inliver233
0fc2a30408
New reference: references/code-virtualization-and-custom-linkers.md
Covers the layer between "packed" and "clean" - manifests whose Application is the
app's own and whose dex is readable, yet a re-signed build dies before user code runs.
Contents: three cheap identification tells (whole classes as native declarations, the
registerNativesForClass/special_clinit pair, SONAME-vs-filename mismatch); the
keep-the-library / delete-the-library deadlock and why both routes fail; the
checker-vs-implementation question that resolves it; the string-redirect technique
(equal-length rewrite of an isolated constant so a loadLibrary call reaches an
already-mapped library, so no offsets move and no death path needs neutralizing);
expected private-loader log shapes; a single-variable experiment to determine whether
the native check reads in-APK certificate material or the PMS-reported signature; and
why a Java-layer signature killer logging success proves nothing.
New tool: scripts/doctor.py
Cheapest first command. Capability report and per-script runnability, with
APKREV_TOOLS support so tools installed off-PATH or as runnable jars are found rather
than reported missing. Surfaces the environment facts that silently poison
experiments: clock skew, leftover adb forward entries, a leftover device proxy, and a
frida process already running on the device (which makes the target look like it
detects instrumentation when nothing has been attached yet).
New tool: scripts/so_constpatch.py
Implements the string-redirect technique. Enforces equal length, refuses to patch a
substring of a longer identifier, reports whether each hit sits in an ELF constant
pool, and patches inside an APK or a bare .so. Field-tested on a real protected
library: find, equal-length replace, unequal-length refusal, multi-hit guard, --force
and absent-name paths all verified.
Updated:
SKILL.md fix question count (ten -> thirteen); index the new reference and
both new scripts; make doctor.py the first step of the workflow;
add the "no packer but still dying" pointer during recon
packers.md cross-reference the new layer so "no packer" is not read as
"editable"
ad-removal.md make log evidence a count rather than an impression (n -> 0 is a
measurement; a tag that goes quiet without reaching zero is a
weaker result)
repack-and-sign.md warn that a full apktool rebuild rewrites obfuscated resource
paths (hundreds of rename pairs) - a tool artifact that must not be
read as breakage - and prefer a dex-only swap when that is the only
edit
pitfalls.md generalize the ServiceLoader example so it is not tied to one
sample's third-party library
README.md document the new files and scripts; document APKREV_TOOLS and the
cross-platform intent
227 lines
15 KiB
Markdown
227 lines
15 KiB
Markdown
# apk-reverse
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An Agent Skill for Android APK reverse engineering, debloating, ad removal, surgical
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dex patching, repacking, and runtime/server analysis.
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It is a **skill**, not a tutorial: it is written to be loaded by an agent (Claude Code,
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Codex, or any harness that supports the Agent Skills format) while it works, so it is
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organized for progressive disclosure — a short decision-oriented `SKILL.md`, detailed
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references loaded only when a step needs them, and parameterized scripts you can run
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directly.
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## What it is good at
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- Deciding **fast** whether a request is even achievable client-side, instead of
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burning hours on a paywall that is enforced by a server.
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- Deciding **what form the deliverable must take** before any work starts — an
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unrooted, self-contained artifact is a different problem from "make it work on this
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machine", and confusing the two is the most expensive drift in this domain.
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- Choosing the **safest patch layer** for a given change, and avoiding the layers that
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break the app.
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- Catching the repack failure that looks like success: an app that installs, launches and
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renders perfectly while **every signed request is rejected**, because the client derives its
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request-signing key from its own signing certificate.
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- Separating **your own mistakes from the app's or the server's problems** — a
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feature-scoped failure (login, registration, payment) is often a TLS/certificate issue on
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one code path, not a consequence of the patch you just built. Device state, a dead device
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server and clock drift masquerade the same way.
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- Establishing **which architecture and which library are actually executing**, rather
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than trusting what the manifest ships or what the device claims.
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- Working through **packed/hardened targets**: identifying the packer, unpacking, and turning a
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memory dump back into a patched, installable APK.
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- Handling a **hardened library that terminates the process on purpose** — including the
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deliberate-crash shape (`fault addr 0x4`) that looks exactly like an ordinary null-dereference
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bug, and the "neutralise it, but never by making it *not return*" rule that decides whether the
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fix works or freezes the whole app in a way that looks nothing like the cause.
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- Knowing **which tools to reach for and where each one lies** — including the ones that only
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exist as a GUI, so you ask for a human instead of silently substituting a weaker method.
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- Making a patched build **stay** patched: neutralising version checks, forced-upgrade dialogs and
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self-update installers so the work cannot be switched off remotely — and recognising the
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hot-update/remote-config channel that can quietly undo it without any version change.
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- Separating a **client-side sign-in gate** (patchable) from an **account-scoped resource** (empty
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because the server has nothing to answer with), and knowing that forging a session produces a state
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worse than being signed out.
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- Keeping a **long task honest**: a live record, graded conclusions, calibrated timeouts, and
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bounded waits, so progress is not lost and the same mistake is not made twice.
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- Avoiding the specific mistakes that produce an APK that builds perfectly and dies at
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runtime.
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## Structure
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```
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SKILL.md decision tree, workflow, hard constraints, indexes
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references/ loaded on demand, one topic each
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recon.md identify packer, SDKs, code location, tamper checks; unpacking
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packers.md hardened targets: rejection signals, measuring the validation
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boundary with single-variable tests, choosing a native host
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code-virtualization-and-custom-linkers.md
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the layer between "packed" and "clean": whole classes turned into
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`native` declarations, a private loader whose SONAME does not match
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its filename, an embedded self-decrypting payload, a Java-layer
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"signature killer" that logs success while a native check kills you.
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The keep-it/drop-it deadlock, how to separate the *checker* from the
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*implementation*, and the string-redirect technique that ends it
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without neutralizing anything
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framework-runtimes.md Flutter / React Native / Unity: which layer owns the UI, and how to
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find logic when there are no symbols (string encoding traps)
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dart-aot.md Dart AOT in depth: version pinning and building a matching decompiler,
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the object pool and reference indexes, register/boolean conventions,
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the three signatures that identify business logic, locating, patching
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native-and-so.md .so hosts, DT_NEEDED vs JNI_OnLoad, relocation limits,
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relocation-free bootstrapping, replacing Java methods natively,
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and which ABI/library is *actually loaded and executing*
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native-tamper-and-suicide.md how a hardened library kills its own process: the visible
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mechanisms, how to tell which one actually fires, how to find the
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site, forged section headers, function boundaries from
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PT_GNU_EH_FRAME, scanner traps, and neutralising safely
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detection-and-anti-analysis.md when the app fights back or the tool cannot run here: telling
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detection apart from a broken environment, deciding by cost instead
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of escalating, recognising an environment where dynamic analysis
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simply does not work, and keeping the "blocks my analysis" question
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separate from "blocks the deliverable"
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toolchain.md what to install, how to invoke it non-interactively, which tools
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are GUI-only, version-alignment traps, working offline
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long-task-discipline.md live record, conclusion grading, drift control, timeout and
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wait calibration, deliverable-form drift, handover
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ad-removal.md ad taxonomy, wrapper mapping, callback trap, global gates, verification
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updates-and-forced-upgrade.md keeping a patched build alive: locating the version check, the
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two-layer patch (no-op the routine, neutralise the comparison), what not
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to touch (manifest version, installer permission, host blocking),
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self-update and hot-update/remote-config channels, verifying that no
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version request is issued at all
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account-gates.md sign-in walls, forced phone binding, guest mode: telling a client-side
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gate (patchable) apart from an account-scoped resource (not), why
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fabricating a session is worse than staying signed out, and the
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unavoidable session loss after a reinstall
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signature-derived-keys.md when the app's own signing certificate is used as key material:
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detection greps, why offline extraction is unreliable, the
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hardcode-then-verify procedure
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membership-and-limits.md server vs client authority; what is and is not patchable
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server-api.md probe an app's API; prove who owns the gate
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tls-and-cert.md feature-scoped network failures: expired certs, dual trust chains
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third-party-builds.md auditing a "cracked"/"modded" APK before trusting it
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dex-patching.md patch-layer table + dexlib2 technique in depth
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patch-audit.md proving a patch *landed* and is *legal*: length-vs-bytes
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comparison, the equal-length-replacement blind spot, verifier-level
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legality (move-result adjacency) checked statically, text-matching
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patch traps, and reporting a missing patch
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repack-and-sign.md repack rules, unpack-and-repack, signing, post-install hazards
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runtime-data.md DataStore / SharedPreferences / SQLite / protobuf; when the app
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rewrites your edit, and decoding a value that looks encrypted
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dynamic-frida.md Frida setup, version pinning, the four-layer probe, hook strategy
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environment.md device/emulator setup, root, ADB, offline devices, log signals,
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emulator console control and recovery, preflight, look-at-the-screen
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verification.md the claim ladder; what "done" means
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pitfalls.md the failure catalogue -- read before building
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scripts/ parameterized, path-agnostic
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doctor.py run this first: capability report + per-script runnability, finds
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tools installed off-PATH or as runnable jars, and surfaces the
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environment facts that poison experiments (clock skew, leftover
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adb forward / proxy, a device-side frida process already running)
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so_constpatch.py same-length in-place rewrite of an isolated string constant, for
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redirecting a library load instead of defeating a check
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smtool.py baksmali/smali wrapper with a configurable classpath
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dexpatch/ dexlib2 method-level surgical rewriter (preferred tool)
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patch_smali.py method-body replacement in a smali tree
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dex_strpatch.py byte-level string patch with a string_ids ordering guard
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dex_classdiff.py prove a dex edit was surgical
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dex_strings.py strings/URLs/SDK markers without a decompiler
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dart_pool_strings.py recover literals from a Dart AOT snapshot (framed entries, the
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one-byte vs UTF-16 split, file offsets, run-length noise filter)
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dart_pprefs.py build/query the object-pool -> code-site index for a Dart snapshot
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dart_disasm.py annotated windowed disassembly of Dart AOT code + B/BL caller index
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find_refs.py count callers of a method before patching it
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repack.py rebuild APK, strip only signatures, sign, verify
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devsh.py quoting-safe ADB shell helper
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usb_net_proxy.py give an offline device network over USB
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datastore_inject.py encode/inject AndroidX DataStore preferences safely
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probe_api.py probe an HTTP API with the right headers
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grab_crash.py recover stacks hidden by a crash-reporter SDK
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install_test.py install + launch health check with logcat signal scan
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frida_probe.js four-layer runtime probe (app net layer + OkHttp + java.net + exceptions)
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run_probe.py inject the probe, stream it to a log file, stay resident
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tls_check.py strict certificate check for one or more hosts
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preflight.py environment check before every experiment block (device, root,
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ABI/translation, clock skew, leftover proxy/forwards, dead server)
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lib_map.py what is *actually mapped* into a live process: per-library path,
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base, architecture, and whether it came from the APK or was
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materialized at runtime
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elf_plt.py resolve a PLT stub to its imported symbol (x86_64 + aarch64) from
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the relocation table; list a symbol's callers; byte-diff two
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libraries and name the symbol each changed stub belongs to
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apk_diff.py entry-level diff of two builds: changed / added / removed, by
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content hash so same-size replacements are caught
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native_crash.py locate a native death from a log or tombstone: signal, fault
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address, registers, frames split app vs system, the faulting
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instruction, and a flag when the fault looks *arranged*
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blob_decode.py search, don't guess, the framing of a stored value
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(base64/hex x rotation x deflate); re-encode the edited payload
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snap.py bounded burst screenshots + control-tree capture with a stall
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detector, and a verdict on whether the tree is usable at all
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sig_probe.py find the exact signatures[0].toCharsString() value — offline
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candidates from an APK, or the authoritative read from a device
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```
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## Install
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Drop this directory where your agent finds skills, e.g.:
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```
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<skills-dir>/apk-reverse/SKILL.md
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```
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The agent loads `SKILL.md` when a task matches its description, and pulls in
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`references/*` only as needed. No global state, no machine-specific paths.
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## Requirements
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Nothing is mandatory; each script checks what it needs. `scripts/doctor.py` reports which of these
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are present here, which scripts can therefore run, and — usefully — which tools exist somewhere other
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than PATH.
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If your toolchain lives outside PATH (a project-local `tools/` directory, a versioned SDK folder, a
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runnable `.jar` instead of a command), set `APKREV_TOOLS` to one or more directories and `doctor.py`
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will find them:
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```
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set APKREV_TOOLS=D:\tools;D:\android\build-tools\35.0.0 # Windows
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export APKREV_TOOLS=/opt/tools:/opt/android/build-tools # POSIX
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```
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The scripts themselves are plain `python3` and are intended to work identically on Windows, macOS and
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Linux; where a snippet is POSIX-only it is labelled. Nothing here assumes a Unix shell.
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| Tool | Used for |
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| Python 3.9+ | all scripts |
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| `baksmali` / `smali` + `dexlib2` jars | disassembly, assembly, surgical patching |
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| JDK (`javac`, `java`) | building/running the dexlib2 patcher |
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| Android SDK build-tools (`aapt`, `zipalign`, `apksigner`) | manifest info, alignment, signing |
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| `uber-apk-signer` (optional) | one-step align + sign |
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| ADB | device work |
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| Frida (host package + matching on-device server) | dynamic analysis |
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| a rooted device or emulator | anything beyond static analysis |
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## Read this first
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`references/pitfalls.md`. It is the most valuable file here — every entry is a failure
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that produced a broken artifact while looking completely healthy.
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The four that hurt most:
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1. Stripping the whole `META-INF/` during a repack deletes ServiceLoader registrations
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and the app dies at startup with an error that names an unrelated library.
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2. Patching a byte-level string without preserving `string_ids` ordering gets the whole
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dex rejected, while checksums and signatures verify perfectly.
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3. Rebuilding a dex with a whole-tree smali round-trip damages R8 output invisibly —
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class tables compare clean, and it only blows up at runtime.
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4. Neutralising a native terminate path by making it **not return**. A spinning stub does not
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suppress the check; it freezes the caller and every thread behind it. The app hangs with *no
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crash record at all*, and the eventual death gets blamed on whatever killed the frozen process.
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## Scope
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Built for working on your own applications, on samples you are authorized to analyze,
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and in CTF/competition sandboxes. It contains no vendored third-party binaries and no
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target-specific data.
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