CVE-2021-26411
Microsoft Internet Explorer Memory Corruption Vulnerability
Description
CVE-2021-26411 is a high-severity use-after-free vulnerability in Microsoft Internet Explorer. Internet Explorer contains a memory corruption vulnerability that can be exploited by an attacker to execute arbitrary code on the affected system. This vulnerability is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, confirming active exploitation in the wild. Notably, this vulnerability is known to be used in ransomware campaigns. With an EPSS score of 92.47% (99.7th percentile), exploitation is near-certain for unpatched systems.
KEV Information
CVSS Score
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:LOpen in CalculatorAffected Products
| Vendor | Product | Version |
|---|---|---|
| microsoft | edge | - |
| microsoft | internet explorer | 11; 9 |
Multiple CVSS Assessments
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
References
Weakness Type
CWE-416: Use After Free
Use After Free (UAF) is a vulnerability that occurs when software continues to reference memory after it has been freed, potentially allowing an attacker to corrupt data or execute arbitrary code. In the case of Microsoft Internet Explorer, the browser engine improperly handles objects in memory during certain operations, leading to a condition where freed memory can be manipulated by an attacker to gain code execution.
Learn more: CWE-416 — Use After Free
Impact Analysis
CVE-2021-26411 carries a CVSS 3.1 score of 8.8 (HIGH), indicating a severe threat that requires prompt action. The vulnerability is exploitable over the network with low attack complexity, requires no authentication, but does require user interaction — typically convincing the victim to visit a malicious website. Confidentiality (Low): The initial exploitation provides limited data exposure, though post-exploitation activities can escalate access. Integrity (High): Successful exploitation allows the attacker to execute arbitrary code, enabling full modification of the affected system and installation of malware. Availability (Low): The vulnerability can cause browser crashes and limited service disruption. The scope is changed, meaning exploitation in Internet Explorer can affect resources beyond the browser itself, potentially compromising the underlying operating system. Ransomware risk: This vulnerability is known to be leveraged in ransomware campaigns, significantly increasing the urgency of remediation.
Exploit Maturity
CVE-2021-26411 is confirmed as actively exploited through its listing in the CISA Known Exploited Vulnerabilities catalog, with a remediation deadline of 2021-11-17. The vulnerability is known to be associated with ransomware campaigns, adding a significant financial and operational risk dimension. The EPSS score of 92.47% (99.7th percentile) indicates near-certain exploitation activity, placing it among the most actively targeted vulnerabilities in the wild. While no specific public exploit links were found in the references, the extremely high EPSS score and ransomware association confirm that reliable exploitation techniques are widely available to threat actors.
Remediation
- Apply vendor security updates immediately as required by the CISA KEV directive: Apply updates per vendor instructions. The remediation deadline was 2021-11-17.
- Update all affected Microsoft products — Internet Explorer (versions 9 and 11) and Microsoft Edge (Legacy) — to the latest security patches. Consider migrating users away from Internet Explorer entirely, as it has reached end of life.
- If immediate patching is not possible, restrict Internet Explorer usage through Group Policy or application control policies, and enforce the use of modern browsers with better security features.
- Deploy network-level protections such as web content filtering and intrusion prevention systems (IPS) to block access to known malicious domains used in exploitation campaigns.
- Monitor endpoint systems for indicators of ransomware activity, including unusual file encryption patterns, ransom notes, and suspicious process behavior. Ensure offline backups are current and tested.
Technical Details
CVE-2021-26411 is a use-after-free vulnerability (CWE-416) in the Microsoft Internet Explorer browser engine. The vulnerability is triggered when the browser improperly handles objects in memory during certain DOM manipulation operations. When a user visits a specially crafted web page, the browser frees memory that is subsequently referenced, allowing an attacker to execute arbitrary code in the context of the current user. The CVSS vector (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L) indicates network-based exploitation with low complexity and no privileges required, though user interaction is needed. The changed scope means that while the vulnerability exists in the browser, successful exploitation can affect the underlying operating system. This vulnerability was reportedly used in targeted attacks by advanced persistent threat (APT) groups before being publicly disclosed.
Frequently Asked Questions
Is CVE-2021-26411 being actively exploited?
Yes. CVE-2021-26411 is listed in the CISA Known Exploited Vulnerabilities catalog, confirming active exploitation. It is also known to be used in ransomware campaigns. The EPSS score of 92.47% (99.7th percentile) indicates near-certain exploitation.
What products are affected by CVE-2021-26411?
CVE-2021-26411 affects Microsoft Internet Explorer versions 9 and 11, as well as Microsoft Edge (Legacy). Organizations still running Internet Explorer are strongly encouraged to migrate to modern browsers.
How do I fix CVE-2021-26411?
Apply the security updates provided by Microsoft for Internet Explorer and Edge. The most effective long-term remediation is migrating away from Internet Explorer to a modern, supported browser. Implement web filtering and restrict IE usage through Group Policy in the interim.
How severe is CVE-2021-26411?
CVE-2021-26411 has a CVSS 3.1 score of 8.8 (HIGH). The vulnerability is remotely exploitable, known to be used in ransomware campaigns, and has a 92.47% EPSS score. Organizations should treat it as a critical remediation priority.
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