Threat Intelligence / CVE / CVE-2021-26857

CVE-2021-26857 — Microsoft Exchange Server

CISA KEVRansomware

CVSS v3.1 7.8 (HIGH) · EPSS 44.7% (higher than 97.5% of all scored CVEs) · Priority 10/10 · Published 2021-03-03

As of 2025-12-18, CVE-2021-26857 is a HIGH-severity vulnerability in Microsoft Exchange Server, CVSS v3.1 7.8, EPSS 44.7% (97.5th 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 2 tracked threat campaigns to CVE-2021-26857, most recently “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”.

Last updated: 2025-12-18

What is CVE-2021-26857?

Microsoft Exchange Server Remote Code Execution Vulnerability

The record classifies CVE-2021-26857 under weakness class CWE-502. Its CVSS v3 base vector states that the flaw requires local access to the host, needs no prior authentication, needs a user to take an action first, and has high impact on confidentiality, integrity, availability. 25 affected-product entries are recorded, across 1 vendor, listed below. The identifier was first published 2020 days ago.

Severity and exploitation probability

CVSS v3.1 base score: 7.8 — HIGH
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
EPSS (FIRST): 44.7% probability of exploitation in the next 30 days, higher than 97.5% 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-2021-26857 being exploited?

CISA added CVE-2021-26857 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 39 in the Threadlinqs vulnerability feed.

Affected products and versions

Microsoft: Exchange Server 2010, Exchange Server 2013, Exchange Server 2016, Exchange Server 2019

Showing 4 of 25 recorded product entries.

How to fix CVE-2021-26857

The record marks a vendor fix as available for CVE-2021-26857. Patch reference: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26857. Vendor advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26857. Because CVE-2021-26857 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-2021-26857

2 tracked threats in the Threadlinqs corpus reference CVE-2021-26857, either in the campaign’s CVE list or as an indicator on the campaign record.

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
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-2021-26857 are available via the Threadlinqs MCP server (Purple tier). View plans →

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