CISA KEV Critical
SSRF exploit reference
CVE-2021-26855 — Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability
100
Urgent
Who exploits it and how urgent it is
A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.
CISA KEV · Nov 3, 2021 R used by ransomware (CISA) ☠ APT41☠ Magic Hound☠ Threat Group-3390☠ ToddyCat Detection templates (Nuclei) · 1Metasploit modules · 3Exploit-DB · 4Proofs of concept on GitHub · 51
How the score was calculated
- Confirmed exploitation (CISA KEV)+35
- Used in ransomware campaigns (CISA)+10
- Exploited by 4 threat actor(s) (MITRE ATT&CK)+10
- Public exploit/PoC (nuclei, metasploit, exploitdb, github)+20
- EPSS 100% (30-day exploitation probability)+15
- Technical severity (critical)+10
- Automatable exploitation (SSVC)+3
The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).
Triage
Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.
Exploitation
Confirmed
CISA KEV, listed Nov 3, 2021
EPSS FIRST
100%
ahead of 100% of scored CVEs · Oct 15, 2026
Affects
Microsoft
Exchange Server and 24 more
CVSS base
9.1
Critical · 4 source(s)
Prioritization reference: 9.8 provisional
CISA SSVC assessment CISA
Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.
Exploitation
Active
none · proof-of-concept · active
Automatable
Yes
can an attacker script the kill-chain steps
Technical impact
Total
partial · total control of the vulnerable component
Remediation
Fixes and mitigations documented by the sources — linked to the original advisory.
- Documented patch — msrc.microsoft.com (NIST National Vulnerability Database)
- Documented patch — portal.msrc.microsoft.com (NIST National Vulnerability Database)
CISA required action CISA
Apply updates per vendor instructions.
US federal deadline: May 3, 2022 — passed 1627 day(s) ago
First 24 hours
Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.
- Identify exposed assets running the affected vendor/product/version combinations (Microsoft Exchange Server, Microsoft Microsoft Exchange Server 2013 Cumulative Update 21, Microsoft Microsoft Exchange Server 2013 Cumulative Update 22…).
- Treat as emergency remediation: the CVE is in CISA KEV (confirmed exploitation).
- The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
- Apply the fixed version or the patch documented under Remediation.
- EPSS ahead of 100% of scored CVEs: prioritize it in the patch queue.
- Search available logs for exploit probes matching the vulnerability class (ssrf), errors and authentication anomalies.
Affected scope
Vendor, product and versions as the sources word them.
| Vendor | Product | Versions | Status | Source |
|---|---|---|---|---|
| Microsoft | Exchange Server |
version 2013-cumulative_update_22version 2013-cumulative_update_21version 2019-cumulative_update_8version 2019-cumulative_update_7version 2019-cumulative_update_6version 2019-cumulative_update_5version 2019-cumulative_update_4version 2019-cumulative_update_3 and 16 moreversion 2019-cumulative_update_2version 2019-cumulative_update_1version 2019version 2016-cumulative_update_9version 2016-cumulative_update_8version 2016-cumulative_update_19version 2016-cumulative_update_18version 2016-cumulative_update_17version 2016-cumulative_update_16version 2016-cumulative_update_15version 2016-cumulative_update_14version 2016-cumulative_update_13version 2016-cumulative_update_12version 2016-cumulative_update_11version 2016-cumulative_update_10version 2013-cumulative_update_23 |
Vulnerable | NIST National Vulnerability Database |
| Microsoft | Microsoft Exchange Server 2013 Cumulative Update 21 | ≥ 15.00.0 and < 15.00.1395.012 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2013 Cumulative Update 22 | ≥ 15.00.0 and < 15.00.1473.006 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2013 Cumulative Update 23 | ≥ 15.00.0 and < 15.00.1497.012 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 10 | ≥ 15.01.0 and < 15.01.1531.012 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 11 | ≥ 15.01.0 and < 15.01.1591.018 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 12 | ≥ 15.01.0 and < 15.01.1713.010 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 13 | ≥ 15.01.0 and < 15.01.1779.008 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 14 | ≥ 15.01.0 and < 15.01.1847.012 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 15 | ≥ 15.01.0 and < 15.01.1913.012 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 16 | ≥ 15.01.0 and < 15.01.1979.008 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 17 | ≥ 15.01.0 and < 15.01.2044.013 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 18 | ≥ 15.01.0 and < 15.01.2106.013 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 19 | ≥ 15.01.0 and < 15.01.2176.009 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 8 | ≥ 15.01.0 and < 15.01.1415.010 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 9 | ≥ 15.01.0 and < 15.01.1466.016 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2019 | ≥ 15.02.0 and < 15.02.0221.018 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 1 | ≥ 15.02.0 and < 15.01.1979.006 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 2 | ≥ 15.02.0 and < 15.02.0397.011 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 3 | ≥ 15.02.0 and < 15.02.0464.015 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 4 | ≥ 15.02.0 and < 15.02.0659.012 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 5 | ≥ 15.02.0 and < 15.02.0595.008 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 6 | ≥ 15.02.0 and < 15.02.0529.013 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 7 | ≥ 15.02.0 and < 15.02.0721.013 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 8 | ≥ 15.02.0 and < 15.02.0792.010 | Vulnerable | CVE Program (cve.org) e CISA ADP |
Attack characteristics
The CVSS vector, decoded. It describes the attack, not your exposure to it.
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None
Every base score collected
Sources score independently and may disagree; each row says who scored it and under which version.
| Score | Version | Severity | Expl. | Impact | Scored by |
|---|---|---|---|---|---|
| 9.8 | CVSS 3.1 | Critical | 3.9 | 5.9 | nvd@nist.gov via NIST National Vulnerability Database |
| 9.8 | CVSS 3.1 | Critical | — | — | GitHub, Inc. via GitHub Security Advisories |
| 9.1 | CVSS 3.1 | Critical | 3.9 | 5.2 | secure@microsoft.com via NIST National Vulnerability Database |
| 9.1 | CVSS 3.1 | Critical | — | — | microsoft via CVE Program (cve.org) e CISA ADP |
| 7.5 | CVSS 2.0 | High | 10.0 | 6.4 | nvd@nist.gov via NIST National Vulnerability Database |
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N (score shown at the top: secure@microsoft.com)
Scoring entities (NIST, CNA, ADP) score independently; each score is shown with its origin. For prioritization we use 9.8 (NVD (NIST)) as a conservative provisional reference — not a definitive official score.
Weakness MITRE
Weakness class according to MITRE’s CWE catalog.
CISA KEV details CISA
CISA lists a CVE once it has evidence of exploitation against real targets.
Listed
Nov 3, 2021
Federal deadline
May 3, 2022
passed 1627 day(s) ago
Ransomware use
Known
CISA records ransomware use only for campaigns it has confirmed, so “Unknown” means unrecorded, not ruled out. The deadline binds US federal agencies; for everyone else it is a reference date.
Public exploits, PoCs and detection templates
Public code that demonstrates or tests the flaw raises the odds of mass exploitation — and Nuclei templates help find vulnerable assets in your environment.
Detection templates (Nuclei)
Metasploit modules
- Microsoft Exchange ProxyLogon Collector normal Mar 2, 2021
- Microsoft Exchange ProxyLogon Scanner normal Mar 2, 2021
- Microsoft Exchange ProxyLogon RCE excellent Mar 2, 2021
Exploit-DB
- Microsoft Exchange 2019 - Server-Side Request Forgery (Proxylogon) (PoC) Mar 11, 2021
- Microsoft Exchange 2019 - Server-Side Request Forgery Mar 14, 2021
- Microsoft Exchange 2019 - Unauthenticated Email Download May 18, 2021
- Microsoft Exchange 2019 - Unauthenticated Email Download (Metasploit) May 21, 2021
Proofs of concept on GitHub — 51 repositories; showing the 10 most starred
- C# POC for CVE-2021-26855 aka ProxyLogon, supports the classically semi-interactive web shell as well as shellcode injection ★ 247 Mar 29, 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. ★ 176 Apr 14, 2021
- A fast tool to mass scan for a vulnerability on Microsoft Exchange Server that allows an attacker bypassing the authentication and impersonating as the admin (CVE-2021-26855). ★ 166 Mar 8, 2021
- ProxyLogon(CVE-2021-26855+CVE-2021-27065) Exchange Server RCE(SSRF->GetWebShell) ★ 124 Mar 17, 2021
- Detect webshells dropped on Microsoft Exchange servers exploited through "proxylogon" group of vulnerabilites (CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, CVE-2021-27065) ★ 98 Mar 5, 2021
- h4x0r-dz/CVE-2021-26855 ★ 98 Mar 9, 2021
- PoC of proxylogon chain SSRF(CVE-2021-26855) to write file by testanull, censored by github ★ 59 Mar 11, 2021
- alt3kx/CVE-2021-26855_PoC ★ 53 Mar 10, 2021
- Proof-of-concept exploit for CVE-2021-26855 and CVE-2021-27065. Unauthenticated RCE in Exchange. ★ 52 Mar 24, 2021
- Microsoft Exchange Server SSRF漏洞(CVE-2021-26855) ★ 36 Mar 6, 2021
Warning: public PoCs are sometimes disguised malware. Use them only for authorized testing in an isolated environment. This portal does not host exploit code — it only points to the sources.
Threat actors exploiting this CVE MITRE ATT&CK
According to MITRE ATT&CK.
APT41 G0096Magic Hound G0059Threat Group-3390 G0027ToddyCat G1022EPSS over time FIRST
Oct 13, 2026 → Oct 15, 2026 · 3 reading(s) · max 100.00%
Detection
Read off the CVSS vector and the weakness class. Starting points, not tested rules.
- Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
- Monitor outbound requests from the server to internal IPs and cloud metadata endpoints.
- Hunt retroactively in logs from before Nov 3, 2021 (KEV listing): exploitation may have started earlier.
Timeline
What happened to this CVE, newest first.
- 2026
- Fix documented
- Affected products added · microsoft/exchange server
- Source status changed · Published → Analyzed
- Record published
- Affected products added · microsoft/microsoft exchange server 2013 cumulative update 21, microsoft/microsoft exchange server 2013 cumulative updat
- CVSS changed · 9.1
- Source status changed · → Published
- SSVC assessment updated
- Added to CISA KEV (detected)
- Record created in this database
- 2021
- Added to CISA KEV catalog
- CVE published
Source activity
Repeated readings are counted rather than listed.
- 3× References added · Oct 13, 2026 – Oct 13, 2026
- 1× Official description updated · Oct 13, 2026 – Oct 13, 2026
References
9 on the record
- msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26855
- portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855
- www.cisa.gov/known-exploited-vulnerabilities-catalog
- github.com/advisories/GHSA-6784-2mh5-cq56
- packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.html
- packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.html
- packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.html
- nvd.nist.gov/vuln/detail/CVE-2021-26855
- packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.h
Elsewhere on this site
Related CVEs (same products)
- High CVE-2026-96940 Weak authorization in Microsoft Exchange Server allows an authenticated attacker to elevate... 13d
- Critical CVE-2026-69641 Microsoft Exchange Server Elevation of Privilege Vulnerability Sep 8, 2026
- Medium CVE-2026-69382 Microsoft Exchange Server Information Disclosure Vulnerability Sep 8, 2026
- High CVE-2026-69380 Microsoft Exchange Server Elevation of Privilege Vulnerability Sep 8, 2026
- Medium CVE-2026-69375 Microsoft Exchange Server Tampering Vulnerability Sep 8, 2026
- High CVE-2026-69378 Microsoft Exchange Server Denial of Service Vulnerability Sep 8, 2026
Not in any source we poll
Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.
- No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
- No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
- No exploit packet captures, log samples, or incident case IDs supplied.
Answered from this record
Text assembled only from the fields above — no additional claims.
What should defenders know first?
CVE-2021-26855 is "Microsoft Exchange Server Remote Code Execution Vulnerability", rated Critical. It affects Exchange Server (Microsoft); versions are in the scope table. It is in CISA KEV: there is evidence of real-world exploitation. CVSS 9.1 describes technical severity. EPSS 100.00% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.
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