urllib3: Chunked Deflate streaming can enter an infinite loop

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.

Scores

EPSS (exploitation in 30 days)

0.29% · top 78% of all CVEs (7 Nov 2026)

CISA SSVC

Exploitation: none · Automatable: yes · Technical impact: partial

The list uses the assigner's score when there is one, then CISA's, then NVD's: Medium 6.9.

Weakness

• CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop') (opens in a new tab)All about CWE-835 and its CVEs

Affected products

• urllib3 · urllib3

  • affected >= 2.6.2, < 2.8.0

Affected packages

From reviewed advisories of the GitHub Advisory Database.

• PyPIurllib3GHSA-gh4c-6fx4-qh6g (opens in a new tab)

References (3)

• https://github.com/urllib3/urllib3/security/advisories/GHSA-gh4c-6fx4-qh6g (opens in a new tab)x_refsource_CONFIRM
• https://github.com/urllib3/urllib3/commit/ea2ad7b21a80da3632f80016526a18864586077f (opens in a new tab)x_refsource_MISC
• https://github.com/urllib3/urllib3/releases/tag/2.8.0 (opens in a new tab)x_refsource_MISC

Sources

The same CVE in the primary databases and aggregators, to check against.

• CVE.org record (opens in a new tab)
• CVE List JSON (GitHub) (opens in a new tab)
• NVD (opens in a new tab)
• FIRST EPSS (opens in a new tab)
• OSV.dev (opens in a new tab)
• GitHub Advisory GHSA-gh4c-6fx4-qh6g (opens in a new tab)
• CIRCL Vulnerability-Lookup (opens in a new tab)

Badge for a README or a ticket

[![CVE-2026-97688](https://trackcve.app/badge/CVE-2026-97688.svg)](https://trackcve.app/cve/CVE-2026-97688)

Record last updated 29 Sept 2026, 15:42 UTC.