CVE-2021-26855 — Exchange Server
CRITICALCVE-2021-26855 is a critical-severity (CVSS 9.1) Server-Side Request Forgery (SSRF) vulnerability in microsoft exchange server. It is in CISA's Known Exploited Vulnerabilities catalog (added 2021-11-03) and has been used in ransomware campaigns — treat it as actively exploited and patch now. No vendor fix is recorded yet; mitigation options are listed below.
Microsoft Exchange Server Remote Code Execution Vulnerability
Exploitation Status
Actively exploited in the wild
- Confirmed by CISA's Known Exploited Vulnerabilities catalog on 2021-11-03 — including use in ransomware campaigns. Federal agencies were required to remediate by 2022-05-03.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA (KEV catalog and SSVC triage) for CVE-2021-26855.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-26855 by exploitation likelihood (EPSS) against impact (CVSS). In the shaded patch-first corner (EPSS 50%+, CVSS 7.0+).
Where this sits among everything scored
Of 394,803 CVEs with a current EPSS score, this one falls in the ≥ 90% band (highlighted). Counts from FIRST.org, log-scaled.
Description
Microsoft Exchange Server Remote Code Execution Vulnerability
Affected Products
exchange servermicrosoftResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
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Detection & mitigation playbook
Vendor / applianceDetect
Inventory every microsoft exchange server deployment and check each version against the affected-products list above.
Remediation status
No patch has shipped for CVE-2021-26855 yet — track the microsoft exchange server advisory for a fixed release and apply the workarounds below in the meantime.
Mitigate without a patch
Restrict outbound requests from the affected component to an allowlist of hosts, block access to link-local and internal address ranges at the network layer, and require authentication on internal services so a forged request cannot reach them unauthenticated. Alongside that, restrict the management interface to trusted networks and apply the vendor's recommended configuration mitigations.
How to detect CVE-2021-26855
A community-maintained Nuclei template exists for this CVE. You can scan for it directly:
nuclei -id cve-2021-26855 -u https://target- Template
- Microsoft Exchange Server SSRF Vulnerability
- Severity
- critical
- Impact
- Successful exploitation of this vulnerability could lead to unauthorized access to sensitive information, remote code execution, or further compromise of the affected system.
- Remediation
- Apply the appropriate security update.
Template by ProjectDiscovery nuclei-templates (madrobot), MIT licensed. View the full template. Scan only systems you are authorised to test.
Frequently Asked Questions
Is CVE-2021-26855 being exploited in your environment?
Detect and block the exploit chain at execution, on systems you cannot patch yet.