that desynchronise a decode, neutralise a branch vs redirect it, dex header integrity field order, and the verifier's move-result rule packers.md hardened targets: rejection signals, measuring the validation boundary with single-variable tests, choosing a native host code-virtualization-and-custom-linkers.md the layer between "packed" and "clean": whole classes turned into `native` declarations, a private loader whose SONAME does not match its filename, an embedded self-decrypting payload, a Java-layer "signature killer" that logs success while a native check kills you. The keep-it/drop-it deadlock, how to separate the *checker* from the *implementation*, and the string-redirect technique that ends it without neutralizing anything framework-runtimes.md Flutter / React Native / Unity: which layer owns the UI, and how to find logic when there are no symbols (string encoding traps) dart-aot.md Dart AOT in depth: version pinning and building a matching decompiler, the object pool and reference indexes, register/boolean conventions, the three signatures that identify business logic, locating, patching native-and-so.md .so hosts, DT_NEEDED vs JNI_OnLoad, relocation limits, relocation-free bootstrapping, replacing Java methods natively, and which ABI/library is *actually loaded and executing* native-tamper-and-suicide.md how a hardened library kills its own process: the visible mechanisms, how to tell which one actually fires, how to find the site, forged section headers, function boundaries from PT_GNU_EH_FRAME, scanner traps, and neutralising safely detection-and-anti-analysis.md when the app fights back or the tool cannot run here: telling detection apart from a broken environment, deciding by cost instead of escalating, recognising an environment where dynamic analysis simply does not work, and keeping the "blocks my analysis" question separate from "blocks the deliverable" toolchain.md what to install, how to invoke it non-interactively, which tools are GUI-only, version-alignment traps, working offline, **"not on PATH" is not "not installed"**, and which signer to use long-task-discipline.md live record, conclusion grading, drift control, timeout and wait calibration, deliverable-form drift, captures-you-never-looked-at, long-context decay, handover ad-removal.md ad taxonomy, wrapper mapping, callback trap, global gates, verification updates-and-forced-upgrade.md keeping a patched build alive: locating the version check, the two-layer patch (no-op the routine, neutralise the comparison), what not to touch (manifest version, installer permission, host blocking), self-update and hot-update/remote-config channels, verifying that no version request is issued at all account-gates.md sign-in walls, forced phone binding, guest mode: telling a client-side gate (patchable) apart from an account-scoped resource (not), why fabricating a session is worse than staying signed out, and the unavoidable session loss after a reinstall signature-derived-keys.md when the app's own signing certificate is used as key material: detection greps, why offline extraction is unreliable, the hardcode-then-verify procedure membership-and-limits.md server vs client authority; what is and is not patchable server-api.md probe an app's API; prove who owns the gate tls-and-cert.md feature-scoped network failures: expired certs, dual trust chains third-party-builds.md auditing a "cracked"/"modded" APK before trusting it dex-patching.md patch-layer table + dexlib2 technique in depth patch-audit.md proving a patch *landed* and is *legal*: length-vs-bytes comparison, the equal-length-replacement blind spot, verifier-level legality (move-result adjacency) checked statically, text-matching patch traps, and reporting a missing patch repack-and-sign.md repack rules, unpack-and-repack, signing, post-install hazards runtime-data.md DataStore / SharedPreferences / SQLite / protobuf; when the app rewrites your edit, and decoding a value that looks encrypted dynamic-frida.md Frida setup, version pinning, the four-layer probe, hook strategy environment.md device/emulator setup, root, ADB, offline devices, log signals, emulator console control and recovery, preflight, look-at-the-screen verification.md the claim ladder; what "done" means pitfalls.md the failure catalogue -- read before building scripts/ parameterized, path-agnostic doctor.py run this first: capability report + per-script runnability, finds tools installed off-PATH or as runnable jars, and surfaces the environment facts that poison experiments (clock skew, leftover adb forward / proxy, a device-side frida process already running) dexutil.py dependency-free dex reader: structural walk + exact instruction decode, dex header recompute/verify (correct checksum/signature order), branch-target and operand helpers. Library shared by the dex scripts, also runs standalone to dump one method with offsets dex_find_insn.py locate an instruction by decoded semantics and print its exact byte offset with context and both sides of any branch -- how you find a patch site instead of guessing offsets dex_patch_bytes.py equal-length byte patches from a JSON spec: semantic match, polarity pin via expect_next, equal-length enforcement, verifier check, dex header recompute, re-decode to prove it landed (--dry-run first) dex_check_verifier.py tier-3 check: does any conditional branch target a move-result (bypassing its producer)? Compares two builds and separates pre-existing findings from regressions your patch introduced coldstart.py cold-launch capture: timed screenshot burst + logcat signals + installed-build facts + launch timing, and warns when the foreground activity is not your app so_constpatch.py same-length in-place rewrite of an isolated string constant, for redirecting a library load instead of defeating a check smtool.py baksmali/smali wrapper with a configurable classpath dexpatch/ dexlib2 method-level rewriter (for changes that need new instructions) patch_smali.py method-body replacement in a smali tree dex_strpatch.py byte-level string patch with a string_ids ordering guard dex_classdiff.py prove a dex edit was surgical dex_strings.py strings/URLs/SDK markers without a decompiler dart_pool_strings.py recover literals from a Dart AOT snapshot (framed entries, the one-byte vs UTF-16 split, file offsets, run-length noise filter) dart_pprefs.py build/query the object-pool -> code-site index for a Dart snapshot dart_disasm.py annotated windowed disassembly of Dart AOT code + B/BL caller index find_refs.py count callers of a method before patching it repack.py rebuild APK, strip only signatures, keep META-INF/services/, write a 4-byte-aligned archive (resources.arsc STORED+aligned), sign, verify devsh.py quoting-safe ADB shell helper usb_net_proxy.py give an offline device network over USB datastore_inject.py encode/inject AndroidX DataStore preferences safely probe_api.py probe an HTTP API with the right headers grab_crash.py recover stacks hidden by a crash-reporter SDK install_test.py install + launch health check with logcat signal scan frida_probe.js four-layer runtime probe (app net layer + OkHttp + java.net + exceptions) run_probe.py inject the probe, stream it to a log file, stay resident tls_check.py strict certificate check for one or more hosts preflight.py environment check before every experiment block (device, root, ABI/translation, clock skew, leftover proxy/forwards, dead server) lib_map.py what is *actually mapped* into a live process: per-library path, base, architecture, and whether it came from the APK or was materialized at runtime elf_plt.py resolve a PLT stub to its imported symbol (x86_64 + aarch64) from the relocation table; list a symbol's callers; byte-diff two libraries and name the symbol each changed stub belongs to apk_diff.py entry-level diff of two builds: changed / added / removed, by content hash so same-size replacements are caught native_crash.py locate a native death from a log or tombstone: signal, fault address, registers, frames split app vs system, the faulting instruction, and a flag when the fault looks *arranged* blob_decode.py search, don't guess, the framing of a stored value (base64/hex x rotation x deflate); re-encode the edited payload snap.py bounded burst screenshots + control-tree capture with a stall detector, and a verdict on whether the tree is usable at all sig_probe.py find the exact signatures[0].toCharsString() value — offline candidates from an APK, or the authoritative read from a device ``` ## Install This repository is a **skills repository**: the skill lives at `skills/apk-reverse/`, which is the layout the `skills` CLI resolves, and it is installed by name rather than by copying a directory: ``` npx skills add newliver666/apk-reverse # install every skill in the repo npx skills add newliver666/apk-reverse --list # list what is here, install nothing npx skills add newliver666/apk-reverse --skill apk-reverse -y npx skills use newliver666/apk-reverse@apk-reverse # use it once, without installing ``` The CLI symlinks the skill into your agent's skills directory by default (`--copy` makes independent copies instead), and `-g` installs for every project rather than the current one. With one skill in the repository, `--skill apk-reverse` is redundant today; it is written out here because it is what selects a single skill once a second one exists. Once installed, the agent loads `SKILL.md` when a task matches its description, and pulls in `references/*` only as needed. No global state, no machine-specific paths, and no build step. ## Requirements Nothing is mandatory; each script checks what it needs. `skills/apk-reverse/scripts/doctor.py` reports which of these are present here, which scripts can therefore run, and — usefully — which tools exist somewhere other than PATH. If your toolchain lives outside PATH (a project-local `tools/` directory, a versioned SDK folder, a runnable `.jar` instead of a command), set `APKREV_TOOLS` to one or more directories and `doctor.py` will find them: ``` set APKREV_TOOLS=; # Windows, e.g. an SDK or project-local tools dir export APKREV_TOOLS=: # POSIX ``` The scripts themselves are plain `python3` and are intended to work identically on Windows, macOS and Linux; where a snippet is POSIX-only it is labelled. Nothing here assumes a Unix shell. | Tool | Used for | |---|---| | Python 3.9+ | all scripts | | `ddc` (optional but recommended) | single-binary dex→Java decompiler with query subcommands (`info`, `findrefs`, `strings --with-locations`, per-class decompile). No JVM. Turns string cross-referencing from a crawl into a lookup, and reports package identity reliably — see `skills/apk-reverse/references/toolchain.md` | | `baksmali` / `smali` + `dexlib2` jars | disassembly, assembly, surgical patching | | JDK (`javac`, `java`) | building/running the dexlib2 patcher; also provides `keytool`/`jarsigner` | | Android SDK build-tools (`aapt`, `zipalign`, `apksigner`) | manifest info, alignment, signing. **`apksigner` is the signer to use** — `jarsigner` rewrites the archive and breaks the alignment Android R+ requires | | `uber-apk-signer` (optional) | one-step align + sign | | ADB | device work | | Frida (host package + matching on-device server) | dynamic analysis | | a rooted device or emulator | anything beyond static analysis | None of these need to be on `PATH`: every script accepts an explicit path for the tools it shells out to, and `skills/apk-reverse/references/toolchain.md` covers finding an install that `PATH` does not know about (the common case for `apksigner` and `keytool`). ## Read this first `skills/apk-reverse/references/pitfalls.md`. It is the most valuable file here — every entry is a failure that produced a broken artifact while looking completely healthy. The four that hurt most: 1. Stripping the whole `META-INF/` during a repack deletes ServiceLoader registrations and the app dies at startup with an error that names an unrelated library. 2. Patching a byte-level string without preserving `string_ids` ordering gets the whole dex rejected, while checksums and signatures verify perfectly. 3. Rebuilding a dex with a whole-tree smali round-trip damages R8 output invisibly — class tables compare clean, and it only blows up at runtime. 4. Neutralising a native terminate path by making it **not return**. A spinning stub does not suppress the check; it freezes the caller and every thread behind it. The app hangs with *no crash record at all*, and the eventual death gets blamed on whatever killed the frozen process. ## Scope Built for working on your own applications, on samples you are authorized to analyze, and in CTF/competition sandboxes. It contains no vendored third-party binaries and no target-specific data. What it covers, and what it deliberately does not, is stated at the top of `SKILL.md` under **Coverage**. The short version: Android only (no iOS), and deep on the layers that have been worked through for real — dex patching, repacking, packers and custom loaders, native tamper response, and Flutter/Dart AOT. Unity/IL2CPP logic recovery, React Native/Hermes bytecode internals, and defeating a server-side authority are **not** covered, and the skill is written to say so and stop rather than apply the nearest documented procedure to a target it was not written for. ## Repository maintenance Three tools live at the root and are not part of the installed skill: ``` check_repo.py every skill discovered, frontmatter valid, scripts runnable, documented paths resolve, README paths explicit and existing check_refs.py every `file.md` §section cross-reference reaches a real heading build_scripts.py audit for machine-specific leftovers (absolute paths, credentials) ```