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.
Published: 2026-09-29
Score: 6.9 Medium
EPSS: n/a
KEV: No
Impact: n/a
Action: n/a
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Remediation

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History

Tue, 29 Sep 2026 15:45:00 +0000

Type Values Removed Values Added
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.
Title urllib3: Chunked Deflate streaming can enter an infinite loop
Weaknesses CWE-835
References
Metrics cvssV4_0

{'score': 6.9, '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'}


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cve-icon MITRE

Status: PUBLISHED

Assigner: GitHub_M

Published:

Updated: 2026-09-29T15:42:34.502Z

Reserved: 2026-09-24T21:43:37.208Z

Link: CVE-2026-97688

cve-icon Vulnrichment

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cve-icon NVD

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cve-icon Redhat

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cve-icon OpenCVE Enrichment

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Weaknesses