CVE-2023-21745 Overview
CVE-2023-21745 is a spoofing vulnerability affecting Microsoft Exchange Server. The flaw allows an authenticated attacker on an adjacent network to spoof identities and compromise the confidentiality, integrity, and availability of the mail infrastructure. Microsoft addressed the issue in the January 2023 Patch Tuesday release. The vulnerability is associated with [CWE-502] Deserialization of Untrusted Data, indicating the underlying weakness involves unsafe object handling during message or request processing.
Critical Impact
An authenticated adjacent attacker can spoof Exchange Server components, potentially gaining high-impact access to mailbox data and altering server behavior across affected on-premises Exchange deployments.
Affected Products
- Microsoft Exchange Server 2016 Cumulative Update 23
- Microsoft Exchange Server 2019 Cumulative Update 11
- Microsoft Exchange Server 2019 Cumulative Update 12
Discovery Timeline
- 2023-01-10 - CVE-2023-21745 published to the National Vulnerability Database
- 2023-01-10 - Microsoft published advisory and security update for CVE-2023-21745
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2023-21745
Vulnerability Analysis
CVE-2023-21745 is a spoofing vulnerability in Microsoft Exchange Server. Microsoft classifies the issue under [CWE-502] Deserialization of Untrusted Data, which suggests the server processes attacker-controlled serialized objects without sufficient validation. An adjacent-network attacker with valid credentials can craft requests that cause Exchange to act on spoofed identities or trusted contexts.
The attack does not require user interaction. The attacker must already hold low-privilege credentials and be able to reach the Exchange server from the same logical network segment. Successful exploitation impacts confidentiality, integrity, and availability of the mail environment.
Root Cause
The root cause is improper handling of serialized data within an Exchange Server component. When the server deserializes objects without validating their origin or contents, an attacker can submit crafted payloads that the server treats as legitimate. This permits identity spoofing and unauthorized actions in the context of higher-privileged Exchange processes.
Attack Vector
Exploitation requires network adjacency, such as access to the same internal subnet as the Exchange server. The attacker authenticates with low privileges, then issues a crafted request to a vulnerable Exchange endpoint. Because attack complexity is low and no user interaction is required, internal attackers and adversaries who have already gained a foothold on the corporate network can reliably trigger the flaw.
No public proof-of-concept code is currently available for CVE-2023-21745. Refer to the Microsoft CVE-2023-21745 Advisory for vendor technical detail.
Detection Methods for CVE-2023-21745
Indicators of Compromise
- Unexpected authenticated requests to Exchange endpoints originating from internal hosts that do not normally interact with Exchange services.
- Anomalous serialized payloads or oversized requests in Internet Information Services (IIS) logs on Exchange front-end and back-end roles.
- Process activity from w3wp.exe spawning unusual child processes such as cmd.exe or powershell.exe on Exchange servers.
- Authentication events involving low-privileged accounts performing Exchange administrative or mailbox-impersonation actions.
Detection Strategies
- Inventory Exchange Server build numbers and flag any host still running CU23 (2016) or CU11/CU12 (2019) without the January 2023 security update applied.
- Correlate IIS request logs with Windows authentication events to identify spoofed identity patterns and lateral access attempts from adjacent hosts.
- Monitor .NET deserialization exceptions and unhandled errors in Exchange application logs, which can indicate exploitation attempts.
Monitoring Recommendations
- Forward Exchange IIS, application, and Windows Security event logs to a centralized analytics platform for retention and correlation.
- Baseline normal client and admin traffic to Exchange servers, then alert on deviations from expected source subnets and user agents.
- Enable PowerShell script block logging on Exchange servers to capture post-exploitation tooling.
How to Mitigate CVE-2023-21745
Immediate Actions Required
- Apply the January 2023 Microsoft security update for Exchange Server 2016 CU23 and Exchange Server 2019 CU11 and CU12 without delay.
- Restrict network access to Exchange management endpoints, allowing only known administrative subnets and jump hosts.
- Audit Exchange administrative accounts and remove unused or stale credentials that could enable adjacent-network exploitation.
- Review recent authentication logs for low-privilege accounts accessing Exchange in unexpected ways.
Patch Information
Microsoft released a fix for CVE-2023-21745 as part of the January 2023 security updates. Administrators should obtain the appropriate Security Update from the Microsoft CVE-2023-21745 Advisory and apply it to all affected on-premises Exchange Server roles. After installation, run the Setup.exe /PrepareSchema and Exchange health checks as required by Microsoft guidance to ensure the update fully applies.
Workarounds
- No official workaround replaces patching. Where immediate patching is not possible, segment Exchange servers from general user subnets to limit adjacent-network exposure.
- Enforce multi-factor authentication on all accounts with mailbox or Exchange administrative rights to reduce the value of stolen low-privilege credentials.
- Disable legacy authentication protocols on Exchange where business requirements allow.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
