CVE-2026-55005: Microsoft Exchange Server Buffer Overflow

CVE-2026-55005 Overview

CVE-2026-55005 is a heap-based buffer overflow vulnerability in Microsoft Exchange Server. An authenticated attacker can trigger memory corruption over the network to execute arbitrary code in the context of the Exchange process. The flaw is classified under [CWE-122] Heap-based Buffer Overflow. Microsoft published guidance through its Security Update Guide, and the vulnerability carries a CVSS 3.1 base score of 8.8 with attack vector AV:N and low privileges required.

Critical Impact
Successful exploitation grants remote code execution on Exchange Server, exposing mailboxes, credentials, and downstream Active Directory infrastructure to compromise.

Affected Products

Microsoft Exchange Server (see the Microsoft Security Update Guide for specific build ranges)

Discovery Timeline

2026-07-14 - CVE-2026-55005 published to the National Vulnerability Database
2026-07-15 - Last updated in the NVD database

Technical Details for CVE-2026-55005

Vulnerability Analysis

The vulnerability resides in Microsoft Exchange Server code paths that allocate and populate buffers on the heap when processing attacker-influenced input. Insufficient bounds validation permits a write beyond the allocated buffer, corrupting adjacent heap metadata or object pointers. An authenticated attacker who can reach the Exchange service over the network can shape the heap layout, then trigger the overflow to redirect execution flow. Because Exchange runs with high privileges and integrates tightly with Active Directory, code execution on the server offers a pivot into mailbox data, service accounts, and directory objects. The current EPSS probability is 0.66%, indicating limited near-term exploitation likelihood, but the impact profile keeps it a priority patch target.

Root Cause

The root cause is a missing or incorrect length check preceding a memory copy into a heap-allocated buffer [CWE-122]. When the source data exceeds the destination size, the copy operation overwrites adjacent heap structures. Microsoft has not published low-level technical details, and no proof-of-concept code is publicly available at time of writing.

Attack Vector

Exploitation requires network access to an Exchange endpoint and valid credentials (PR:L). The attacker sends a crafted request containing input that reaches the vulnerable parsing or serialization path. No user interaction is required (UI:N). Because privileges required are low, any authenticated mailbox user or compromised service account represents a realistic threat model. See the Microsoft Security Update Guide for vendor-provided details.

No verified public exploit code is available. Refer to the Microsoft advisory
for technical guidance and affected build ranges.

Detection Methods for CVE-2026-55005

Indicators of Compromise

Unexpected crashes, faulting modules, or Watson dumps generated by w3wp.exe or Exchange worker processes
Child processes such as cmd.exe, powershell.exe, or rundll32.exe spawned by Exchange service accounts
Outbound network connections from Exchange servers to previously unseen external hosts following authenticated requests

Detection Strategies

Hunt for anomalous authenticated requests to Exchange endpoints from accounts that do not typically use those interfaces
Correlate Exchange application crash events with subsequent process creation and lateral movement activity
Review authentication logs for credential reuse patterns that could seed the low-privilege access required for exploitation

Monitoring Recommendations

Forward Exchange, IIS, and Windows Security event logs to a centralized analytics platform for retention and correlation
Alert on process lineage anomalies where Exchange service accounts launch scripting or LOLBin executables
Monitor for new scheduled tasks, services, or web shell files under Exchange virtual directories

How to Mitigate CVE-2026-55005

Immediate Actions Required

Apply the Microsoft security update referenced in the Security Update Guide to all affected Exchange Server builds
Restrict Exchange administrative interfaces to trusted networks and enforce multi-factor authentication on all mailbox accounts
Audit Exchange service accounts and rotate credentials for any account that could authenticate to a vulnerable server

Patch Information

Microsoft published the fix through its monthly security update channel. Administrators should install the update on all Exchange Server roles, verify the build number post-installation, and confirm the Health Checker script reports the server as patched. Refer to the Microsoft advisory for KB numbers and cumulative update prerequisites.

Workarounds

No official workaround replaces the security update; reduce exposure by limiting network reachability to Exchange services
Enforce conditional access and IP allow-lists on Outlook Web Access and Exchange Web Services until patching completes
Disable legacy authentication protocols that provide additional attack surface for authenticated exploitation

bash

# Verify installed Exchange build after patching
Get-ExchangeServer | Select-Object Name, AdminDisplayVersion

# Run the Exchange Health Checker to confirm patch status
.\HealthChecker.ps1 -Server <ExchangeServerName>