CVE-2021-26855: Microsoft Exchange Server Remote Code Execution Vulnerability
Vulnerability summary
CVE-2021-26855 is a critical (CVSS 9.1) Server-Side Request Forgery (SSRF) vulnerability in Microsoft Exchange Server. It is known to be exploited: CISA lists it, first on 03-11-2021. It has been used in ransomware campaigns. EPSS estimates a 100.00% chance of exploitation in the next 30 days, higher than 99.9% of scored vulnerabilities. 11 public exploits are recorded.
Timeline
- 02-03-2021PublishedVulnerability first disclosed.
- 03-11-2021Added to CISA KEVMicrosoft Exchange Server Remote Code Execution Vulnerability
- 03-05-2022CISA remediation dueApply updates per vendor instructions.
- 19-08-2026Last modifiedVulnerability information updated.
Description
Microsoft Exchange Server Remote Code Execution Vulnerability
CVSS metrics
3 scores · 2 sources · 2 versions- NVDNIST9.1CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
EPSS trend
Probability
100.00%
Chance of exploitation in the next 30 days
Percentile
100%
Versus all other CVEs
30-day change
0.00 pp
From 100.00% on 20-09-2026
Known exploited
Compare KEV catalogs
CISA KEVListed- Added
- 03-11-2021
- Federal due date
- 03-05-2022
CISA required action
Apply updates per vendor instructions.
Public exploits
11 foundExploit Reference (packetstormsecurity.com)
Maturity: unknown
Exploit Reference (packetstormsecurity.com)
Maturity: unknown
Exploit Reference (packetstormsecurity.com)
Maturity: unknown
Show all 11 exploits
Exploit Reference (packetstormsecurity.com)
Maturity: unknown
Microsoft Exchange 2019 - Server-Side Request Forgery
Maturity: poc
Microsoft Exchange 2019 - Server-Side Request Forgery (Proxylogon) (PoC)
Maturity: poc
Microsoft Exchange 2019 - Unauthenticated Email Download
Maturity: poc
Microsoft Exchange 2019 - Unauthenticated Email Download (Metasploit)
Maturity: poc
Microsoft Exchange ProxyLogon Collector
auxiliary/gather/exchange_proxylogon_collector
Maturity: poc
Microsoft Exchange ProxyLogon Scanner
auxiliary/scanner/http/exchange_proxylogon
Maturity: poc
Microsoft Exchange ProxyLogon RCE
exploit/windows/http/exchange_proxylogon_rce
Maturity: weaponized
Weaknesses
CWE · root causeAffected systems
Microsoft25 products
- exchange_serverEOL2013:cumulative_update_21 | 2013:cumulative_update_22 | 2013:cumulative_update_23 | 2016:cumulative_update_10 | 2016:cumulative_update_11 | 2016:cumulative_update_12 | 2016:cumulative_update_13 | 2016:cumulative_update_14 | 2016:cumulative_update_15 | 2016:cumulative_update_16 | 2016:cumulative_update_17 | 2016:cumulative_update_18 | 2016:cumulative_update_19 | 2016:cumulative_update_8 | 2016:cumulative_update_9 | 2019 | 2019:cumulative_update_1 | 2019:cumulative_update_2 | 2019:cumulative_update_3 | 2019:cumulative_update_4 | 2019:cumulative_update_5 | 2019:cumulative_update_6 | 2019:cumulative_update_7 | 2019:cumulative_update_8
- microsoft exchange server 2013 cumulative update 21≥ 15.00.0, < 15.00.1395.012
- microsoft exchange server 2013 cumulative update 22≥ 15.00.0, < 15.00.1473.006
- microsoft exchange server 2013 cumulative update 23≥ 15.00.0, < 15.00.1497.012
- microsoft exchange server 2016 cumulative update 10≥ 15.01.0, < 15.01.1531.012
1 vendor · 25 products
References
7 links
- portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855X Refsource MISCX TransferredPatchVendor Advisory
- packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.htmlX Refsource MISCX TransferredExploitThird Party AdvisoryVDB Entry
- packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.htmlX Refsource MISCX TransferredExploitThird Party AdvisoryVDB Entry
- packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.htmlX Refsource MISCX TransferredExploitThird Party AdvisoryVDB Entry
- packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.htmlX Refsource MISCX TransferredExploitThird Party AdvisoryVDB Entry
- www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26855Government ResourceUS Government Resource