CVE-2025-66418VulnerabilityPublished 4 Feb 2026Updated 8 Oct 2026

Red Hat Security Advisory: python3.12-urllib3 security update

Affects Red HatSource GCVE Database

Mentioned vulnerabilities

Not known to be exploited : upgrade to 9.4.2 soon.

Data as of 26 Oct 2026, 06:26 UTC

Severity

High

Rated by Threat Radar · No CVSS score from NVD or the CNA

EPSS: 0.6% chance of exploitation in 30 days – higher than 52% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 9.4.2

Exploitation evidence

Disclosed: evidence found.
CVE published 4 Feb 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 2026-10-26.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade Red Hat to 9.4.2.

Affected software

Product	Affected	Fixed in
Red Hat Product Security Red Hat	Affected >=9.4 <9.4.2 Red Hat Red Hat Enterprise Linux Red Hat Enterprise Linux AppStream EUS (v.9.4) src python3.12-urllib3-0:1.26.18-2.el9_4.2.src	Fixed in 9.4.2

AI-suggested

Red Hat has released updated python3.12-urllib3 packages (version 1.26.18-2.el9_4.2) that address these vulnerabilities. Users of affected Red Hat Enterprise Linux 9.4 Extended Update Support systems should apply the security update as detailed in the Red Hat advisory RHSA-2026:1957 and the referenced article https://access.redhat.com/articles/11258. Applying this update mitigates the described vulnerabilities.

Description

Python is an interpreted, interactive, object-oriented programming language, which includes modules, classes, exceptions, very high level dynamic data types and dynamic typing. Python supports interfaces to many system calls and libraries, as well as to various windowing systems. Security Fix(es): * urllib3: urllib3: Unbounded decompression chain leads to resource exhaustion (CVE-2025-66418) * urllib3: urllib3 Streaming API improperly handles highly compressed data (CVE-2025-66471) * urllib3: urllib3 vulnerable to decompression-bomb safeguard bypass when following HTTP redirects (streaming API) (CVE-2026-21441) For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Impact

These vulnerabilities can lead to resource exhaustion due to unbounded decompression chains and improper handling of compressed data, potentially impacting availability of applications using urllib3. The decompression-bomb safeguard bypass may allow attackers to circumvent protections designed to prevent denial-of-service conditions caused by maliciously crafted compressed data. No known exploits in the wild have been reported at this time.

Summary

The Red Hat Security Advisory RHSA-2026:1957 addresses three vulnerabilities in the python3.12-urllib3 package: CVE-2025-66418 involves an unbounded decompression chain that can cause resource exhaustion; CVE-2025-66471 concerns improper handling of highly compressed data by the urllib3 Streaming API; and CVE-2026-21441 describes a decompression-bomb safeguard bypass in the Streaming API when following HTTP redirects. These vulnerabilities affect Red Hat Enterprise Linux 9.4 Extended Update Support versions prior to 9.4.2. The advisory provides updated packages to mitigate these issues.

CWE-770Allocation of Resources Without Limits or Throttling (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #25
The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
Requirements
Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
Architecture and Design
Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
Architecture and Design
Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
OWASP ASVS 4.0.3
V8.1.4 (opens in a new tab)
V13.4.1 (opens in a new tab)
V11.1.4 (opens in a new tab)
Attack patterns
XML Ping of the Death (opens in a new tab)
Exponential Data Expansion (opens in a new tab)
Serialized Data Parameter Blowup (opens in a new tab)
Serialized Data with Nested Payloads (opens in a new tab)
Oversized Serialized Data Payloads (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-409Improper Handling of Highly Compressed Data (Data Amplification) (opens in a new tab)
Base weakness
The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.2 (opens in a new tab)
V12.1.1 (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 17 Aug 2026, 17:46 UTC

WeaknessesCWE-770, CWE-409

Technical details

References

Community reviews

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