HighCVE-2026-21441

    Urllib3: Improper Handling of Highly Compressed Data (Data Amplification) (CVE-2026-21441)

    Published Last modified Sources: Red Hat, SUSE, OSV, CVE.org, Msrc Update Guide, BSI

    What is affected, what to do

    A flaw was found in urllib3. When processing HTTP redirect responses during data streaming, the library automatically reads and decompresses the full response body without enforcing configured size limits.

    Update urllib3 to version 2.6.3, Security Verify Access to version 10.0.9.2.

    Affected
    • Urllib3 gte 1.22, < 2.6.3..*

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

    Metrics

    8.9

    Priority

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

    Source: NVD, CVSS v4

    7.5

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: Red Hat, SUSE, OSV, CVE.org, Msrc Update Guide, BSI

    2.9 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 86.9

    Show all metrics
    Severity
    High
    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
    118.40 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
    8.9 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
    7.5
    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
    8.9
    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
    2.9 %
    Moderate — model estimates 1-10% 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
    86.9
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    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
    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-409

    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
    High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
    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. When processing HTTP redirect responses during data streaming, the library automatically reads and decompresses the full response body without enforcing configured size limits. A malicious server can exploit this by serving an excessively compressed response, bypassing decompression bomb safeguards. This issue leads to high memory and processor consumption, resulting in a Denial of Service (DoS) on the client.

    Source: redhat_csaf, suse_csaf, OSV (Open Source Vulnerabilities), NVD (NIST), CVE.org (MITRE), Microsoft Security Update Guide, cvelistv5, BSI WID Portal, BSI CSAF

    Affected products

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

    Affected products
    VendorProductAffected versionsFixed in
    urllib3urllib3
    • ≥ 1.22, < 2.6.3
    • —
    pythonurllib3
    • 1.22 – < 2.6.3
    • 2.6.3
    IBMQRadar SIEM
    • <7.5.0 UP15
    • —
    IBMSecurity Verify Access
    • < 10.0.9.2
    • 10.0.9.2
    OracleCommunications
    • 7.5.0
    • 15.0.1.0.0
    • 26.1.202
    • 7.7.0
    • 25.1.100
    • 47.0.0.2.0
    • 7.0.0
    • 6
    • 6.1
    • 7.8.0
    • 24.2.4-25.2.000
    • 7.5.1
    • 25.2.100
    • 8.0-8.1
    • 8.0.1
    • 7.6.0
    • 14
    • 15.1.0.0.0
    • 25.1.200
    • 6.1.1-7.0.0
    • 25.2.200
    • 15.2.0.0.0
    • 11
    • 26.1.200
    • 25.2.202
    • 25.2.201
    • 15.0.0.0.0
    • 8.0-8.3
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    OracleFinancial Services Applications
    • 8.1.3.0
    • 8.1.2.5.0
    • 8.1.2.9
    • 8.1.2.7
    • 14.5.0-14.8.0
    • 8.1.3.1
    • 8.0.8.7.0
    • 8.0.7.9.0
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    OracleUtilities Applications
    • 7.0.0.0.8
    • 4.5.0.0.0
    • 4.4.0.2.0-4.4.0.4.0
    • 2.4.0.1.0-2.4.0.1.32
    • 4.5.0.0.0-4.5.0.1.1
    • 4.5.0.2.0
    • 4.5.0.1.3
    • 4.3.0.5.0-4.3.0.6.0
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —

    Weakness classes (CWE)

    • CWE-409Improper Handling of Highly Compressed Data (Data Amplification)

      Base

      The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output.

      Entry at MITRE CWE-409

    References & sources

    Reanalysis & status changes

    Chronological NVD audit events for this CVE — reanalyses, CVSS updates, CPE diffs.

    1. CVE Modified0b0ca135-0b70-47e7-9f44-1890c2a1c46c

      • Affected: after Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/21xxx/CVE-2026-21441.json">CVE-2026-21441</a>

    2. CVE Modified0b0ca135-0b70-47e7-9f44-1890c2a1c46c

      • Affected: after Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/21xxx/CVE-2026-21441.json">CVE-2026-21441</a>

    3. CVE Modified0b0ca135-0b70-47e7-9f44-1890c2a1c46c

      • Affected: after Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/21xxx/CVE-2026-21441.json">CVE-2026-21441</a>

    4. CVE Modified0b0ca135-0b70-47e7-9f44-1890c2a1c46c

      • Affected: after Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/21xxx/CVE-2026-21441.json">CVE-2026-21441</a>

    5. CVE Modified0b0ca135-0b70-47e7-9f44-1890c2a1c46c

      • Affected: after Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/21xxx/CVE-2026-21441.json">CVE-2026-21441</a>
      • Reference: after https://access.redhat.com/errata/RHSA-2026:61628
    Embed badge

    Show the severity and KEV status of this vulnerability on your site – with a link back here. The image is updated hourly.

    Badge preview for CVE-2026-21441
    <a href="https://intel.neosec.eu/en/cve/CVE-2026-21441"><img src="https://intel.neosec.eu/badge/cve/CVE-2026-21441.svg?lang=en" alt="CVE-2026-21441 – NEOSEC Intel" height="20"></a>