MediumCVE-2021-3733

    Python: Uncontrolled Resource Consumption (CVE-2021-3733)

    Published Last modified Sources: Siemens ProductCERT, CVE.org, OSV

    What is affected, what to do

    There's a flaw in urllib's AbstractBasicAuthHandler class. An attacker who controls a malicious HTTP server that an HTTP client (such as web browser) connects to, could trigger a Regular Expression Denial of Service (ReDOS) during an authentication request with a specially crafted payload that is sent by the server to the client.

    Update python to version 3.7.11.

    Affected
    • Python eq Fixed in python v3.6.14, python v3.7.11, python v3.8.10, python v3.9.5...Fixed in python v3.6.14, python v3.7.11, python v3.8.10, python v3.9.5.

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    Metrics

    6.5

    Priority

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

    Source: NVD, CVSS v3

    6.5

    CVSS v3

    Severity Medium on the scale from 0 to 10.

    Source: Siemens ProductCERT, CVE.org, OSV

    4.8 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 91.9

    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
    100.61 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
    6.5 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
    6.5
    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
    4.8 %
    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
    91.9
    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
    CISA assessment (SSVC)
    Exploitation: none known · automatable · Technical impact: partial
    Source: CISA Vulnrichment, as of 8 October 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-400

    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

    There's a flaw in urllib's AbstractBasicAuthHandler class. An attacker who controls a malicious HTTP server that an HTTP client (such as web browser) connects to, could trigger a Regular Expression Denial of Service (ReDOS) during an authentication request with a specially crafted payload that is sent by the server to the client. The greatest threat that this flaw poses is to application availability.

    Source: siemens_advisory, cvelistv5, NVD (NIST), OSV (Open Source Vulnerabilities)

    Affected products

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

    Affected products
    Affected versionsFixed in
    —python
    • Fixed in python v3.6.14, python v3.7.11, python v3.8.10, python v3.9.5.
    • —
    fedoraprojectextra_packages_for_enterprise_linux
    • 7.0
    • —
    netappmanagement_services_for_element_software_and_netapp_hci
    • —
    • —
    netappontap_select_deploy_administration_utility
    • —
    • —
    netappsolidfire\,_enterprise_sds_\&_hci_storage_node
    • —
    • —
    pythonpython
    • 3.10.0
    • 3.7.0 – < 3.7.11
    • 3.9.0 – < 3.9.5
    • < 3.6.14
    • 3.8.0 – < 3.8.10
    • —
    • 3.7.11
    • 3.9.5
    • 3.6.14
    • 3.8.10
    redhatcodeready_linux_builder
    • 8.0
    • —
    redhatcodeready_linux_builder_for_ibm_z_systems
    • 8.0
    • —
    redhatcodeready_linux_builder_for_power_little_endian
    • 8.0
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Linuxredhatenterprise_linux8.0
    Linuxredhatenterprise_linux_eus8.4
    Linuxredhatenterprise_linux_for_ibm_z_systems8.0
    Linuxredhatenterprise_linux_for_ibm_z_systems_eus8.4
    Linuxredhatenterprise_linux_for_power_little_endian8.0
    Linuxredhatenterprise_linux_for_power_little_endian_eus8.4
    Linuxredhatenterprise_linux_server_aus8.4
    Linuxredhatenterprise_linux_server_for_power_little_endian_update_services_for_sap_solutions8.4
    Linuxredhatenterprise_linux_server_tus8.4
    Linuxredhatenterprise_linux_server_update_services_for_sap_solutions8.4
    Otherfedoraprojectfedora33
    Otherfedoraprojectfedora34
    Otherfedoraprojectfedora35
    Otherfedoraprojectfedora36
    Othernetapphci_compute_node_firmware—

    Weakness classes (CWE)

    • CWE-400Uncontrolled Resource Consumption

      Class

      The product does not properly control the allocation and maintenance of a limited resource.

      Entry at MITRE CWE-400

    References & sources

    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).

    1. Record modifiedSiemens ADP

      Show 2 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Affected versions (publisher)added
      —
      New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2021/3xxx/CVE-2021-3733.json">CVE-2021-3733</a>
      Referenceadded
      —
      https://cert-portal.siemens.com/productcert/html/ssa-398330.html

      Source identifier: 0b142b55-0307-4c5a-b3c9-f314f3fb7c5e

    2. Record modifiedRed Hat

      Show 22 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://bugs.python.org/issue43075
      Referenceadded
      —
      https://bugzilla.redhat.com/show_bug.cgi?id=1995234
      Referenceadded
      —
      https://github.com/python/cpython/commit/7215d1ae25525c92b026166f9d5cac85fb
      Referenceadded
      —
      https://github.com/python/cpython/pull/24391
      Referenceadded
      —
      https://lists.debian.org/debian-lts-announce/2023/05/msg00024.html
      Referenceadded
      —
      https://lists.debian.org/debian-lts-announce/2023/06/msg00039.html
      Referenceadded
      —
      https://security.netapp.com/advisory/ntap-20220407-0001/
      Referenceadded
      —
      https://ubuntu.com/security/CVE-2021-3733
      Referenceremoved
      https://bugs.python.org/issue43075
      —
      Referenceremoved
      https://bugzilla.redhat.com/show_bug.cgi?id=1995234
      —
      Referenceremoved
      https://github.com/python/cpython/commit/7215d1ae25525c92b026166f9d5cac85fb
      —
      Referenceremoved
      https://github.com/python/cpython/pull/24391
      —
      Referenceremoved
      https://lists.debian.org/debian-lts-announce/2023/05/msg00024.html
      —
      Referenceremoved
      https://lists.debian.org/debian-lts-announce/2023/06/msg00039.html
      —
      Referenceremoved
      https://security.netapp.com/advisory/ntap-20220407-0001/
      —
      Referenceremoved
      https://ubuntu.com/security/CVE-2021-3733
      —
      Reference typeremoved
      https://bugs.python.org/issue43075 Types: Exploit, Issue Tracking, Patch, Vendor Advisory
      —
      Reference typeremoved
      https://bugzilla.redhat.com/show_bug.cgi?id=1995234 Types: Issue Tracking, Third Party Advisory
      —
      Reference typeremoved
      https://github.com/python/cpython/commit/7215d1ae25525c92b026166f9d5cac85fb Types: Patch, Third Party Advisory
      —
      Reference typeremoved
      https://github.com/python/cpython/pull/24391 Types: Patch, Third Party Advisory
      —
      Reference typeremoved
      https://security.netapp.com/advisory/ntap-20220407-0001/ Types: Third Party Advisory
      —
      Reference typeremoved
      https://ubuntu.com/security/CVE-2021-3733 Types: Patch, Third Party Advisory
      —

      Source identifier: secalert@redhat.com

    3. Record modifiedCISA ADP

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

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      SSVC assessmentadded
      —
      {"id":"CVE-2021-3733","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-10-08T20:57:59.983466Z"}

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

    4. Record modifiedCVE Program

      Show 24 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://bugs.python.org/issue43075
      Referenceadded
      —
      https://bugzilla.redhat.com/show_bug.cgi?id=1995234
      Referenceadded
      —
      https://github.com/python/cpython/commit/7215d1ae25525c92b026166f9d5cac85fb
      Referenceadded
      —
      https://github.com/python/cpython/pull/24391
      Referenceadded
      —
      https://lists.debian.org/debian-lts-announce/2023/05/msg00024.html
      Referenceadded
      —
      https://lists.debian.org/debian-lts-announce/2023/06/msg00039.html
      Referenceadded
      —
      https://lists.debian.org/debian-lts-announce/2024/12/msg00000.html
      Referenceadded
      —
      https://security.netapp.com/advisory/ntap-20220407-0001/
      Referenceadded
      —
      https://ubuntu.com/security/CVE-2021-3733
      Referenceremoved
      https://bugs.python.org/issue43075
      —
      Referenceremoved
      https://bugzilla.redhat.com/show_bug.cgi?id=1995234
      —
      Referenceremoved
      https://github.com/python/cpython/commit/7215d1ae25525c92b026166f9d5cac85fb
      —
      Referenceremoved
      https://github.com/python/cpython/pull/24391
      —
      Referenceremoved
      https://lists.debian.org/debian-lts-announce/2023/05/msg00024.html
      —
      Referenceremoved
      https://lists.debian.org/debian-lts-announce/2023/06/msg00039.html
      —
      Referenceremoved
      https://lists.debian.org/debian-lts-announce/2024/12/msg00000.html
      —
      Referenceremoved
      https://security.netapp.com/advisory/ntap-20220407-0001/
      —
      Referenceremoved
      https://ubuntu.com/security/CVE-2021-3733
      —
      Reference typeremoved
      https://bugs.python.org/issue43075 Types: Exploit, Issue Tracking, Patch, Vendor Advisory
      —
      Reference typeremoved
      https://bugzilla.redhat.com/show_bug.cgi?id=1995234 Types: Issue Tracking, Third Party Advisory
      —
      Reference typeremoved
      https://github.com/python/cpython/commit/7215d1ae25525c92b026166f9d5cac85fb Types: Patch, Third Party Advisory
      —
      Reference typeremoved
      https://github.com/python/cpython/pull/24391 Types: Patch, Third Party Advisory
      —
      Reference typeremoved
      https://security.netapp.com/advisory/ntap-20220407-0001/ Types: Third Party Advisory
      —
      Reference typeremoved
      https://ubuntu.com/security/CVE-2021-3733 Types: Patch, Third Party Advisory
      —

      Source identifier: af854a3a-2127-422b-91ae-364da2661108

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