Threat Intelligence / CVE / CVE-2021-26855

CVE-2021-26855 — Microsoft Exchange Server

CISA KEVRansomware

CVSS v3.1 9.1 (CRITICAL) · EPSS 93.9% (higher than 99.8% of all scored CVEs) · Priority 10/10 · Published 2021-03-03

As of 2025-12-18, CVE-2021-26855 is a CRITICAL-severity vulnerability in Microsoft Exchange Server, CVSS v3.1 9.1, EPSS 93.9% (99.8th percentile). It is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), with a US federal remediation deadline of 2022-05-03, and CISA links it to known ransomware campaigns. Threadlinqs Intelligence links 8 tracked threat campaigns to CVE-2021-26855, most recently “Recorded Future H1 2026 Report: Actively Exploited CVEs Up 34%, Ransomware Adopts BYOVD and Post-Quantum Crypto”.

Last updated: 2025-12-18

What is CVE-2021-26855?

Microsoft Exchange Server Remote Code Execution Vulnerability

The record classifies CVE-2021-26855 under weakness class CWE-918. Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on confidentiality, integrity. 24 affected-product entries are recorded, across 1 vendor, listed below. The identifier was first published 2019 days ago.

Severity and exploitation probability

CVSS v3.1 base score: 9.1 — CRITICAL
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
EPSS (FIRST): 93.9% probability of exploitation in the next 30 days, higher than 99.8% of all scored CVEs
CISA KEV: Listed since 2021-11-03, federal remediation deadline 2022-05-03 — used in known ransomware campaigns
Threadlinqs priority: 10/10 — CISA lists it as used in ransomware, which Threadlinqs scores at the maximum
Published: 2021-03-03, last modified 2025-12-18

Is CVE-2021-26855 being exploited?

CISA added CVE-2021-26855 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 2022-05-03 under BOD 22-01. CISA flags the vulnerability as one used in known ransomware campaigns. It currently carries a trending score of 40 in the Threadlinqs vulnerability feed.

Affected products and versions

Microsoft: Exchange Server 2013, Exchange Server 2016, Exchange Server 2019

Showing 3 of 24 recorded product entries.

How to fix CVE-2021-26855

The record marks a vendor fix as available for CVE-2021-26855. Patch reference: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855. Vendor advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855. Because CVE-2021-26855 is KEV-listed, US federal civilian agencies were required to apply the vendor fix, or stop using the product, by 2022-05-03. 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-2021-26855

8 tracked threats in the Threadlinqs corpus reference CVE-2021-26855, either in the campaign’s CVE list or as an indicator on the campaign record.

Recorded Future H1 2026 Report: Actively Exploited CVEs Up 34%, Ransomware Adopts BYOVD and Post-Quantum Crypto — HIGH · 2026-09-03
StrikeShark Campaign: SharkLoader Dropper Targets Governments and Software Developers via N-Day Exploits and Trojanized Installers to Deploy Cobalt Strike — HIGH · 2026-07-11
SharkLoader Malware Campaign Uses Fake Cisco AnyConnect and Google Update Installers to Deploy Cobalt Strike — HIGH · 2026-07-03
SharkLoader Malware Deploys Cobalt Strike Beacon via DLL Side-Loading in StrikeShark Campaign — HIGH · 2026-06-27
AhnLab ASEC April 2026 APT Group Trend Report: State-Sponsored Espionage Campaigns (CVE-2026-32202, CVE-2025-20333/20362, CVE-2021-26855) — HIGH · 2026-06-10
SHADOW-EARTH-053 — China-Aligned Cyberespionage Campaign Exploiting Microsoft Exchange (ProxyLogon CVE-2021-26855/26857/26858/27065) and IIS to Deploy GODZILLA Web Shells and ShadowPad — HIGH · 2026-05-11
APT41/Silver Dragon Expanding Enterprise Attack Surface — Google Drive C2, AppDomain Hijacking, Cloud/Supply Chain Targeting — HIGH · 2026-03-27
UAT-8099 (China) BadIIS Malware Campaign Targeting IIS Servers for SEO Fraud — HIGH · 2026-02-02

Sources

Seeded from nvd and not yet processed by the Threadlinqs enrichment pipeline, so blank CVSS, EPSS or KEV fields above mean NOT MEASURED rather than measured-absent.

Full detection coverage & IOCs for threats exploiting CVE-2021-26855 are available via the Threadlinqs MCP server (Purple tier). View plans →

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