Internet Explorer Memory Corruption Vulnerability (CVE-2021-26411)
Published Last modified In CISA KEV since Sources: SingCERT, CVE.org
What is affected, what to do
Internet Explorer Memory Corruption Vulnerability
The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. The available sources name no fixed version. Check the vendor guidance in the references below. Immediate actions, mitigation and detection rules are in the runbook below.
Conflicting version data — version display is hidden until the data is resolved.
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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: SingCERT, CVE.org
81.7 %
EPSS score
High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
Source: FIRST EPSS, percentile 99.64
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- 248.43 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- 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.
Read in glossary- 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.
Read in glossary- 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“.
Read in glossary- 81.7 %
- High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
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- 99.64
- 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- 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.
Read in glossary- CISA assessment (SSVC)
- Exploitation: active · not automatable · Technical impact: partial
- Source: CISA Vulnrichment, as of 12 August 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-416
Response and mitigation
Why act now?
Prioritisation rationale
The flaw is network-reachable without authentication and needs only user interaction, such as visiting a crafted page or opening a document. A scope change gives code execution in the user's context. It has been used in drive-by campaigns and targeted spear-phishing, CISA flags known use in ransomware campaigns, and exploitation likelihood is very high. Although Internet Explorer is retired, the MSHTML engine is still reachable through IE mode, Office and legacy systems. Organisations running legacy clients, OT engineering stations or unpatched Server 2008 hosts should treat this as critical; fully updated estates with IE disabled can treat it as high, with the focus on producing compliance evidence.
Runbook step 1
Immediate response (0-24 h)
- Deploy the cumulative Windows/Internet Explorer security update to all clients and servers. Target versions per the advisory are at least 10.0.17134.2087 for IE 11 and EdgeHTML-based Edge, and at least 6.0.6003.21070 for IE 9 on Windows Server 2008. Look up the KB number per OS build in the Microsoft Security Update Guide.
- Run an inventory sweep via EDR, Intune or SCCM: query the file version of
%SystemRoot%\System32\mshtml.dllandiexplore.exe, and flag anything below target as non-compliant. Pay special attention to legacy hosts such as Server 2008, Windows 10 1803, embedded/kiosk devices and OT engineering workstations. - Where patching must wait, disable Internet Explorer via GPO ("Disable Internet Explorer 11 as a standalone browser") and redirect users to Chromium-based Edge. Restrict Edge IE mode to explicitly listed internal sites using the Enterprise Mode Site List.
- Cut direct internet access for unpatched systems and force them through a filtering proxy. Admin workstations and jump hosts come first.
- If you suspect successful exploitation, isolate the endpoint and preserve memory and the EDR timeline. Then reset the logged-on user's credentials, including session tokens and stored browser credentials.
Runbook step 2
Mitigation layers
- Network/proxy: Restrict outbound HTTP(S) carrying a
Trident/7.0orMSIEuser agent to an allowlist of internal destinations, and block it everywhere else. - Endpoint hardening: Enable Microsoft Defender ASR rules in block mode, notably "Block all Office applications from creating child processes" and "Block Office applications from injecting code into other processes". MSHTML is also reachable through Office documents and embedded web content.
- Configuration: Use GPO to disable Active Scripting and ActiveX in the Internet zone (IE security zones, zone 3). Enable Enhanced Protected Mode and 64-bit EPM processes to contain the blast radius of an exploit.
- IAM/least privilege: Run end users without local admin rights. Technically prevent privileged accounts (domain admins, tier 0) from browsing, for example by blocking those accounts at the proxy or enforcing a PAW model.
- Mail/web gateway: Quarantine HTML and MHT attachments, as well as Office documents with external OLE or MSHTML references. Enable vendor IPS signatures for CVE-2021-26411 (Suricata/Snort, Palo Alto, Fortinet) and set them to drop.
- Network/proxy: Restrict outbound HTTP(S) carrying a
Runbook step 3
Detection rules
- Sysmon EID 1 / EDR process tree:
iexplore.exeor an Office process spawningcmd.exe,powershell.exe,rundll32.exe,regsvr32.exe,mshta.exeorwscript.exe. Sigma shape:ParentImage|endswith: '\iexplore.exe' AND Image|endswith: ['\powershell.exe','\cmd.exe','\rundll32.exe']. - KQL (Defender for Endpoint):
DeviceProcessEvents | where InitiatingProcessFileName =~ "iexplore.exe" and FileName in~ ("powershell.exe","cmd.exe","rundll32.exe","regsvr32.exe") - Windows Application log EID 1000/1001:
iexplore.execrashes with faulting modulemshtml.dllorjscript9.dll. Clusters of crashes after visits to external URLs suggest failed exploit attempts. - Sysmon EID 11 / EID 3:
iexplore.exewriting.dll,.exeor.datfiles into%TEMP%or%APPDATA%, followed immediately by outbound connections to previously unseen domains. - Proxy/Zeek HTTP logs:
Trident/7.0requests to newly registered or low-reputation domains, especially from hosts where IE is officially retired. Any internet-bound IE traffic from those hosts is an anomaly worth triaging.
- Sysmon EID 1 / EDR process tree:
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- 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 Memory Corruption Vulnerability
Source: CSA/SingCERT, cvelistv5, NVD (NIST), CISA KEV Catalog, CVE.org (MITRE)
Affected products
The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.
| Affected versions | Fixed in | ||
|---|---|---|---|
| Microsoft | Internet Explorer 11 |
|
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| Microsoft | Internet Explorer 9 |
|
|
| Microsoft | Microsoft Edge (EdgeHTML-based) |
|
|
| microsoft | edge |
|
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| microsoft | internet_explorer |
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|
Vendors and products
Weakness classes (CWE)
CWE-416Use After Free
Variant
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
Entry at MITRE CWE-416
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.
| CISA KEV Catalog | CISA Known Exploited Vulnerability (opens in a new tab)cisa.gov | actively exploited |
References & sources
- portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2021-26411x_refsource_MISC, x_transferred
- cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26411government-resource
- Microsoft Security Update GuideInternet Explorer Memory Corruption VulnerabilityVendor advisory, Patch
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).
Record modifiedMicrosoft
Show 3 original changes
Individual changes of this event as published by NVD Type Action Before After Reference added —https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26411Reference removed https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26411—Reference type removed https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26411 Types: Patch, Vendor Advisory—Source identifier: secure@microsoft.com
Record modifiedCISA ADP
CISA assessment (SSVC): exploitation: active, automatable: no
Show 5 original changes
Individual changes of this event as published by NVD Type Action Before After Reference added —https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26411Reference removed https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26411—Reference type removed https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26411 Types: US Government Resource—SSVC assessment removed {"id":"CVE-2021-26411","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2021-03-05T00:00:00+00:00"}—SSVC assessment added —{"id":"CVE-2021-26411","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-08-12T03:55:30.330990Z"}Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0
Record modifiedCVE Program
Show 3 original changes
Individual changes of this event as published by NVD Type Action Before After Reference added —https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26411Reference removed https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26411—Reference type removed https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26411 Types: Patch, Vendor Advisory—Source identifier: af854a3a-2127-422b-91ae-364da2661108
Record modifiedMicrosoft
Show 4 original changes
Individual changes of this event as published by NVD Type Action Before After Reference added —https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26411Reference removed https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26411—Reference type removed https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26411 Types: Patch, Vendor Advisory—Affected versions (publisher) changed Internet Explorer 9, Internet Explorer 11, Microsoft Edge (EdgeHTML-based)Internet Explorer 11, Internet Explorer 9, Microsoft Edge (EdgeHTML-based)Source identifier: secure@microsoft.com
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