CVE-2024-21410Known exploitationNetwork attack vector

Account impersonation via unbound NTLM relay

Published Feb 13, 2024 · Updated Aug 10, 2026

Improper authentication in Microsoft Exchange Server 2016 CU23 and 2019 CU13 and CU14 allows remote attackers to impersonate NTLM users. Exchange accepts relayed NTLM authentication outside the client's originating TLS channel because Extended Protection channel binding is not enforced by default. Remote attackers must first induce an NTLM client such as Outlook to authenticate and then relay that response to act with the victim's Exchange privileges.

CVSS severity9.8
Critical
EPSS probability12.56%
Next 30 days · Sep 16, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Improper authentication in Microsoft Exchange Server 2016 CU23 and 2019 CU13 and CU14 allows remote attackers to impersonate NTLM users. Exchange accepts relayed NTLM authentication outside the client's originating TLS channel because Extended Protection channel binding is not enforced by default. Remote attackers must first induce an NTLM client such as Outlook to authenticate and then relay that response to act with the victim's Exchange privileges.

The record

CVE
CVE-2024-21410
Published
Feb 13, 2024
Updated
Aug 10, 2026
Vendor
Microsoft
Product
Microsoft Exchange Server 2019 Cumulative Update 14
Classifications
CWE-287, T1557
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 13, 2024CVE publishedPublication date reported by the CVE source.
  2. Feb 15, 2024Exploitation reportedCISA Known Exploited Vulnerabilities Catalog entry
  3. 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=15.02.0 <15.2.1544.004

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Microsoft · Microsoft Exchange Server 2019 Cumulative Update 14version=15.02.0 <15.2.1544.004
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 13version=15.02.0 <15.2.1544.004
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 23version=15.01.0 <15.01.2507.037

Published CVSS scores

9.8Microsoft CorporationCVSS: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 IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationReported
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-287Improper Authentication
T1557Adversary-in-the-Middle

Public exploit references

No public exploit references are available in this record.

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

Reported exploitation

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo