CVE-2021-26855

CRITICALNVD 9.19.8▲Trending — 4 sources updated this weekExploited in the wild

9.1

Microsoft Exchange Server Remote Code Execution Vulnerability

CVSS v3: 9.1
EG Score: 9.8(medium)
EPSS: 100.0%
KEV: ⚠ Exploited

Published

March 3, 2021

Last Modified

December 18, 2025

Advisory Details (5)

Auto-updated Jun 2, 2026

Patch available. Sources: microsoft.

microsoft Patch Available

Security Update Guide - Microsoft Security Response Center

Security Update Guide - Microsoft Security Response Center. Patch available via Microsoft Security Update

https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855

generic

Packet Storm

http://packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.html

generic

Packet Storm

http://packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.html

generic

Packet Storm

http://packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.html

generic

Packet Storm

http://packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.html

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

CWE-918Server-Side Request Forgery (SSRF)

All Vendor Advisories

(1)

Every vendor that published an advisory referencing this CVE — pulled from our cve_vendor_advisories aggregation. Click any row for the vendor's original advisory page.

Microsoft MSRCCVE-2021-268552021-03-02
Microsoft Exchange Server Remote Code Execution Vulnerability

Data Freshness Timeline

(refreshed 20× in last 7d / 20× in last 30d)

Every time one of our enrichment pipelines (NVD, MITRE cvelistV5, EPSS, CISA KEV, GHSA, OSV, vendor advisories) ran against this CVE. Most recent first.

2026-06-04 04:43 UTCEG score recompute
2026-06-04 04:43 UTCVendor advisory
2026-06-04 04:43 UTCGHSA enrichment
2026-06-04 00:54 UTCEG score recompute
2026-06-04 00:54 UTCVendor advisory
2026-06-04 00:54 UTCGHSA enrichment
2026-06-03 21:05 UTCEG score recompute
2026-06-03 21:05 UTCVendor advisory
2026-06-03 21:05 UTCGHSA enrichment
2026-06-03 19:09 UTCkev_newly_listed
2026-06-03 17:16 UTCEG score recompute
2026-06-03 17:16 UTCVendor advisory
2026-06-03 17:16 UTCGHSA enrichment
2026-06-03 13:27 UTCEG score recompute
2026-06-03 13:27 UTCVendor advisory
2026-06-03 13:27 UTCGHSA enrichment
2026-06-03 09:38 UTCEG score recompute
2026-06-03 09:38 UTCVendor advisory
2026-06-03 09:38 UTCGHSA enrichment
2026-06-03 05:49 UTCEG score recompute

Publicly available exploits

(10 references)

Working exploit code is in the public domain (6 GitHub PoCs) (4 Exploit-DB entries). Defenders should treat patch urgency accordingly — public PoCs typically lead to mass-exploitation within 24-72 hours.

Exploit-DBEDB-49895
First seen May 21, 2021
Microsoft Exchange 2019 - Unauthenticated Email Download (Metasploit)
Open source ↗
Exploit-DBEDB-49879
First seen May 18, 2021
Microsoft Exchange 2019 - Unauthenticated Email Download
Open source ↗
GitHub PoChosch3n/ProxyVulns
First seen Apr 14, 2021
[ProxyLogon] CVE-2021-26855 & CVE-2021-27065 Fixed RawIdentity Bug Exploit. [ProxyOracle] CVE-2021-31195 & CVE-2021-31196 Exploit Chains. [ProxyShell] CVE-2021-34473 & CVE-2021-34523 & CVE-2021-31207 Exploit Chains.
Open source ↗
GitHub PoCFlangvik/SharpProxyLogon
First seen Mar 29, 2021
C# POC for CVE-2021-26855 aka ProxyLogon, supports the classically semi-interactive web shell as well as shellcode injection
Open source ↗
GitHub PoCpraetorian-inc/proxylogon-exploit
First seen Mar 24, 2021
Proof-of-concept exploit for CVE-2021-26855 and CVE-2021-27065. Unauthenticated RCE in Exchange.
Open source ↗
GitHub PoCp0wershe11/ProxyLogon
First seen Mar 17, 2021
ProxyLogon(CVE-2021-26855+CVE-2021-27065) Exchange Server RCE(SSRF->GetWebShell)
Open source ↗
Exploit-DBEDB-49663
First seen Mar 14, 2021
Microsoft Exchange 2019 - Server-Side Request Forgery
Open source ↗
Exploit-DBEDB-49637
First seen Mar 11, 2021
Microsoft Exchange 2019 - Server-Side Request Forgery (Proxylogon) (PoC)
Open source ↗
GitHub PoChackerschoice/CVE-2021-26855
First seen Mar 11, 2021
PoC of proxylogon chain SSRF(CVE-2021-26855) to write file by testanull, censored by github
Open source ↗
GitHub PoCalt3kx/CVE-2021-26855_PoC
First seen Mar 10, 2021
Open source ↗

Frequently asked(6)

What is CVE-2021-26855?

CVE-2021-26855 is a critical vulnerability published on March 3, 2021. Microsoft Exchange Server Remote Code Execution Vulnerability

When was CVE-2021-26855 disclosed?

CVE-2021-26855 was first published in the National Vulnerability Database on March 3, 2021, with the most recent update on December 18, 2025. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.

Is CVE-2021-26855 actively exploited?

Yes. CISA added CVE-2021-26855 to the Known Exploited Vulnerabilities catalog on November 3, 2021, affecting Microsoft Exchange Server. KEV listing indicates confirmed exploitation in the wild; this CVE warrants immediate patching attention.

What is the CVSS score of CVE-2021-26855?

CVE-2021-26855 has a CVSS v3 base score of 9.1 (NVD). EchelonGraph synthesises NVD + CISA KEV + FIRST EPSS + GHSA into a combined EG score of 9.8.

Which products are affected by CVE-2021-26855?

CVE-2021-26855 affects Microsoft Exchange Server. The full affected-products list, including version ranges and fixed versions, is shown in the Affected Packages section of this page.

How do I remediate CVE-2021-26855?

Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-26855, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

See which npm, PyPI, Go, and Maven packages are affected by CVE-2021-26855

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Is Your Infrastructure Affected by CVE-2021-26855?

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CVE-2021-26855

Published

Last Modified

Advisory Details (5)

Security Update Guide - Microsoft Security Response Center

Packet Storm

Packet Storm

Packet Storm

Packet Storm

Weakness Classification(1)

All Vendor Advisories

Data Freshness Timeline

Publicly available exploits

Frequently asked(6)

Dependency Blast Radius

Is Your Infrastructure Affected by CVE-2021-26855?

Same vendor

Same CWE

CVE-2021-26855

📅 Published

🔄 Last Modified

📋 Advisory Details (5)

Security Update Guide - Microsoft Security Response Center

Packet Storm

Packet Storm

Packet Storm

Packet Storm

Weakness Classification(1)

All Vendor Advisories

Data Freshness Timeline

Publicly available exploits

❓ Frequently asked(6)

Dependency Blast Radius

Is Your Infrastructure Affected by CVE-2021-26855?

🔗 Related CVEs(same vendor + same CWE)

Same vendor

Same CWE

Published

Last Modified

Advisory Details (5)

Frequently asked(6)

Related CVEs(same vendor + same CWE)