CriticalCVE-2021-27085

    Internet Explorer Remote Code Execution Vulnerability (CVE-2021-27085)

    Published Last modified In CISA KEV since Sources: CVE.org

    What is affected, what to do

    Internet Explorer Remote Code Execution Vulnerability

    The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Internet Explorer 11 to version 10.0.17134.2087. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Internet Explorer 11 lt 1.0.0..10.0.17134.2087
    Fixed in
    • Microsoft Internet Explorer 11 10.0.17134.2087

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

    Metrics

    8.8

    Priority

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

    Source: NVD, CVSS v3

    8.8

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: CVE.org

    5.5 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 92.7

    Show all metrics
    Severity
    Critical
    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
    212.75 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.

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    actively exploited
    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.

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    8.8 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.

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    8.8
    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“.

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    5.5 %
    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.

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    92.7
    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.

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    Actively exploited
    Published
    In CISA KEV since

    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.

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    CISA assessment (SSVC)
    Exploitation: active · not automatable · Technical impact: partial
    Source: CISA Vulnrichment, as of 15 March 2024

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw is network-reachable without authentication but requires a user to open a crafted web page or a document embedding web content; the scope change and high integrity impact make it a textbook initial-access vector for drive-by and phishing campaigns. Its exploitation likelihood sits well above the average CVE, while CISA has not flagged known ransomware use. Typical targets are organisations still dependent on IE 11 or MSHTML for legacy line-of-business apps, and unpatched legacy hosts in OT-adjacent or critical-infrastructure environments. Wherever IE 11 is still enabled and internet-facing, treat remediation as high priority; in estates with IE fully disabled and a current patch baseline, a verification scan is sufficient.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory: Identify every endpoint and server where Internet Explorer 11 or the MSHTML engine is active (Intune/SCCM/Defender for Endpoint software inventory). Pay particular attention to Windows 10 1803 (build 10.0.17134.x), LTSC editions and Windows Server hosts with IE 11.
      • Patch: Deploy the cumulative Windows/IE security update that brings IE 11 to at least 10.0.17134.2087 (or the equivalent fixed level on other Windows builds). The KB number differs per OS version — look it up in the Microsoft Security Update Guide entry for CVE-2021-27085. Treat any host that has not received updates since early 2021 as at risk of compromise.
      • Isolate unpatchable legacy hosts: Move out-of-support endpoints (Windows 10 1803 is end-of-service) into a restricted VLAN and block direct internet access via proxy policy.
      • Disable IE 11 as a standalone browser: Apply the GPO "Disable Internet Explorer 11 as a standalone browser" or remove the feature (Disable-WindowsOptionalFeature -Online -FeatureName Internet-Explorer-Optional-amd64) on all clients without a business need.
      • Retro-hunt: On unpatched hosts, search historical telemetry for suspicious child processes of iexplore.exe (see detection rules) and preserve forensic evidence on any hit.

    2. Runbook step 2

      Mitigation layers

      • Configuration / application layer: For legacy apps, use Microsoft Edge IE mode with a tightly scoped Enterprise Site List — only explicitly approved internal URLs may render in Trident/MSHTML. Enforce Enhanced Protected Mode and 64-bit tab processes for IE via GPO.
      • Network / proxy: On the secure web gateway, block or at minimum log outbound requests carrying the IE user agent (Trident/7.0) to non-allow-listed external destinations. Deny "Newly Registered Domains" and "Uncategorized" categories for IE traffic.
      • Endpoint hardening: Enable Defender ASR rules, notably "Block Office applications from creating child processes" and "Block process creations originating from PSExec and WMI commands", since MSHTML content is frequently pulled in via Office documents. Enforce Exploit Protection (CFG, DEP, ASLR, Arbitrary Code Guard) for iexplore.exe.
      • Email gateway: Quarantine attachments carrying embedded HTML/MHT/web-archive content (.mht, .mhtml, .hta, .url) and enable URL rewriting/detonation, as exploitation requires user interaction.
      • IAM / least privilege: Remove local admin rights from clients (LAPS) so that code execution in user context does not translate directly into full host takeover.

    3. Runbook step 3

      Detection rules

      • Sysmon EID 1 / Security EID 4688 — process ancestry: iexplore.exe spawning cmd.exe, powershell.exe, wscript.exe, cscript.exe, mshta.exe, rundll32.exe or regsvr32.exe. KQL: DeviceProcessEvents | where InitiatingProcessFileName =~ "iexplore.exe" and FileName in~ ("cmd.exe","powershell.exe","mshta.exe","rundll32.exe","regsvr32.exe","wscript.exe")
      • Sysmon EID 11 — file drops: Executables or scripts written by iexplore.exe outside the user Downloads folder, e.g. into %TEMP%, %APPDATA% or INetCache. Sigma shape: Image|endswith: '\iexplore.exe' AND TargetFilename|endswith: ['.exe','.dll','.ps1','.vbs'].
      • Sysmon EID 7 — image loads: iexplore.exe loading DLLs from user-writable paths, indicating a staged payload.
      • Proxy / Zeek logs: Sessions with user agent Trident/7.0 to first-seen domains, immediately followed by downloads typed application/octet-stream or application/x-msdownload. SPL: index=proxy user_agent="*Trident/7.0*" content_type IN ("application/octet-stream","application/x-msdownload") | stats count by src_ip, dest_host
      • Crash telemetry: Clusters of iexplore.exe crashes (Windows EID 1000 with faulting module mshtml.dll or jscript9.dll) can indicate failed exploitation attempts.

    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.

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    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    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
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    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

    Internet Explorer Remote Code Execution Vulnerability

    Source: cvelistv5, CISA KEV Catalog, NVD (NIST), CVE.org (MITRE)

    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
    MicrosoftInternet Explorer 11
    • 1.0.0 – < 10.0.17134.2087
    • 10.0.17134.2087
    microsoftinternet_explorer
    • 11
    • —

    Vendors and products

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited

    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 modifiedMicrosoft

      Show 4 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-27085
      Referenceremoved
      https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-27085
      —
      Reference typeremoved
      https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-27085 Types: Patch, Vendor Advisory
      —
      Affected versions (publisher)changed
      Internet Explorer 11
      Internet Explorer 11

      Source identifier: secure@microsoft.com

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