MediumCVE-2026-97688

urllib3: Chunked Deflate streaming can enter an infinite loop (CVE-2026-97688)

Published 29.09.2026Last modified 13.10.2026Sources: Red Hat, SUSE, Ubuntu, OSV, Hersteller, CVE.org

What is affected, what to do

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.

Update urllib3 to version 2.8.0.

Affected: Urllib3 gte 2.6.2, < 2.8.0..*

Not sure whether and how you are affected? Ask a question about this vulnerability

Metrics

6.9

Priority

XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

Source: Vendor (CNA), CVSS v4

5.3

CVSS v3

Severity Medium on the scale from 0 to 10.

Source: Red Hat, SUSE, Ubuntu, OSV, Hersteller, CVE.org

0.3 %

EPSS score

Low — model estimates < 1% exploitation likelihood.

Source: FIRST EPSS, percentile 21.8

Show all metrics

Severity: Medium
Priority (0–325) The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first? Read in glossary: 68.26 Methodology
Exploit maturity An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack. Read in glossary: no public PoC known
XIEM Effective CVSS XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command. Read in glossary: 6.9 Methodology
CVSS v3 CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions. Read in glossary: 5.3
CVSS v4 CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions. Read in glossary: 6.9
EPSS score The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“. Read in glossary: 0.3 %; Low — model estimates < 1% exploitation likelihood.
EPSS percentile The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking. Read in glossary: 21.8; Elevated — this CVE ranks above at least 10% of all CVEs scored today.
KEV status The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching. Read in glossary: —
Published: 29.09.2026
CISA assessment (SSVC): Exploitation: none known · automatable · Technical impact: partial; Source: CISA Vulnrichment, as of 29 September 2026
CWE CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect. Read in glossary: CWE-835

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2 NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management. Read in glossary: Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
CRA The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates. Read in glossary: From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001 ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security. Read in glossary: Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

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.

Source: redhat_csaf, suse_csaf, Ubuntu Security Notices, OSV (Open Source Vulnerabilities), GitHub Security Advisories, NVD (NIST), cvelistv5

Affected products

The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

Affected products
		Affected versions	Fixed in
urllib3	urllib3	`≥ 2.6.2, < 2.8.0`	—
python	urllib3	`2.6.2 – < 2.8.0`	`2.8.0`

Vendors and products

Affected operating systems

Affected operating systems

Linux	ubuntu	python-urllib3	`bionic`
Linux	ubuntu	python-urllib3	`focal`
Linux	ubuntu	python-urllib3	`jammy`
Linux	ubuntu	python-urllib3	`noble`
Linux	ubuntu	python-urllib3	`resolute`

Weakness classes (CWE)

CWE-835Loop with Unreachable Exit Condition ('Infinite Loop')
Base
The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop.
Entry at MITRE CWE-835

References & sources

github.comurllib3/urllib3/security/advisories/GHSA-gh4c-6fx4-qh6gx_refsource_CONFIRM
github.comurllib3/urllib3/commit/ea2ad7b21a80da3632f80016526a18864586077fx_refsource_MISC
github.comurllib3/urllib3/releases/tag/2.8.0x_refsource_MISC
NVD (NIST)vuln/detail/CVE-2026-97688advisory
github.comurllib3/urllib3package
pypi.orgproject/urllib3package
GitHub Security Advisoriesadvisories/GHSA-gh4c-6fx4-qh6gadvisory

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

13.10.2026Initial analysis by NVDNVD

Newly listed as affected:

Python Software Foundation Python urllib3 (2.6.2 – before 2.8.0)

CVSS V3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

New references:

Show 5 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
CVSS	added	—	AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Product configuration	added	—	OR cpe:2.3:a:python:urllib3::::::::* versions from (including) 2.6.2 up to (excluding) 2.8.0
Reference type	added	—	GitHub, Inc.: https://github.com/urllib3/urllib3/commit/ea2ad7b21a80da3632f80016526a18864586077f Types: Patch
Reference type	added	—	GitHub, Inc.: https://github.com/urllib3/urllib3/releases/tag/2.8.0 Types: Release Notes
Reference type	added	—	GitHub, Inc.: https://github.com/urllib3/urllib3/security/advisories/GHSA-gh4c-6fx4-qh6g Types: Mitigation, Vendor Advisory

Source identifier: nvd@nist.gov

29.09.2026ReceivedGitHub

CVSS V4.0: AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

Weakness type: CWE-835

Description added or changed

New references:

Show 7 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Description	added	—	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.
CVSS	added	—	AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Weakness type (CWE)	added	—	CWE-835
Affected versions (publisher)	added	—	New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/97xxx/CVE-2026-97688.json">CVE-2026-97688</a>
Reference	added	—	https://github.com/urllib3/urllib3/commit/ea2ad7b21a80da3632f80016526a18864586077f
Reference	added	—	https://github.com/urllib3/urllib3/releases/tag/2.8.0
Reference	added	—	https://github.com/urllib3/urllib3/security/advisories/GHSA-gh4c-6fx4-qh6g

Source identifier: security-advisories@github.com

29.09.2026Record modifiedCISA ADP

CISA assessment (SSVC): exploitation: none known, automatable: yes

Show the original change

Individual changes of this event as published by NVD
Type	Action	Before	After
SSVC assessment	added	—	{"id":"CVE-2026-97688","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-09-29T15:42:28.931695Z"}

Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

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