Add content from: GodFather - Part 1 - A multistage dropper

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src/generic-methodologies-and-resources/basic-forensic-methodology/specific-software-file-type-tricks/zips-tricks.md

@@ -14,11 +14,168 @@ The [Zip file format specification](https://pkware.cachefly.net/webdocs/casestud
 
 It's crucial to note that password-protected zip files **do not encrypt filenames or file sizes** within, a security flaw not shared with RAR or 7z files which encrypt this information. Furthermore, zip files encrypted with the older ZipCrypto method are vulnerable to a **plaintext attack** if an unencrypted copy of a compressed file is available. This attack leverages the known content to crack the zip's password, a vulnerability detailed in [HackThis's article](https://www.hackthis.co.uk/articles/known-plaintext-attack-cracking-zip-files) and further explained in [this academic paper](https://www.cs.auckland.ac.nz/~mike/zipattacks.pdf). However, zip files secured with **AES-256** encryption are immune to this plaintext attack, showcasing the importance of choosing secure encryption methods for sensitive data.
 
+---
+
+## Anti-reversing tricks in APKs using manipulated ZIP headers
+
+Modern Android malware droppers use malformed ZIP metadata to break static tools (jadx/apktool/unzip) while keeping the APK installable on-device. The most common tricks are:
+
+- Fake encryption by setting the ZIP General Purpose Bit Flag (GPBF) bit 0
+- Abusing large/custom Extra fields to confuse parsers
+- File/directory name collisions to hide real artifacts (e.g., a directory named `classes.dex/` next to the real `classes.dex`)
+
+### 1) Fake encryption (GPBF bit 0 set) without real crypto
+
+Symptoms:
+- `jadx-gui` fails with errors like:
+  
+  ```
+  java.util.zip.ZipException: invalid CEN header (encrypted entry)
+  ```
+- `unzip` prompts for a password for core APK files even though a valid APK cannot have encrypted `classes*.dex`, `resources.arsc`, or `AndroidManifest.xml`:
+  
+  ```bash
+  unzip sample.apk
+  [sample.apk] classes3.dex password:
+    skipping: classes3.dex                          incorrect password
+    skipping: AndroidManifest.xml/res/vhpng-xhdpi/mxirm.png  incorrect password
+    skipping: resources.arsc/res/domeo/eqmvo.xml            incorrect password
+    skipping: classes2.dex                          incorrect password
+  ```
+
+Detection with zipdetails:
+
+```bash
+zipdetails -v sample.apk | less
+```
+
+Look at the General Purpose Bit Flag for local and central headers. A telltale value is bit 0 set (Encryption) even for core entries:
+
+```
+Extract Zip Spec      2D '4.5'
+General Purpose Flag  0A09
+  [Bit 0]   1 'Encryption'
+  [Bits 1-2] 1 'Maximum Compression'
+  [Bit 3]   1 'Streamed'
+  [Bit 11]  1 'Language Encoding'
+```
+
+Heuristic: If an APK installs and runs on-device but core entries appear "encrypted" to tools, the GPBF was tampered with.
+
+Fix by clearing GPBF bit 0 in both Local File Headers (LFH) and Central Directory (CD) entries. Minimal byte-patcher:
+
+```python
+# gpbf_clear.py – clear encryption bit (bit 0) in ZIP local+central headers
+import struct, sys
+
+SIG_LFH = b"\x50\x4b\x03\x04"  # Local File Header
+SIG_CDH = b"\x50\x4b\x01\x02"  # Central Directory Header
+
+def patch_flags(buf: bytes, sig: bytes, flag_off: int):
+    out = bytearray(buf)
+    i = 0
+    patched = 0
+    while True:
+        i = out.find(sig, i)
+        if i == -1:
+            break
+        flags, = struct.unpack_from('<H', out, i + flag_off)
+        if flags & 1:  # encryption bit set
+            struct.pack_into('<H', out, i + flag_off, flags & 0xFFFE)
+            patched += 1
+        i += 4  # move past signature to continue search
+    return bytes(out), patched
+
+if __name__ == '__main__':
+    inp, outp = sys.argv[1], sys.argv[2]
+    data = open(inp, 'rb').read()
+    data, p_lfh = patch_flags(data, SIG_LFH, 6)  # LFH flag at +6
+    data, p_cdh = patch_flags(data, SIG_CDH, 8)  # CDH flag at +8
+    open(outp, 'wb').write(data)
+    print(f'Patched: LFH={p_lfh}, CDH={p_cdh}')
+```
+
+Usage:
+
+```bash
+python3 gpbf_clear.py obfuscated.apk normalized.apk
+zipdetails -v normalized.apk | grep -A2 "General Purpose Flag"
+```
+
+You should now see `General Purpose Flag  0000` on core entries and tools will parse the APK again.
+
+### 2) Large/custom Extra fields to break parsers
+
+Attackers stuff oversized Extra fields and odd IDs into headers to trip decompilers. In the wild you may see custom markers (e.g., strings like `JADXBLOCK`) embedded there.
+
+Inspection:
+
+```bash
+zipdetails -v sample.apk | sed -n '/Extra ID/,+4p' | head -n 50
+```
+
+Examples observed: unknown IDs like `0xCAFE` ("Java Executable") or `0x414A` ("JA:") carrying large payloads.
+
+DFIR heuristics:
+- Alert when Extra fields are unusually large on core entries (`classes*.dex`, `AndroidManifest.xml`, `resources.arsc`).
+- Treat unknown Extra IDs on those entries as suspicious.
+
+Practical mitigation: rebuilding the archive (e.g., re-zipping extracted files) strips malicious Extra fields. If tools refuse to extract due to fake encryption, first clear GPBF bit 0 as above, then repackage:
+
+```bash
+mkdir /tmp/apk
+unzip -qq normalized.apk -d /tmp/apk
+(cd /tmp/apk && zip -qr ../clean.apk .)
+```
+
+### 3) File/Directory name collisions (hiding real artifacts)
+
+A ZIP can contain both a file `X` and a directory `X/`. Some extractors and decompilers get confused and may overlay or hide the real file with a directory entry. This has been observed with entries colliding with core APK names like `classes.dex`.
+
+Triage and safe extraction:
+
+```bash
+# List potential collisions (names that differ only by trailing slash)
+zipinfo -1 sample.apk | awk '{n=$0; sub(/\/$/,"",n); print n}' | sort | uniq -d
+
+# Extract while preserving the real files by renaming on conflict
+unzip normalized.apk -d outdir
+# When prompted:
+# replace outdir/classes.dex? [y]es/[n]o/[A]ll/[N]one/[r]ename: r
+# new name: unk_classes.dex
+```
+
+Programmatic detection post-fix:
+
+```python
+from zipfile import ZipFile
+from collections import defaultdict
+
+with ZipFile('normalized.apk') as z:
+    names = z.namelist()
+
+collisions = defaultdict(list)
+for n in names:
+    base = n[:-1] if n.endswith('/') else n
+    collisions[base].append(n)
+
+for base, variants in collisions.items():
+    if len(variants) > 1:
+        print('COLLISION', base, '->', variants)
+```
+
+Blue-team detection ideas:
+- Flag APKs whose local headers mark encryption (GPBF bit 0 = 1) yet install/run.
+- Flag large/unknown Extra fields on core entries (look for markers like `JADXBLOCK`).
+- Flag path-collisions (`X` and `X/`) specifically for `AndroidManifest.xml`, `resources.arsc`, `classes*.dex`.
+
+---
+
 ## References
 
 - [https://michael-myers.github.io/blog/categories/ctf/](https://michael-myers.github.io/blog/categories/ctf/)
+- [GodFather – Part 1 – A multistage dropper (APK ZIP anti-reversing)](https://shindan.io/blog/godfather-part-1-a-multistage-dropper)
+- [zipdetails (Archive::Zip script)](https://metacpan.org/pod/distribution/Archive-Zip/scripts/zipdetails)
+- [ZIP File Format Specification (PKWARE APPNOTE.TXT)](https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT)
 
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