CVE-2025-26670: Windows 10 1507 LDAP Use-After-Free Flaw

CVE-2025-26670 Overview

CVE-2025-26670 is a use-after-free vulnerability [CWE-416] in the Windows Lightweight Directory Access Protocol (LDAP) client implementation. The flaw allows an unauthenticated remote attacker to execute arbitrary code over a network without any user interaction. Microsoft addressed the issue in the April 2025 Patch Tuesday release. The vulnerability affects a broad range of Windows client and server editions, including Windows Server 2025 and Windows 11 24H2.

Critical Impact

Successful exploitation grants remote code execution in the context of the LDAP client process, enabling full compromise of the targeted system without credentials or user interaction.

Affected Products

• Microsoft Windows 10 (1507, 1607, 1809, 21H2, 22H2)
• Microsoft Windows 11 (22H2, 23H2, 24H2)
• Microsoft Windows Server 2008, 2012, 2016, 2019, 2022, 2022 23H2, and 2025

Discovery Timeline

• 2025-04-08 - CVE-2025-26670 published to NVD and Microsoft releases security patch
• 2025-07-09 - Last updated in NVD database

Technical Details for CVE-2025-26670

Vulnerability Analysis

The vulnerability resides in the Windows LDAP client component responsible for parsing protocol responses. A use-after-free condition occurs when the LDAP client references memory that has already been released during response handling. An attacker who controls or impersonates an LDAP server can trigger the dangling pointer dereference and redirect execution flow.

Exploitation requires the victim to initiate or accept an LDAP connection to attacker-controlled infrastructure. Because the LDAP client runs with elevated privileges in many enterprise scenarios, code execution typically inherits SYSTEM-level rights on domain-joined hosts. The EPSS score of 1.117% places this vulnerability in the 78th percentile for exploitation likelihood.

Root Cause

The root cause is improper memory lifecycle management in the LDAP message parser. Specific response fields are freed during processing but remain referenced by subsequent code paths. Crafted server responses can manipulate the heap state between the free and reuse operations, enabling controlled overwrites of function pointers or virtual table entries.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker positions a malicious LDAP server and induces a vulnerable Windows host to query it. Common triggers include DNS poisoning, rogue domain controllers in misconfigured environments, or coercion techniques that force outbound LDAP connections. The high attack complexity reflects the need to reliably win the memory reuse race and bypass modern exploit mitigations such as ASLR and CFG.

// No verified public proof-of-concept code is available for CVE-2025-26670.
// Refer to the Microsoft Security Response Center advisory for additional technical details.

Detection Methods for CVE-2025-26670

Indicators of Compromise

• Unexpected outbound LDAP (TCP/389) or LDAPS (TCP/636) connections from workstations to non-domain-controller hosts
• Crashes or abnormal terminations in wldap32.dll recorded in Windows Error Reporting logs
• Suspicious child processes spawned by lsass.exe or other processes that consume the LDAP client library
• Anomalous DNS queries for SRV records (_ldap._tcp) resolving to untrusted IP addresses

Detection Strategies

• Monitor process memory integrity for use-after-free crash signatures involving wldap32.dll
• Hunt for processes loading the LDAP client library and then performing memory allocation patterns consistent with heap grooming
• Correlate authentication coercion techniques (PetitPotam, DFSCoerce) with outbound LDAP traffic to non-trusted endpoints
• Inspect LDAP response payloads at the network layer for malformed BER-encoded structures

Monitoring Recommendations

• Enable Windows Defender Exploit Guard logging for processes invoking LDAP client APIs
• Deploy network segmentation alerts when endpoints initiate LDAP sessions outside expected directory infrastructure
• Aggregate Microsoft-Windows-LDAP-Client/Operational event logs into a central SIEM for retroactive hunting
• Track patch deployment status across all affected Windows versions to identify unpatched assets

How to Mitigate CVE-2025-26670

Immediate Actions Required

• Apply the April 2025 Microsoft security updates to all affected Windows client and server systems
• Prioritize patching of domain controllers, jump hosts, and systems that frequently initiate LDAP queries
• Restrict outbound LDAP traffic from endpoints to authorized domain controllers only via host and perimeter firewalls
• Audit the environment for legacy systems (Windows Server 2008, 2012) that may require Extended Security Updates

Patch Information

Microsoft released patches for all affected versions on April 8, 2025. Refer to the Microsoft Security Update CVE-2025-26670 advisory for KB numbers corresponding to each supported Windows build. Systems running unsupported versions without Extended Security Updates remain permanently vulnerable.

Workarounds

• Enforce LDAP channel binding and LDAP signing to reduce the attack surface for rogue server scenarios
• Block outbound TCP/389 and TCP/636 from endpoints that do not require direct directory communication
• Disable mDNS and LLMNR to prevent name resolution hijacking that could redirect LDAP queries
• Apply network-level authentication and segmentation to limit lateral movement to vulnerable hosts

# Restrict outbound LDAP traffic to approved domain controllers (PowerShell)
New-NetFirewallRule -DisplayName "Block Outbound LDAP" `
  -Direction Outbound -Protocol TCP -RemotePort 389,636 `
  -RemoteAddress Any -Action Block

# Allow only approved DCs
New-NetFirewallRule -DisplayName "Allow LDAP to DCs" `
  -Direction Outbound -Protocol TCP -RemotePort 389,636 `
  -RemoteAddress 10.0.0.10,10.0.0.11 -Action Allow