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How the Mentioned CVE Works
The vulnerability exploits the chunked transfer encoding mechanism in the aiohttp web server. Normally, this allows a client to stream data in a series of “chunks.” The `request.read()` method processes these chunks sequentially. An attacker can craft a malicious HTTP request containing an extremely large number of very small chunks (e.g., thousands of 1-byte chunks). For each chunk received, the server incurs blocking CPU overhead for parsing headers and managing buffers. When `request.read()` is called, it must synchronously process every single chunk before returning the complete body. This design forces the server’s event loop to block for a moderate but significant duration (e.g., 1 second) per malicious request. During this blocking period, the single worker thread is occupied, preventing it from handling other incoming connections. This turns a standard feature into a resource exhaustion attack. The server’s ability to accept new requests is stalled, leading to a denial-of-service condition. The impact is amplified if multiple such requests are sent concurrently, potentially tying up all available workers.
DailyCVE Form
Platform: Python AIOHTTP library
Version: Pre-patch versions
Vulnerability: Chunked Message DoS
Severity: Moderate
date: 2026-01-05
Prediction: Patch by 2026-01-12
What Undercode Say
Analytics
Monitor for hanging requests with high CPU ps aux | grep aiohttp | grep -v grep netstat -tunlp | grep :8080 Simulate chunked traffic (educational) echo "GET / HTTP/1.1\r\nTransfer-Encoding: chunked\r\n\r\n" | nc localhost 8080 Check server logs for slow requests tail -f /var/log/aiohttp/access.log | grep -E "(read|chunk)"
How Exploit
An attacker sends an HTTP POST request with the `Transfer-Encoding: chunked` header. The request body is crafted as hundreds of tiny chunks (e.g., “1\r\na\r\n” repeated). Each chunk requires the server to parse its size and content. When the vulnerable endpoint calls await request.read(), the server blocks, consuming CPU time to assemble the full message. Concurrent malicious requests exhaust server capacity.
Protection from this CVE
Apply the official patches: `aio-libs/aiohttp@dc3170b` and aio-libs/aiohttp@4ed97a4. Upgrade the aiohttp library to the version containing these fixes. Avoid using `request.read()` for untrusted client data. Implement request timeouts and size limits. Use a reverse proxy (e.g., nginx) for rate limiting.
Impact
Denial of Service (DoS) through CPU resource exhaustion. Server unresponsive for ~1 second per malicious request. Reduced availability for legitimate users. Potential for amplification in concurrent attack scenarios.
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Sources:
Reported By: github.com
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