Detect APK files

detect-apk.py

#!/usr/bin/python3
# encoding: utf-8
# SPDX-FileCopyrightText: 2024 FC (Fay) Stegerman <flx@obfusk.net>
# SPDX-License-Identifier: AGPL-3.0-or-later

import fnmatch
import os
import struct
import sys

from collections import namedtuple
from typing import List, Optional

ZipData = namedtuple("ZipData", ("cd_offset", "eocd_offset", "cd_and_eocd"))

APK_FILES = (
    b"AndroidManifest.xml",     # required
    b"META-INF/com/android/build/gradle/app-metadata.properties",
    b"META-INF/MANIFEST.MF",    # v1 signature
    b"resources.arsc",
    b"classes.dex",
    b"assets/dexopt/baseline.prof",
    b"assets/dexopt/baseline.profm",
    b"DebugProbesKt.bin",
)
APK_FNMATCH = (
    b"res/*.xml",
    b"res/drawable/*.xml",
    b"lib/arm64-v8a/*.so",
    b"lib/armeabi-v7a/*.so",
    b"lib/x86/*.so",
    b"lib/x86_64/*.so",
    b"META-INF/androidx.*.version",
    b"META-INF/*.SF",
)
ZIPFLINGER_VIRTUAL_ENTRY = b"PK\x03\x04\x00\x00\x00\x00\x00\x00\x21\x08\x21\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x66\x00" + b"\x00" * 102


class DetectionError(Exception):
    pass


def detect_apk(apkfile: str) -> Optional[List[str]]:
    """
    Detect APK files; returns a list of things detected, None if the file does
    not seem to be an APK, or raises a DetectionError if processing the file
    fails.
    """
    zdata = zip_data(apkfile)
    results = ["ZIP file"]
    seen = set()
    with open(apkfile, "rb") as fh:
        if fh.read(len(ZIPFLINGER_VIRTUAL_ENTRY)) == ZIPFLINGER_VIRTUAL_ENTRY:
            results.append("zipflinger virtual entry")
        fh.seek(zdata.cd_offset - 16)
        if fh.read(16) == b"APK Sig Block 42":
            results.append("APK Signing Block")
        while fh.tell() < zdata.eocd_offset:
            hdr = fh.read(46)
            if hdr[:4] != b"\x50\x4b\x01\x02":
                raise DetectionError("Expected central directory file header signature")
            n, m, k = struct.unpack("<HHH", hdr[28:34])
            if fh.tell() + n + m + k > zdata.eocd_offset:
                raise DetectionError("Invalid length(s)")
            filename = fh.read(n)
            if filename in APK_FILES:
                results.append(filename.decode())
            for pattern in APK_FNMATCH:
                if pattern not in seen and fnmatch.fnmatch(filename, pattern):
                    results.append(pattern.decode())
                    seen.add(pattern)
            fh.seek(m + k, os.SEEK_CUR)
    return None if APK_FILES[0].decode() not in results else results


def zip_data(apkfile: str, count: int = 4096) -> ZipData:
    with open(apkfile, "rb") as fh:
        fh.seek(-min(os.path.getsize(apkfile), count), os.SEEK_END)
        data = fh.read()
        pos = data.rfind(b"\x50\x4b\x05\x06")
        if pos == -1:
            raise DetectionError("Expected end of central directory record (EOCD)")
        fh.seek(pos - len(data), os.SEEK_CUR)
        eocd_offset = fh.tell()
        fh.seek(16, os.SEEK_CUR)
        cd_offset = int.from_bytes(fh.read(4), "little")
        if cd_offset >= eocd_offset:
            raise DetectionError("Expected CD before EOCD")
        fh.seek(cd_offset)
        cd_and_eocd = fh.read()
    return ZipData(cd_offset, eocd_offset, cd_and_eocd)


def main() -> None:
    import argparse
    desc = "Detect APK files; exits with 0 if all files are detected as APKs, 1 otherwise."
    parser = argparse.ArgumentParser(prog="detect-apk.py", description=desc)
    parser.add_argument("-v", "--verbose", action="count", default=0,
                        help="increase verbosity level")
    parser.add_argument("apks", metavar="APK", nargs="+")
    args = parser.parse_args()
    not_an_apk = 0
    for apkfile in args.apks:
        if not os.path.exists(apkfile):
            print(f"Error: file {apkfile!r} does not exist.", file=sys.stderr)
            sys.exit(1)
        if args.verbose:
            print(f"Processing {apkfile!r}...")
        try:
            results = detect_apk(apkfile)
        except DetectionError as e:
            not_an_apk += 1
            if args.verbose:
                print(f"  Detection error: {e}.")
            continue
        if results:
            if args.verbose:
                print(f"  Detected as APK ({len(results)} matches).")
                if args.verbose > 1:
                    for result in results:
                        print(f"  Found: {result}.")
        else:
            not_an_apk += 1
            if args.verbose:
                print("  Not detected as APK.")
    if not_an_apk:
        sys.exit(1)


if __name__ == "__main__":
    main()

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