CVE-2021-26855Known exploitationPublic exploitNetwork attack vector

Authentication bypass via attacker-controlled backend routing

Published Mar 2, 2021 · Updated Aug 19, 2026

SSRF in Microsoft Exchange Server allows remote attackers to bypass authentication by routing crafted HTTPS requests through the server. The frontend static-resource handler derives its backend destination directly from attacker-controlled routing cookies, then authenticates the relayed request with the Exchange machine account. Internet-facing on-premises servers are reachable without credentials; code execution requires chaining this flaw with a separate post-authentication vulnerability.

CVSS severity9.8
Critical
EPSS probability100.00%
Next 30 days · Sep 16, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

SSRF in Microsoft Exchange Server allows remote attackers to bypass authentication by routing crafted HTTPS requests through the server. The frontend static-resource handler derives its backend destination directly from attacker-controlled routing cookies, then authenticates the relayed request with the Exchange machine account. Internet-facing on-premises servers are reachable without credentials; code execution requires chaining this flaw with a separate post-authentication vulnerability.

The record

CVE
CVE-2021-26855
Published
Mar 2, 2021
Updated
Aug 19, 2026
Vendor
Microsoft
Product
Microsoft Exchange Server 2013 Cumulative Update 22
Classifications
CWE-918, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 2, 2021Exploitation reportedHAFNIUM exploitation of on-premises Exchange
  2. Mar 2, 2021CVE publishedPublication date reported by the CVE source.
  3. Nov 3, 2021Exploitation reportedCISA Known Exploited Vulnerabilities listing
  4. Aug 19, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=15.00.0 <15.00.1473.006

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Microsoft · Microsoft Exchange Server 2013 Cumulative Update 22version=15.00.0 <15.00.1473.006
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 10version=15.01.0 <15.01.1531.012
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 4version=15.02.0 <15.02.0659.012
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 18version=15.01.0 <15.01.2106.013
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 16version=15.01.0 <15.01.1979.008
Microsoft · Microsoft Exchange Server 2013 Cumulative Update 23version=15.00.0 <15.00.1497.012
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 3version=15.02.0 <15.02.0464.015
Microsoft · Microsoft Exchange Server 2013 Cumulative Update 21version=15.00.0 <15.00.1395.012
Microsoft · Microsoft Exchange Server 2019version=15.02.0 <15.02.0221.018
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 14version=15.01.0 <15.01.1847.012
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 8version=15.01.0 <15.01.1415.010
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 19version=15.01.0 <15.01.2176.009
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 15version=15.01.0 <15.01.1913.012
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 11version=15.01.0 <15.01.1591.018
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 13version=15.01.0 <15.01.1779.008
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 6version=15.02.0 <15.02.0529.013
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 5version=15.02.0 <15.02.0595.008
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 1version=15.02.0 <15.01.1979.006
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 17version=15.01.0 <15.01.2044.013
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 2version=15.02.0 <15.02.0397.011
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 8version=15.02.0 <15.02.0792.010
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 7version=15.02.0 <15.02.0721.013
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 12version=15.01.0 <15.01.1713.010
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 9version=15.01.0 <15.01.1466.016

Published CVSS scores

9.8NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsShadowserver observations through Sep 16, 2026. Gaps indicate unavailable data.
Sep 10, 2026Sep 16, 2026
View daily counts
Reporting dayUnique IPs
Sep 10, 202617
Sep 11, 202613
Sep 12, 202613
Sep 13, 202613
Sep 14, 20262
Sep 15, 202615
Sep 16, 202612
Latest reporting daySep 16, 2026
Latest daily unique IPs12
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationReported
Public exploitPublished

Weakness, pattern, technique

CWE-918Server-Side Request Forgery (SSRF)
T1190Exploit Public-Facing Application

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Reported exploitation

Technologies

Your stack

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Your stack

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