How severe is CVE-2021-26855?

CVE-2021-26855 has a CVSS score of 9.1/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-26855?

Affected package information for CVE-2021-26855 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2021-26855?

No fixed version has been published for CVE-2021-26855 yet. Monitor the advisory for updates, apply mitigations recommended by the vendor, and use O3 Security to track when a fix becomes available in your stack.

Is CVE-2021-26855 actively exploited in the wild?

Yes. There are 90 known exploit references for CVE-2021-26855, including 4 documented proof-of-concept exploits on Exploit-DB and 53 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-26855?

CVE-2021-26855 has an EPSS (Exploit Prediction Scoring System) score of 94.3%, placing it in the 100th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. A score above 50% indicates significantly elevated exploitation risk — immediate patching is strongly recommended.

Is CVE-2021-26855 in the CISA Known Exploited Vulnerabilities catalog?

Yes. CVE-2021-26855 (Microsoft Exchange Server Remote Code Execution Vulnerability) was added to CISA's Known Exploited Vulnerabilities (KEV) catalog on 2021-11-03. CISA required all US federal civilian agencies to remediate this vulnerability by 2022-05-03. The required action was: Apply updates per vendor instructions. This CVE has been used in known ransomware campaigns. Being listed in the KEV catalog is the strongest possible confirmation of active exploitation — treat this as a critical priority.

CISA KEV·Added 2021-11-03 — agencies required to remediate by 2022-05-03 · Ransomware

CVE-2021-26855

Q: Can O3 Security detect CVE-2021-26855 in my codebase?

Yes. O3 Security's Impact-Aware SCA scanner identifies CVE-2021-26855 across all affected ecosystems (multiple ecosystems), correlates it against your dependency graph, and uses function-level reachability to confirm whether the vulnerable code path is actually reachable in your application — eliminating false positives. You can scan your stack for CVE-2021-26855 directly from the O3 platform.

CRITICAL

Microsoft Exchange Server Remote Code Execution Vulnerability

Published

Mar 3, 2021

Updated

Dec 18, 2025

Affected

0 pkgs

Patched

None yet

Exploits

90 known

EPSS Exploitation Probability

via FIRST.org ↗

94.3%probability of exploitation in next 30 days

Very High Risk100th percentile-0.02%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Description

Microsoft Exchange Server Remote Code Execution Vulnerability

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

EDB-49879webappswindows

Microsoft Exchange 2019 - Unauthenticated Email Download

by Gonzalo Villegas · May 18, 2021

Exploit-DB Source

EDB-49663remotewindows

Microsoft Exchange 2019 - Server-Side Request Forgery

by F5 · Mar 14, 2021

Exploit-DB Source

EDB-49895webappswindows

Microsoft Exchange 2019 - Unauthenticated Email Download (Metasploit)

by mekhalleh · May 21, 2021

Exploit-DB Source

EDB-49637webappswindows

Microsoft Exchange 2019 - Server-Side Request Forgery (Proxylogon) (PoC)

by testanull · Mar 11, 2021

Exploit-DB Source

Frequently Asked Questions

Microsoft Exchange Server Remote Code Execution Vulnerability

O3 Security · Impact-Aware SCA

Is CVE-2021-26855 in your stack?

O3 detects CVE-2021-26855 across dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my stack How O3 SCA works

CVSS Score

9.1/10

CVSS v3 base score

Vector breakdown

AV:Attack Vector

Network

AC:Attack Complexity

Low

PR:Privileges Required

None

UI:User Interaction