Vulnerability · Published Sep 29, 2026 · Assigned by GitHub_M
CVE-2026-97688urllib3: Chunked Deflate streaming can enter an infinite loop
Medium 6.9
In plain English
A medium-severity infinite loop in urllib3 from urllib3. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 22% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:14 | python-urllib3 | 2.8.0-1 | DEBIAN-CVE-2026-97688 |
| PyPI | urllib3 | 2.8.0 | GHSA-gh4c-6fx4-qh6g |
| PyPI | urllib3 | 2.8.0 | PYSEC-2026-4176 |
| Ubuntu:26.10 | python-pip | no fix listed | UBUNTU-CVE-2026-97688 |
| Ubuntu:22.04:LTS | python-urllib3 | 1.26.5-1~exp1ubuntu0.8 | USN-8926-1 |
| Ubuntu:24.04:LTS | python-urllib3 | 2.0.7-1ubuntu0.8 | USN-8926-1 |
| Ubuntu:26.04:LTS | python-urllib3 | 2.6.3-1ubuntu1.2 | USN-8926-1 |
| Ubuntu:26.04:LTS | python-urllib3 | 2.6.3-1ubuntu1.2 | UBUNTU-CVE-2026-97688 |
Affected products (as reported)
Weakness: CWE-835 · Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
References
- https://github.com/urllib3/urllib3/security/advisories/GHSA-gh4c-6fx4-qh6g x_refsource_CONFIRM
- https://github.com/urllib3/urllib3/commit/ea2ad7b21a80da3632f80016526a18864586077f x_refsource_MISC
- https://github.com/urllib3/urllib3/releases/tag/2.8.0 x_refsource_MISC