CVE-2025-27487: Remote Desktop Client RCE Vulnerability

CVE-2025-27487 Overview

CVE-2025-27487 is a heap-based buffer overflow vulnerability in the Microsoft Remote Desktop Client. An authorized attacker can exploit this flaw to execute arbitrary code over a network when a user connects to a malicious or compromised Remote Desktop Protocol (RDP) server. The vulnerability is tracked under [CWE-122] (Heap-based Buffer Overflow) and [CWE-787] (Out-of-bounds Write). Microsoft published the advisory on April 8, 2025, and the entry was last updated on July 7, 2025. The flaw affects the Remote Desktop Client, the Windows App, and the built-in RDP client across all supported Windows desktop and server releases.

Critical Impact

Successful exploitation allows remote code execution on the client machine with the privileges of the connected user, enabling full compromise of confidentiality, integrity, and availability.

Affected Products

• Microsoft Remote Desktop Client and Windows App
• Windows 10 (1507, 1607, 1809, 21H2, 22H2) and Windows 11 (22H2, 23H2, 24H2)
• Windows Server 2008 R2 SP1 through Windows Server 2025, including Server 2012, 2016, 2019, 2022, and 2022 23H2

Discovery Timeline

• 2025-04-08 - CVE-2025-27487 published to NVD
• 2025-04-08 - Microsoft releases security patch via Microsoft Security Update CVE-2025-27487
• 2025-07-07 - Last updated in NVD database

Technical Details for CVE-2025-27487

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow in the Remote Desktop Client. When the client processes data received from an RDP server, it writes data past the bounds of a heap-allocated buffer. This memory corruption can be steered to overwrite adjacent heap metadata or function pointers, redirecting execution flow to attacker-controlled code. Exploitation requires the victim to initiate or accept an RDP connection to a server controlled by the attacker. The attacker must hold valid authorization on the target RDP server, which is consistent with scenarios where adversaries stand up rogue RDP infrastructure or compromise legitimate jump hosts and lure users to connect.

Root Cause

The root cause lies in improper validation of length fields or message sizes during the parsing of RDP virtual channel or protocol data on the client side. The client allocates a heap buffer based on expected data dimensions but fails to enforce strict bounds when copying server-supplied data into it. This out-of-bounds write ([CWE-787]) corrupts heap structures and enables code execution.

Attack Vector

The attack vector is network-based and requires user interaction. A user must connect their Remote Desktop Client to a malicious server. The attacker then returns crafted RDP responses that trigger the overflow during session negotiation or virtual channel processing. Because execution occurs in the context of the client user, an attacker who controls a server commonly used by administrators can gain code execution on privileged workstations.

No public proof-of-concept exploit is currently available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. See the Microsoft Security Update CVE-2025-27487 for vendor technical details.

Detection Methods for CVE-2025-27487

Indicators of Compromise

• Crashes, exceptions, or unexpected terminations of mstsc.exe or the Windows App immediately after connecting to an RDP server.
• Outbound RDP (TCP/3389) connections from user workstations to untrusted or newly observed external IP addresses.
• Child processes spawned by mstsc.exe that are not part of normal RDP client behavior, such as cmd.exe, powershell.exe, or rundll32.exe.
• Heap corruption events in Windows Error Reporting telemetry referencing Remote Desktop Client modules.

Detection Strategies

• Hunt for anomalous process lineage where mstsc.exe is the parent of script interpreters, LOLBins, or unsigned binaries.
• Correlate RDP client crash events (Application Error 1000) with subsequent network connections or file writes by the same user session.
• Inspect EDR telemetry for memory protection violations and access-violation exceptions inside Remote Desktop Client modules.

Monitoring Recommendations

• Restrict and log outbound RDP connections at the perimeter and only permit them to known, trusted destinations.
• Enable command-line and module load auditing on endpoints that frequently initiate RDP sessions, particularly administrator workstations.
• Forward Sysmon and Windows Defender Exploit Guard events to a centralized data lake for retroactive hunting once additional indicators emerge.

How to Mitigate CVE-2025-27487

Immediate Actions Required

• Apply the April 2025 Microsoft security updates for all affected Windows client and server versions immediately.
• Update the Windows App and standalone Remote Desktop Client packages on managed and BYOD devices.
• Inventory all systems running legacy Windows Server 2008 R2 and Server 2012 to confirm they receive the relevant Extended Security Update.
• Block outbound RDP to untrusted networks and require RDP traffic to traverse a Remote Desktop Gateway.

Patch Information

Microsoft released fixes through the April 8, 2025 Patch Tuesday cycle. Administrators should consult the Microsoft Security Update CVE-2025-27487 advisory for the specific KB articles and build numbers that apply to each affected operating system.

Workarounds

• Prohibit users from initiating RDP sessions to servers outside the corporate network or unmanaged hosts.
• Enforce egress filtering to block TCP/3389 and UDP/3389 to arbitrary internet destinations.
• Require RDP connections to authenticate through a hardened Remote Desktop Gateway with Network Level Authentication enforced.
• Use application control policies to restrict execution of child processes spawned by mstsc.exe.

# Configuration example: block outbound RDP except to approved gateway
New-NetFirewallRule -DisplayName "Block Outbound RDP" `
  -Direction Outbound -Protocol TCP -RemotePort 3389 `
  -Action Block -Profile Any

New-NetFirewallRule -DisplayName "Allow RDP to Gateway" `
  -Direction Outbound -Protocol TCP -RemotePort 3389 `
  -RemoteAddress 10.10.10.5 -Action Allow -Profile Any