CVE-2021-26855
Microsoft Exchange Server Remote Code Execution Vulnerability
Does this matter?
Known to be exploited in the wild (CISA KEV, CISA remediation deadline 3 May 2022). Treat as an emergency change: patch or isolate now, then hunt for prior compromise.
Description
Microsoft Exchange Server Remote Code Execution Vulnerability
- CVSS 3.1
- 9.1 CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
- EPSS
- 100.00% probability · 100th percentile
- Public exploits
- 4 in Exploit-DB · first 11 March 2021
- CISA KEV
- Listed 3 November 2021 · due 3 May 2022 · used in ransomware campaigns
- Weakness
- CWE-918
- Affected
- microsoft/exchange server
- Source
- secure@microsoft.com
EPSS 100.0% since 30 October 2026; no change recorded yet. The dashed line is the ten per cent mark our analysts treat as “patch before the next change window”. Points are recorded when the score first appears and whenever it moves by a percentage point or more.
CISA notes
Apply updates per vendor instructions. Reference CISA's ED 21-02 (https://www.cisa.gov/news-events/directives/ed-21-02-mitigate-microsoft-exchange-premises-product-vulnerabilities) for further guidance and requirements. Note: The due date for addressing this vulnerability aligns with the requirements outlined in ED 21-02. https://nvd.nist.gov/vuln/detail/CVE-2021-26855
Public exploits (4)
Entries in Exploit-DB that cite this CVE. Links go to the Exploit-DB page and to the file in the public repository; nothing is hosted here. A verified tick means the Exploit-DB team confirmed the exploit works against the stated version.
| EDB-ID | Title | Type | Published | Verified |
|---|---|---|---|---|
| EDB-49895source ↗ | Microsoft Exchange 2019 - Unauthenticated Email Download (Metasploit)windows · mekhalleh | webapps | 21 May 2021 | no |
| EDB-49879source ↗ | Microsoft Exchange 2019 - Unauthenticated Email Downloadwindows · Gonzalo Villegas | webapps | 18 May 2021 | no |
| EDB-49663source ↗ | Microsoft Exchange 2019 - Server-Side Request Forgerywindows · F5 | remote | 14 March 2021 | no |
| EDB-49637source ↗ | Microsoft Exchange 2019 - Server-Side Request Forgery (Proxylogon) (PoC)windows · testanull | webapps | 11 March 2021 | no |
References
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26855Patch, Vendor Advisory
- http://packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.htmlExploit, Third Party Advisory, VDB Entry
- http://packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.htmlExploit, Third Party Advisory, VDB Entry
- http://packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.htmlExploit, Third Party Advisory, VDB Entry
- http://packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.htmlExploit, Third Party Advisory, VDB Entry
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855Patch, Vendor Advisory
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26855US Government Resource
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.