Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:NCVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.
Summary
CVE-2021-26855 is a critical-severity SSRF (CWE-918) vulnerability in Microsoft Exchange Server. Its CVSS base score is 9.1 (Critical).
Operationally, exploitation aligns with the MITRE ATT&CK technique LSASS Memory (T1003.001); ranked in the top 0.0% of CVEs by exploit likelihood; CISA has added it to the Known Exploited Vulnerabilities catalog; a public proof-of-concept is referenced.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
CVE-2021-26855 is a remote code execution vulnerability in Microsoft Exchange Server that stems from server-side request forgery (CWE-918). It carries a CVSS 3.1 score of 9.1 reflecting network attack vector, low complexity, no required privileges or user interaction, and high impact on confidentiality and integrity.
An unauthenticated attacker with network access can exploit the flaw to perform SSRF attacks that lead to arbitrary file writes, unauthenticated email downloads, and remote code execution on affected servers, as demonstrated by multiple public proof-of-concept implementations.
The Microsoft Security Response Center advisory linked in the references provides official guidance on patches and mitigations, while additional technical details and exploit artifacts appear in the listed PacketStorm reports.
The associated EPSS score remains elevated at a current value of 0.9431 with a recorded peak of 0.9754.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2021-13639
Vulnerability Data
Microsoft Exchange Server Remote Code Execution Vulnerability
- CWE(s)
- KEV Date Added
- 03 November 2021
Related Threats
Threat-Actor AttributionAI
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
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Likely Mitigating Controls AI
Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.
Penetration testing attempts server-side requests to internal resources, identifying SSRF weaknesses for remediation.
Outbound connections to external resources can be monitored and limited at the boundary, reducing SSRF impact.
Validates server-side URLs and resource references to block SSRF attempts.
Detects server-side request forgery through monitoring of unexpected outbound connections.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Runtime monitoring of web applications and services can detect anomalous outbound requests indicative of SSRF.
Vulnerability identification processes can discover and record SSRF flaws in web applications.
Network segmentation and egress controls can limit the damage from successful SSRF requests.
Secure development practices directly include input validation and destination allow-listing that prevent SSRF.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.