CVE-2026-97688 urllib3: Chunked Deflate streaming can enter an infinite loop
A malicious server can cause urllib3 to enter an infinite loop during chunked deflate decoding, leading to excessive CPU usage and denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-97688?
CVE-2026-97688 is a medium-severity denial of service vulnerability in urllib3, published on 29 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
urllib3 is an HTTP client library for Python. From 2.6.2 until 2.8.0, HTTPResponse.stream and HTTPResponse.read_chunked can enter an infinite loop because the Deflate decoder retains trailing bytes as unconsumed input after reaching end-of-stream and repeatedly decodes them without progress. The issue occurs when an untrusted server sends a chunked Deflate response whose decoded body exceeds a positive finite chunk size and whose encoded body has trailing bytes, specifically a response with Transfer-Encoding: chunked and Content-Encoding: deflate, content decoding enabled, and the positive finite amt=N streaming chunk size. The attack mechanism is that a malicious server returns a compressed chunked response with trailing bytes after the Deflate stream. The impact is excessive CPU usage and a request that does not complete, and network read timeouts do not interrupt the loop because no further socket read occurs. This issue is fixed in version 2.8.0.
Weakness: CWE-835
Which versions are affected by CVE-2026-97688?
3 open source packages ship an affected version, including urllib3. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
urllib3 PyPI | From 2.6.2 | 2.8.0 |
urllib3 PyPI | From 2.6.2 | 2.8.0 |
urllib3 PyPI | From 2.6.2 | 2.8.0 |
Products include Python Urllib3.
How do I fix CVE-2026-97688?
Upgrade urllib3 to 2.8.0 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-97688 being exploited?
There is no known exploitation of CVE-2026-97688 in the wild and no public exploit code.
Other vulnerabilities in urllib3
- CVE-2026-97689 urllib3: HTTPResponse.stream()/read_chunked() buffers an unbounded chunk-size line into memory
- CVE-2026-44432 urllib3: Decompression-bomb safeguards bypassed in parts of the streaming API
- CVE-2026-21441 urllib3 vulnerable to decompression-bomb safeguard bypass when following HTTP redirects (streaming API)
- CVE-2025-66418 urllib3 allows an unbounded number of links in the decompression chain
- CVE-2025-66471 urllib3 Streaming API improperly handles highly compressed data
Sources
Also known as BELL-CVE-2026-97688, CLEANSTART-2026-AD44592, CLEANSTART-2026-BK97819, CLEANSTART-2026-BS13854, CLEANSTART-2026-GA47961, CLEANSTART-2026-TN63603 and 39 more.