CVE-2021-34473Known exploitationPublic exploitNetwork attack vector

Pre-authentication backend access via path confusion

Published Jul 14, 2021 · Updated Aug 10, 2026

SSRF in Microsoft Exchange Server 2013 CU23, 2016 CU19 through CU20, and 2019 CU8 through CU9 allows remote attackers to access backend URLs. EwsAutodiscoverProxyRequestHandler accepts a crafted Email query value, and RemoveExplicitLogonFromUrlAbsoluteUri removes attacker-chosen text with Substring during URL normalization. No credentials or user interaction are required; code execution requires chaining the resulting machine-account backend access with the other ProxyShell flaws.

CVSS severity9.8
Critical
EPSS probability100.00%
Next 30 days · Nov 1, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

SSRF in Microsoft Exchange Server 2013 CU23, 2016 CU19 through CU20, and 2019 CU8 through CU9 allows remote attackers to access backend URLs. EwsAutodiscoverProxyRequestHandler accepts a crafted Email query value, and RemoveExplicitLogonFromUrlAbsoluteUri removes attacker-chosen text with Substring during URL normalization. No credentials or user interaction are required; code execution requires chaining the resulting machine-account backend access with the other ProxyShell flaws.

The record

CVE
CVE-2021-34473
Published
Jul 14, 2021
Updated
Aug 10, 2026
Vendor
Microsoft Corporation
Product
Microsoft Exchange Server 2013 Cumulative Update 23
Classifications
CWE-918, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 14, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 10, 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 <15.00.1497.015

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Microsoft Corporation · Microsoft Exchange Server 2013 Cumulative Update 23version=15.00.0 <15.00.1497.015
Microsoft Corporation · Microsoft Exchange Server 2016 Cumulative Update 19version=15.01.0 <15.01.2176.012
Microsoft Corporation · Microsoft Exchange Server 2019 Cumulative Update 8version=15.02.0 <15.02.0792.013
Microsoft Corporation · Microsoft Exchange Server 2019 Cumulative Update 9version=15.02.0 <15.02.0858.010
Microsoft Corporation · Microsoft Exchange Server 2016 Cumulative Update 20version=15.01.0 <15.01.2242.008

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.

Activity data is delayed. The latest report is Sep 28, 2026.

Daily unique IPsShadowserver observations through Sep 28, 2026. Gaps indicate unavailable data.
Sep 22, 2026Sep 28, 2026
View daily counts
Reporting dayUnique IPs
Sep 22, 20265
Sep 23, 20262
Sep 24, 20264
Sep 25, 20264
Sep 26, 20264
Sep 27, 20264
Sep 28, 2026Unavailable
Latest reporting daySep 28, 2026
Latest daily unique IPsUnavailable
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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