CVE-2024-43550: Windows 10 1507 Auth Bypass Vulnerability

CVE-2024-43550 Overview

CVE-2024-43550 is a spoofing vulnerability in the Windows Secure Channel (Schannel) component, which implements the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols. The flaw allows a network-positioned attacker to impersonate a legitimate endpoint during the TLS handshake, undermining the authenticity guarantees of the channel. Microsoft classifies the issue under CWE-295: Improper Certificate Validation. The vulnerability affects a broad range of Windows client and Windows Server releases still in support. Successful exploitation impacts confidentiality and integrity of communications protected by Schannel.

Critical Impact
A successful spoofing attack lets an adversary masquerade as a trusted TLS peer, enabling interception or modification of data exchanged between Windows systems and supposedly authenticated services.

Affected Products

Microsoft Windows 10 (versions 1507, 1607, 1809, 21H2, 22H2)
Microsoft Windows 11 (versions 21H2, 22H2, 23H2, 24H2)
Microsoft Windows Server 2012, 2012 R2, 2016, 2019, 2022, and 2022 23H2

Discovery Timeline

2024-10-08 - CVE-2024-43550 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2024-43550

Vulnerability Analysis

The vulnerability resides in Schannel, the Windows Security Support Provider that handles SSL/TLS sessions for system components and applications using the Secure Channel API. The defect maps to CWE-295: Improper Certificate Validation, indicating that Schannel does not adequately verify some aspect of the peer certificate or handshake material. As a result, an attacker can present crafted credentials that Schannel accepts as valid. The flaw impacts confidentiality and integrity of TLS-protected traffic, while availability is not affected. Exploitation requires the attacker to be in a position to intercept or relay TLS traffic, such as on the local network segment or in control of an upstream network device.

Root Cause

The root cause is improper validation logic within Schannel during certificate or handshake processing. Schannel fails to reject certain malformed or attacker-controlled certificate material that should not satisfy authentication requirements. This validation gap breaks the chain of trust that TLS relies on to bind a session to a known identity.

Attack Vector

The attack is conducted over the network and does not require authentication or user interaction. Microsoft notes high attack complexity, which reflects the need for specific timing or network conditions to perform the spoof. A typical scenario involves an adversary in a machine-in-the-middle position who presents a forged certificate to a Windows client or server initiating a TLS connection. If Schannel accepts the forged identity, the attacker can decrypt, observe, or alter the session contents. No verified public proof-of-concept code is available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2024-43550

Indicators of Compromise

Unexpected TLS sessions terminating to IP addresses or hostnames that do not match the expected service certificate subject or issuer.
Schannel event log entries (Event IDs 36874, 36887, 36888) showing fatal alerts or unusual certificate chain failures correlated with authentication anomalies.
Reissued or short-lived certificates appearing in network captures for hosts that normally use long-lived enterprise PKI certificates.

Detection Strategies

Inspect outbound and inter-server TLS traffic for certificates signed by unexpected issuers, particularly on management, LDAPS, RDP, and SMB-over-TLS flows.
Correlate Windows System log Schannel events with authentication failures in Security and Microsoft-Windows-CAPI2/Operational logs to surface certificate validation anomalies.
Use TLS inspection or passive certificate logging to flag deviations from baseline certificate fingerprints for critical Windows-to-Windows connections.

Monitoring Recommendations

Enable CAPI2 diagnostic logging on sensitive Windows servers to capture detailed certificate chain build and validation events.
Monitor for newly added trusted root or intermediate certificates in the LocalMachine\Root and LocalMachine\CA stores, which adversaries may abuse to support spoofing.
Alert on lateral TLS connections originating from non-standard processes or service accounts, especially on domain controllers and certificate authority servers.

How to Mitigate CVE-2024-43550

Immediate Actions Required

Apply the October 2024 Microsoft security updates that address CVE-2024-43550 to all affected Windows 10, Windows 11, and Windows Server systems.
Prioritize patching of domain controllers, certificate authorities, and any Internet-facing or management-plane servers that terminate or initiate TLS connections.
Audit trusted certificate stores and remove unauthorized root or intermediate certificates.

Patch Information

Microsoft has released cumulative security updates that remediate the Schannel validation defect. Refer to the Microsoft Security Update Guide for CVE-2024-43550 for the specific KB articles that correspond to each affected Windows build. Apply the update through Windows Update, WSUS, Microsoft Update Catalog, or your standard patch management process.

Workarounds

Enforce certificate pinning or strict issuer allowlists at the application layer where supported, reducing reliance on default Schannel trust decisions.
Restrict network paths that allow machine-in-the-middle positioning, including segmenting management networks and requiring IPsec or mutual TLS for sensitive inter-server traffic.
Disable legacy TLS versions and weak cipher suites in Schannel to reduce the surface available for downgrade-based spoofing attempts.

bash

# Configuration example: disable TLS 1.0 and TLS 1.1 in Schannel via registry
reg add "HKLM\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.0\Server" /v Enabled /t REG_DWORD /d 0 /f
reg add "HKLM\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.0\Client" /v Enabled /t REG_DWORD /d 0 /f
reg add "HKLM\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.1\Server" /v Enabled /t REG_DWORD /d 0 /f
reg add "HKLM\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.1\Client" /v Enabled /t REG_DWORD /d 0 /f