CVE-2023-21745: Exchange Server Spoofing Vulnerability

CVE-2023-21745 Overview

CVE-2023-21745 is a spoofing vulnerability affecting Microsoft Exchange Server. This vulnerability allows an attacker with low privileges on an adjacent network to perform spoofing attacks that could lead to unauthorized access and data manipulation. The vulnerability is associated with insecure deserialization (CWE-502), which can allow attackers to manipulate serialized data to achieve unintended behavior within the Exchange Server environment.

Critical Impact

An attacker exploiting this vulnerability from an adjacent network could achieve high impact to confidentiality, integrity, and availability of the affected Exchange Server, potentially compromising email communications and sensitive organizational data.

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 NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-21745

Vulnerability Analysis

This spoofing vulnerability in Microsoft Exchange Server stems from improper handling of deserialization operations. The vulnerability requires the attacker to be positioned on an adjacent network, meaning they must have some level of network proximity to the target Exchange Server rather than being able to exploit it remotely over the internet.

The attack requires low privileges to execute, making it accessible to authenticated users with minimal access rights. Once exploited, the vulnerability can result in complete compromise of the data confidentiality, integrity, and availability on the affected system. The insecure deserialization weakness (CWE-502) indicates that the Exchange Server improperly processes serialized data, potentially allowing attackers to inject malicious objects that execute arbitrary operations when deserialized.

Root Cause

The root cause of this vulnerability is tied to insecure deserialization practices within Microsoft Exchange Server. When the server processes serialized data without proper validation, attackers can craft malicious serialized objects that, when processed by the server, result in spoofing attacks or unauthorized actions. This class of vulnerability is particularly dangerous in email infrastructure as it can enable attackers to impersonate legitimate users or services.

Attack Vector

The attack vector is classified as Adjacent Network, meaning the attacker must have network-level access to the same network segment as the vulnerable Exchange Server. This could include scenarios such as:

• An attacker who has compromised another system on the same corporate network
• An insider threat with physical or VPN access to the organization's internal network
• An attacker who has gained access through a wireless network connected to the same infrastructure

The attacker requires low-level privileges, such as standard user authentication, to exploit this vulnerability. No user interaction is required for successful exploitation, making this vulnerability particularly concerning in environments where network segmentation is not strictly enforced.

The vulnerability mechanism involves crafting malicious requests that exploit the deserialization flaw. Technical details regarding specific exploitation methods are available in the Microsoft Security Advisory.

Detection Methods for CVE-2023-21745

Indicators of Compromise

• Unusual authentication patterns or spoofed identity attempts in Exchange Server logs
• Unexpected serialized object processing or deserialization errors in application logs
• Anomalous network traffic from adjacent network segments targeting Exchange Server services
• Unauthorized configuration changes or data access patterns within the Exchange environment

Detection Strategies

• Monitor Exchange Server event logs for suspicious deserialization-related errors or exceptions
• Implement network segmentation monitoring to detect lateral movement from adjacent networks
• Deploy behavioral analytics to identify abnormal user authentication patterns
• Configure SentinelOne Singularity XDR to detect exploitation attempts targeting Exchange Server vulnerabilities

Monitoring Recommendations

• Enable detailed logging on Microsoft Exchange Server for authentication and data access events
• Implement network traffic analysis between network segments to identify unauthorized communication patterns
• Configure alerts for failed authentication attempts followed by successful access from adjacent network sources
• Regularly audit Exchange Server configurations for unauthorized modifications

How to Mitigate CVE-2023-21745

Immediate Actions Required

• Apply Microsoft security updates immediately for all affected Exchange Server versions
• Implement strict network segmentation to limit adjacent network access to Exchange servers
• Review and restrict user privileges to ensure principle of least privilege
• Monitor for exploitation attempts using endpoint detection and response solutions

Patch Information

Microsoft has released security updates to address this vulnerability. Organizations should apply the appropriate cumulative updates for their Exchange Server versions:

• Exchange Server 2016: Apply security updates released after Cumulative Update 23
• Exchange Server 2019: Apply security updates released after Cumulative Update 11 and Cumulative Update 12

For detailed patch information and download links, refer to the Microsoft Security Response Center advisory.

Workarounds

• Implement strict network segmentation to isolate Exchange servers from potentially compromised adjacent network segments
• Enable enhanced monitoring and logging for all Exchange Server activities
• Restrict authentication mechanisms and implement multi-factor authentication where possible
• Consider implementing application-layer firewalls to filter potentially malicious serialized data

# Network segmentation verification example
# Review Windows Firewall rules for Exchange Server
Get-NetFirewallRule | Where-Object {$_.Enabled -eq 'True'} | Select-Object DisplayName, Direction, Action

# Check Exchange Server cumulative update version
Get-ExchangeServer | Format-List Name,Edition,AdminDisplayVersion