# CVE-2021-26411 — Microsoft Internet Explorer 11 **CISA KEV** · **Ransomware** > As of 2026-11-07, CVE-2021-26411 is a HIGH-severity vulnerability in Microsoft Internet Explorer 11, CVSS v3.1 8.8, EPSS 81.6% (99.6th percentile). It is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), with a US federal remediation deadline of 2021-11-17, and CISA links it to known ransomware campaigns. Threadlinqs Intelligence links 1 tracked threat campaign to CVE-2021-26411, most recently “Exploit kits remain active in April 2021: RIG, Spelevo, PurpleFox, Underminer, Bottle, Magnitude targeting Internet Explorer (CVE-2020-0674, CVE-2021-26411, CVE-2018-8174, CVE-2018-15982, CVE-2020-1380)”. **Last updated:** 2026-10-01 ## What is CVE-2021-26411? Internet Explorer Memory Corruption Vulnerability The record classifies CVE-2021-26411 under weakness class [CWE-416](https://intel.threadlinqs.com/cwe/CWE-416). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs a user to take an action first, and has high impact on integrity. 3 affected-product entries are recorded, across 1 vendor, listed below. The identifier was first published 2068 days ago. ## Severity and exploitation probability - **CVSS v3.1 base score:** 8.8 — HIGH (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L/E:P/RL:O/RC:C`) - **EPSS (FIRST):** 81.6% probability of exploitation in the next 30 days, higher than 99.6% of all scored CVEs - **CISA KEV:** Listed since 2021-11-03, federal remediation deadline 2021-11-17 — used in known ransomware campaigns - **Threadlinqs priority:** 10/10 — CISA lists it as used in ransomware, which Threadlinqs scores at the maximum - **Published:** 2021-03-11, last modified 2026-10-01 ## Is CVE-2021-26411 being exploited? CISA added CVE-2021-26411 to the Known Exploited Vulnerabilities catalog on 2021-11-03, which means the agency holds evidence of exploitation in the wild; US federal civilian agencies had to remediate it by 2021-11-17 under BOD 22-01. CISA flags the vulnerability as one used in known ransomware campaigns. Weaponised exploit code for CVE-2021-26411 is publicly available. 2 public proof-of-concept repositories are tracked for this identifier. It currently carries a trending score of 48 in the Threadlinqs vulnerability feed. - [CrackerCat/CVE-2021-26411](https://github.com/CrackerCat/CVE-2021-26411) (github) - [trickest/cve](https://github.com/trickest/cve/blob/main/2021/CVE-2021-26411.md) (trickest) ## Affected products and versions - [Microsoft](https://intel.threadlinqs.com/vendors/microsoft): Internet Explorer 11, Internet Explorer 9, Edge (EdgeHTML-based) ## How to fix CVE-2021-26411 The record marks a vendor fix as available for CVE-2021-26411. Patch reference: [https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26411](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26411). Because CVE-2021-26411 is KEV-listed, US federal civilian agencies were required to apply the vendor fix, or stop using the product, by 2021-11-17. Apply the vendor fix referenced above to every affected product listed in this record, then confirm the running version against the vendor advisory. ## Threat activity tracking CVE-2021-26411 1 tracked threat in the Threadlinqs corpus references CVE-2021-26411, either in the campaign’s CVE list or as an indicator on the campaign record. - [Exploit kits remain active in April 2021: RIG, Spelevo, PurpleFox, Underminer, Bottle, Magnitude targeting Internet Explorer (CVE-2020-0674, CVE-2021-26411, CVE-2018-8174, CVE-2018-15982, CVE-2020-1380)](https://intel.threadlinqs.com/threat/TL-2026-5496) — HIGH · 2026-11-07 ## Sources Enriched from CVE.org, NVD, FIRST EPSS, CISA KEV, public proof-of-concept repositories. Last verified by Threadlinqs on 2026-11-07. This product uses the NVD API but is not endorsed or certified by the NVD. **Vendor advisory and patch** - [msrc.microsoft.com — vendor advisory](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26411) Canonical: https://intel.threadlinqs.com/cve/CVE-2021-26411 Every threat comes with one complete detection in Splunk SPL, Microsoft KQL and Sigma, free for everyone; a free account adds daily TL Creds to unlock the rest. Sign up free (Blue): https://threadlinqs.com/?modal=signup Indicator values unlock with TL Creds (https://threadlinqs.com/tl-creds); MCP access for AI agents needs Purple: https://intel.threadlinqs.com/mcp