CVE-2024-43642

What it actually does

CVE-2024-43642 is a Windows Server Message Block (SMB) denial-of-service vulnerability. A remote attacker who can reach the SMB service, typically over TCP/445, can send specially crafted SMB traffic that causes the target SMB service or host to become unavailable. The expected impact is availability loss rather than code execution or data theft: file shares, authentication-dependent file access, named-pipe access, and other SMB-backed operations may hang, fail, or require service or system recovery.

Why it matters to the business

Successful exploitation can interrupt Windows file sharing, application access to shared storage, domain-adjacent operational workflows, backup jobs, administrative tooling, and any business process dependent on SMB. Because SMB is widely deployed in Windows environments and the EPSS score is elevated, organizations should treat externally reachable or broadly accessible internal SMB services as higher priority even though the vulnerability is not currently listed in CISA KEV and there is no confirmed ransomware association.

How an attack unfolds

An attacker first identifies a Windows system exposing SMB, usually by scanning for TCP/445. If the service is reachable, the attacker initiates SMB communication and sends malformed or maliciously structured SMB requests that trigger the denial-of-service condition. The vulnerable system may stop responding to SMB requests, log service failures, disrupt active file-share sessions, or require service restart or host reboot. The attacker can repeat the traffic to sustain the outage. Exploitation does not require the attacker to steal credentials or execute commands if the vulnerable code path is reachable over the network, but network controls that block untrusted access to SMB materially reduce exposure.

How to fix it

Apply the Microsoft security update that addresses CVE-2024-43642 for each affected Windows version, using Windows Update, WSUS, Microsoft Configuration Manager, or the Microsoft Update Catalog. Where exact product builds vary, use the Microsoft Security Update Guide for CVE-2024-43642 to map each Windows edition to the required November 2024 or later cumulative update. Reduce exposure by blocking inbound SMB from untrusted networks, especially TCP/445 at internet boundaries; limiting SMB access to required internal segments; disabling SMBv1 where still enabled; enforcing least-privilege network access; and monitoring critical file servers for abnormal SMB traffic. If immediate patching is not possible, prioritize network segmentation and firewall controls around SMB servers until updates can be deployed.

How to confirm the fix worked

Confirm that each Windows system is running a cumulative update level that includes the November 2024 security fixes or later, and verify the applicable KB/build against Microsoft’s Security Update Guide for CVE-2024-43642. Use vulnerability scanners or endpoint inventory to identify systems missing the relevant Windows update. Validate that TCP/445 is not reachable from the internet or from unnecessary network segments. After remediation, review System and SMB Server event logs for continued LanmanServer crashes or SMB service instability, and perform controlled SMB connectivity tests to confirm normal file-share operation.

Typical attack path

1. Scan for hosts exposing SMB on TCP/445.
2. Fingerprint Windows SMB services and identify systems that may not have the November 2024 or later security updates applied.
3. Send crafted SMB traffic designed to trigger the denial-of-service condition.
4. Cause SMB service disruption, failed file-share access, service instability, or host unavailability.
5. Repeat the request sequence to prolong the outage or disrupt recovery efforts.

How to detect it

Because this is a denial-of-service issue, successful exploitation may leave few durable artifacts beyond network traffic and service failure evidence. Review Windows System logs, Microsoft-Windows-SMBServer/Operational logs, endpoint telemetry, firewall logs, IDS/IPS logs, and NetFlow around the time of SMB outages. Look for repeated inbound TCP/445 connections from the same source, bursts of malformed or failed SMB sessions, SMB service crashes, unexpected LanmanServer failures, host reboots following SMB instability, and correlated user reports of file-share disconnections. Investigate any external source attempting to reach TCP/445 and any internal host generating high-volume or repetitive SMB negotiation/session setup traffic across many systems. If threat hunting retrospectively, focus on the period since publication, especially for unpatched systems with SMB exposed to broad networks.

• TCP/445
• LanmanServer
• The Server service terminated unexpectedly
• Event ID 7031
• Event ID 7034
• Microsoft-Windows-SMBServer/Operational
• SMBServer
• srv2.sys
• srvnet.sys
• System error 64
• The specified network name is no longer available
• STATUS_CONNECTION_DISCONNECTED
• STATUS_NETWORK_NAME_DELETED
• SMB2 NEGOTIATE
• SMB2 SESSION_SETUP

What your security team should do

• Prioritize patching Windows servers and workstations that expose SMB to broad internal networks, partner networks, VPN users, or any untrusted source.
• Immediately block inbound TCP/445 from the internet and restrict SMB traffic to explicitly required management and file-sharing paths.
• Use asset inventory and vulnerability scanning to identify Windows systems missing November 2024 or later cumulative updates.
• Increase monitoring for SMB service crashes, repeated reconnect attempts, abnormal TCP/445 spikes, and file-share availability failures.
• Prepare operational recovery steps for critical SMB servers, including service restart procedures, reboot windows, failover validation, and backup access paths.
• Do not rely on the absence of KEV status as a reason to defer remediation; the elevated EPSS score indicates meaningful exploitation likelihood.