Bug 2543253 (CVE-2026-97688)

Summary: CVE-2026-97688 urllib3: urllib3: Denial of Service via infinite loop during chunked Deflate decoding
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: medium    
Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in urllib3. A malicious server can trigger a Denial of Service (DoS) by returning a specially crafted compressed response containing trailing bytes. When streaming the response in chunks, the decompression logic can enter an infinite loop by repeatedly processing unconsumed data without making progress. This issue leads to excessive CPU utilization and prevents the affected network request from completing.
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Description OSIDB Bzimport 2026-09-29 15:53:58 UTC 
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.