CVE-2025-64667: Exchange Server Auth Bypass Vulnerability

CVE-2025-64667 Overview

CVE-2025-64667 is a spoofing vulnerability in Microsoft Exchange Server caused by user interface misrepresentation of critical information [CWE-451]. An unauthenticated attacker can exploit the flaw over a network to present misleading content to Exchange users. The issue affects Microsoft Exchange Server 2016, Exchange Server 2019, and Exchange Server Subscription Edition. Microsoft published the advisory on December 9, 2025.

The vulnerability does not require authentication or user interaction, but the impact is limited to integrity. Successful exploitation can enable phishing, credential harvesting, or trust abuse scenarios by making malicious content appear legitimate within the Exchange interface.

Critical Impact
Attackers can misrepresent trusted UI elements within Exchange, enabling convincing spoofing attacks against mailbox users without authentication.

Affected Products

Microsoft Exchange Server 2016 (all cumulative updates)
Microsoft Exchange Server 2019 (all cumulative updates)
Microsoft Exchange Server Subscription Edition

Discovery Timeline

2025-12-09 - CVE-2025-64667 published to NVD
2025-12-09 - Microsoft releases security update for CVE-2025-64667
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-64667

Vulnerability Analysis

The vulnerability falls under [CWE-451], user interface misrepresentation of critical information. Exchange Server renders certain user-facing content in a way that obscures or distorts information the user relies on to make trust decisions. An attacker leverages this behavior to present crafted content that appears authentic within the Exchange UI.

The attack occurs over the network without prior authentication or user interaction. The scope is unchanged, confidentiality is not directly impacted, and availability is unaffected. Only integrity is affected because the attacker manipulates displayed information rather than the underlying data store.

The EPSS score is 0.822% with a percentile of 52.869, indicating moderate probability of exploitation activity relative to other CVEs. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog at the time of publication.

Root Cause

The root cause is improper handling and rendering of critical fields within the Exchange Server user interface. Exchange fails to normalize, sanitize, or unambiguously present specific message or interface content, allowing attacker-controlled data to masquerade as trustworthy elements.

Attack Vector

The attack vector is network-based. An unauthenticated attacker sends specially crafted content to an Exchange Server instance, and when the content is rendered to a recipient through the Exchange UI, key indicators such as sender identity, message metadata, or interface labels are misrepresented. No exploit code or public proof-of-concept is currently available. Refer to the Microsoft Security Update Guide entry for CVE-2025-64667 for vendor technical details.

Detection Methods for CVE-2025-64667

Indicators of Compromise

Inbound messages with mismatched or ambiguous sender headers, display names, or Unicode homoglyphs that render differently than the underlying address
User reports of Exchange messages or interface elements appearing to originate from internal or trusted sources but failing header validation
Anomalous SMTP traffic to Exchange servers from previously unseen external hosts targeting mailbox users

Detection Strategies

Inspect message transport logs for header inconsistencies between From, Reply-To, Return-Path, and authentication results such as SPF, DKIM, and DMARC
Correlate Exchange rendering telemetry with outbound user actions such as clicked links or credential submissions following delivery of suspicious messages
Baseline typical sender-to-recipient patterns and alert on new external senders using display names that match internal executives or systems

Monitoring Recommendations

Enable and centralize Exchange message tracking logs and IIS logs in a SIEM for retention and query
Monitor Microsoft Security Response Center advisories and apply Exchange cumulative updates promptly
Track user-reported phishing submissions and correlate with Exchange transport events to identify successful spoofing attempts

How to Mitigate CVE-2025-64667

Immediate Actions Required

Apply the Microsoft security update for CVE-2025-64667 to all affected Exchange Server 2016, 2019, and Subscription Edition instances
Verify the current cumulative update level and confirm the December 2025 security update is installed on every Exchange role server
Enforce SPF, DKIM, and DMARC on inbound mail flow and reject or quarantine messages failing authentication

Patch Information

Microsoft released a security update addressing CVE-2025-64667 on December 9, 2025. Administrators should review the Microsoft Security Update Guide entry for CVE-2025-64667 for exact build numbers and applicable cumulative updates. Exchange Server Subscription Edition receives updates through its standard servicing channel.

Workarounds

No official workaround replaces the security update; deploy the patch as the primary remediation
Strengthen mail authentication policies, external sender warning banners, and anti-phishing controls in Exchange Online Protection or the on-premises transport pipeline
Deliver user awareness training focused on identifying spoofed sender information and reporting suspicious messages

bash

# Verify Exchange Server build after patching
Get-ExchangeServer | Format-List Name, AdminDisplayVersion, Edition

# Confirm anti-spoofing transport rules and external sender tagging are enabled
Get-ExternalInOutlook
Get-TransportRule | Where-Object {$_.FromScope -eq 'NotInOrganization'}