CVE-2026-55007: Microsoft Exchange Server RCE Vulnerability

CVE-2026-55007 Overview

CVE-2026-55007 is a double free vulnerability [CWE-415] in Microsoft Exchange Server. An unauthorized attacker can trigger the flaw over a network to execute arbitrary code on the target server. The issue stems from freeing the same memory region twice, which corrupts heap metadata and enables control of program execution flow.

Microsoft published details in the Microsoft Security Update Guide for CVE-2026-55007. Exchange Server operates as a high-value target because it holds mail data, service accounts, and often bridges internal and external network boundaries.

Critical Impact
Unauthenticated remote code execution against internet-facing Exchange Server infrastructure with full confidentiality, integrity, and availability impact.

Affected Products

Microsoft Exchange Server (specific build ranges listed in the Microsoft Security Update Guide)
On-premises Exchange deployments exposing HTTP/SMTP services to the network
Hybrid Exchange environments bridging on-premises and Microsoft 365

Discovery Timeline

2026-09-08 - CVE-2026-55007 published to the National Vulnerability Database
2026-09-09 - Last updated in NVD database
2026-09-10 - EPSS score recorded at 0.726% (52.019 percentile)

Technical Details for CVE-2026-55007

Vulnerability Analysis

The vulnerability is a double free condition in Microsoft Exchange Server. A double free occurs when the same memory address is passed to the deallocation routine more than once. The second free operation corrupts the heap allocator's internal free-list bookkeeping.

Attackers who control adjacent allocations can shape the heap so that the corrupted free list returns an attacker-influenced pointer during a subsequent allocation. This primitive typically escalates to arbitrary write, followed by control-flow hijack and code execution in the Exchange service context. The attack complexity is rated High, reflecting the timing and heap-grooming precision required for reliable exploitation.

Root Cause

The root cause is improper object lifetime management, classified under [CWE-415: Double Free]. A code path releases a heap-allocated object and then references or releases the same pointer again without clearing it or coordinating ownership between components. Microsoft has not disclosed the specific Exchange component involved.

Attack Vector

Exploitation occurs over the network without authentication and without user interaction. An attacker sends a crafted protocol message to an exposed Exchange service endpoint. The message triggers the code path that frees the same allocation twice, corrupting the heap and enabling subsequent memory manipulation for code execution.

No public proof-of-concept exploit is currently available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. Refer to the Microsoft Security Update Guide for CVE-2026-55007 for vendor-specific technical details.

Detection Methods for CVE-2026-55007

Indicators of Compromise

Unexpected crashes or restarts of w3wp.exe hosting Exchange application pools, especially with heap corruption exception codes such as 0xC0000374.
Anomalous outbound network connections originating from Exchange server processes to unknown hosts.
New or modified files in Exchange virtual directories such as FrontEnd\HttpProxy or ClientAccess\ecp.
Creation of new local accounts, scheduled tasks, or service installations on Exchange hosts shortly after suspicious HTTP or SMTP traffic.

Detection Strategies

Monitor Windows Error Reporting and Application event logs for repeated Exchange service faults consistent with heap corruption.
Inspect IIS logs for abnormally large, malformed, or repeated requests to Exchange endpoints preceding process crashes.
Correlate child process creation from w3wp.exe, EdgeTransport.exe, or other Exchange binaries against a baseline of expected behavior.
Alert on PowerShell, cmd.exe, or scripting engine execution spawned by Exchange service accounts.

Monitoring Recommendations

Enable command-line and module load auditing on all Exchange hosts and forward events to a centralized SIEM.
Track outbound traffic from Exchange servers and treat any egress to non-Microsoft infrastructure as suspicious pending review.
Baseline expected Exchange service behavior and alert on deviations in process trees, loaded DLLs, and file writes.

How to Mitigate CVE-2026-55007

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update Guide for CVE-2026-55007 to all on-premises Exchange servers.
Inventory all internet-exposed Exchange endpoints and prioritize patching Client Access and Edge Transport roles first.
Review Exchange servers for signs of prior compromise before and after patching, including web shells and unexpected accounts.
Rotate credentials for Exchange service accounts and high-privilege administrative accounts if compromise is suspected.

Patch Information

Microsoft has issued a security update for CVE-2026-55007. Administrators should consult the Microsoft Security Update Guide for CVE-2026-55007 for the applicable Cumulative Update or Security Update package for each Exchange Server version and install it during the next available maintenance window. Verify successful installation using the Exchange Management Shell Get-ExchangeServer | Format-List Name,AdminDisplayVersion and comparing against Microsoft's documented fixed build numbers.

Workarounds

Restrict Exchange management and OWA endpoints to trusted networks using firewall rules or reverse proxy access control lists where operationally feasible.
Enforce Extended Protection for Authentication on Exchange virtual directories to reduce exposure of unauthenticated code paths.
Deploy a web application firewall in front of Exchange to inspect and rate-limit malformed protocol requests.
Migrate mailboxes from unsupported or end-of-life Exchange versions to a supported release or Exchange Online where possible.