CVE-2023-38181Network attack vector

NTLM credential relay via unsafe GsmWriter deserialization

Published Aug 8, 2023 · Updated Aug 10, 2026

Deserialization in GsmWriter in Microsoft Exchange Server 2016 and 2019 allows remote authenticated users to relay Net-NTLMv2 credentials. The GsmWriter class deserializes user-supplied data without adequate validation, allowing attacker-controlled objects to trigger outbound NTLM authentication. A valid Exchange account is required; exploitation can interrupt the service or authenticate to another NTLM-accepting service as the victim.

CVSS severity8.8
High
EPSS probability10.80%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Deserialization in GsmWriter in Microsoft Exchange Server 2016 and 2019 allows remote authenticated users to relay Net-NTLMv2 credentials. The GsmWriter class deserializes user-supplied data without adequate validation, allowing attacker-controlled objects to trigger outbound NTLM authentication. A valid Exchange account is required; exploitation can interrupt the service or authenticate to another NTLM-accepting service as the victim.

The record

CVE
CVE-2023-38181
Published
Aug 8, 2023
Updated
Aug 10, 2026
Vendor
Microsoft
Product
Microsoft Exchange Server 2019 Cumulative Update 12
Classifications
CWE-502, T1557
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Aug 8, 2023CVE 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

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Is a vulnerable build present?

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

  1. Affected versionversion=15.02.0 <15.02.1118.037

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Microsoft · Microsoft Exchange Server 2019 Cumulative Update 12version=15.02.0 <15.02.1118.037
Microsoft · Microsoft Exchange Server 2019 Cumulative Update 13version=15.02.0 <15.02.1258.025
Microsoft · Microsoft Exchange Server 2016 Cumulative Update 23version=15.01.0 <15.01.2507.032

Published CVSS scores

8.8Microsoft CorporationCVSS:3.1/AV:N/AC:L/PR:L/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
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-502Deserialization of Untrusted Data
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.

Technologies

Your stack

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Your stack

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