CVE-2021-26855
ProxyLogon
Microsoft Exchange Server Remote Code Execution Vulnerability
- CVSS
- 9.1Criticalscored by the publisher (CNA)
- Exploited in the wild
- Yes, in CISA KEV since 3 Nov 2021CISA deadline for US federal agencies: 3 May 2022known use in ransomware campaignspublic exploit code
- EPSS
- > 99.9%chance of exploitation in the next 30 daystop 0.1% of scored CVEsFIRST score of 11 Nov 2026
- Fix
- Patch availablesee the advisories in the references
- Published
- 3 Mar 2021analysed here 12 Nov 2026
Do first. Apply updates per vendor instructions. (CISA's required action for this KEV entry)
Summary
Triage
The summary, impact and action text on this page are written by our AI analysis pipeline, or taken from the NVD description when a CVE was imported from NVD. They are not the vendor's wording: check them against the sources. A longer AI executive summary and technical analysis are generated for signed-in accounts on the Max plan.
Affected products
| Product | Affected versions | Vendor says fixed in |
|---|---|---|
| Microsoft Microsoft Exchange Server 2013 Cumulative Update 21 | >= 15.00.0 and < 15.00.1395.012Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2013 Cumulative Update 22 | >= 15.00.0 and < 15.00.1473.006Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2013 Cumulative Update 23 | >= 15.00.0 and < 15.00.1497.012Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 10 | >= 15.01.0 and < 15.01.1531.012Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 11 | >= 15.01.0 and < 15.01.1591.018Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 12 | >= 15.01.0 and < 15.01.1713.010Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 13 | >= 15.01.0 and < 15.01.1779.008Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 14 | >= 15.01.0 and < 15.01.1847.012Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 15 | >= 15.01.0 and < 15.01.1913.012Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 16 | >= 15.01.0 and < 15.01.1979.008Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 17 | >= 15.01.0 and < 15.01.2044.013Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 18 | >= 15.01.0 and < 15.01.2106.013Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 19 | >= 15.01.0 and < 15.01.2176.009Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 8 | >= 15.01.0 and < 15.01.1415.010Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 9 | >= 15.01.0 and < 15.01.1466.016Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2019 | >= 15.02.0 and < 15.02.0221.018Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 1 | >= 15.02.0 and < 15.01.1979.006Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 2 | >= 15.02.0 and < 15.02.0397.011Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 3 | >= 15.02.0 and < 15.02.0464.015Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 4 | >= 15.02.0 and < 15.02.0659.012Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 5 | >= 15.02.0 and < 15.02.0595.008Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 6 | >= 15.02.0 and < 15.02.0529.013Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 7 | >= 15.02.0 and < 15.02.0721.013Vendor says fixed in: not stated | |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 8 | >= 15.02.0 and < 15.02.0792.010Vendor says fixed in: not stated |
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.
Products named in our analysis
- microsoft:exchange_server
Fix and mitigation
WAF and other mitigations
Can a WAF block it? Partly. SSRF makes the server call a URL the attacker picks. A WAF can block well-known targets such as cloud metadata addresses, but not every internal host.
Also consider: egress filter · IMDSv2
Based on the weakness type only; no CVE-specific evidence yet. We check Cloudflare, AWS, Google Cloud Armor and Azure; for Akamai, Imperva, Fastly, F5, ask your vendor.
Remediation steps (AI-written)
Step-by-step remediation is generated for accounts on the Max plan, for CVEs that match a technology in their stack. Add this technology to your stack to see the fix checklist.
Exploitation evidence
- CISA KEV catalog
- Listed since 3 Nov 2021. Known use in ransomware campaigns.
- CISA due date
- 3 May 2022, the remediation deadline for US federal agencies
- CISA required action
- Apply updates per vendor instructions.
- Public exploit code
- Yes: 4 on Exploit-DB · 3 Metasploit modules · 53 GitHub proofs of concept · 1 Nuclei template
- CISA SSVC
- exploitation active, automatable yes, technical impact totalAssessed by CISA 1 Mar 2021 (SSVC 2.0.3). Exploitation is none, poc (public proof of concept) or active.
Exploit-DB
- Microsoft Exchange 2019 - Unauthenticated Email Download (Metasploit)webapps, 21 May 2021
- Microsoft Exchange 2019 - Unauthenticated Email Downloadwebapps, 18 May 2021
- Microsoft Exchange 2019 - Server-Side Request Forgeryremote, 14 Mar 2021
- Microsoft Exchange 2019 - Server-Side Request Forgery (Proxylogon) (PoC)webapps, 11 Mar 2021
Metasploit
- exploit/windows/http/exchange_proxylogon_rceexploit, rank excellent
- auxiliary/gather/exchange_proxylogon_collectorauxiliary, rank normal
- auxiliary/scanner/http/exchange_proxylogonauxiliary, rank normal
Nuclei
- Microsoft Exchange Server SSRF Vulnerabilitydetection template, critical
GitHub proofs of concept
- Flangvik/SharpProxyLogon247 stars
- hosch3n/ProxyVulns176 stars
- dwisiswant0/proxylogscan166 stars
- p0wershe11/ProxyLogon123 stars
and 49 more repositories
Listed so you can judge urgency, not as a recommendation to run anything. GitHub “proofs of concept” are unvetted: some are fake or carry malware. Exploit-DB marks the entries its staff verified.
Timeline
- 1 Mar 2021CISA SSVC assessment: exploitation active
- 3 Mar 2021First public exploit or proof of concept2 days later
- 3 Mar 2021Publishedsame day later
- 3 Nov 2021Added to CISA KEV (exploited in the wild)245 days (about 8 months) later
- 3 May 2022CISA remediation due date (US federal agencies)181 days (about 6 months) later
- 19 Aug 2026CVE record last updated1,570 days (about 4 years) later
- 12 Nov 2026Analysed by vulneraibility.com85 days (about 3 months) later
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
| Metric | Value | What it means |
|---|---|---|
| Attack vector | NetworkReachable over the internet or any routable network. | Reachable over the internet or any routable network. |
| Attack complexity | LowRepeatable attack, no special conditions. | Repeatable attack, no special conditions. |
| Privileges required | NoneThe attacker needs no prior access. | The attacker needs no prior access. |
| User interaction | NoneNo victim action needed. | No victim action needed. |
| Scope | UnchangedImpact stays within the vulnerable component. | Impact stays within the vulnerable component. |
| Confidentiality | HighTotal data disclosure possible. | Total data disclosure possible. |
| Integrity | HighComplete modification possible. | Complete modification possible. |
| Availability | NoneThe service stays up. | The service stays up. |
Common questions
Is CVE-2021-26855 being exploited?
Yes. CVE-2021-26855 appears in CISA's Known Exploited Vulnerabilities catalog, which means exploitation in the wild has been confirmed and reported. Treat it as active rather than theoretical, and patch it ahead of higher-scoring vulnerabilities that carry no exploitation evidence.
How severe is CVE-2021-26855?
CVE-2021-26855 is rated Critical with a CVSS score of 9.1. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2021-26855?
Yes. A fix has been recorded for CVE-2021-26855. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2021-26855 affect?
CVE-2021-26855 affects microsoft:exchange_server. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
References
7 references in the CVE record, grouped by the tags the record gives them.
Patches (1)
- msrc.microsoft.com /update-guide/vulnerability/CVE-2021-26855
Other links (6)
- portal.msrc.microsoft.com /en-US/security-guidance/advisory/CVE-2021-26855
- packetstormsecurity.com /files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.html
- packetstormsecurity.com /files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.html
- packetstormsecurity.com /files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.html
- packetstormsecurity.com /files/162736/Microsoft-Exchange-ProxyLogon-Collector.html
- cisa.gov /known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26855
Where the data on this page comes from
- CVE List (CVE.org): description, affected versions, references; CVE Terms of Use
- NVD: CVSS and CPE data
- CISA KEV catalog: exploitation, date added, due date
- CISA Vulnrichment (ADP): SSVC decision points
- FIRST EPSS: exploit probability and percentile
- Exploit-DB: published exploits
- Metasploit Framework: exploit and scanner modules
- Nuclei templates: detection templates
- PoC-in-GitHub: GitHub repositories that claim a proof of concept
Summary, impact and triage tags: written by our AI analysis pipeline from the sources above. Check them against the sources.
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