CVE-2026-40411: Azure Virtual Network Gateway Improper Input Validation RCE

CVE-2026-40411 is a critical vulnerability affecting Azure Virtual Network Gateway. This vulnerability stems from improper input validation, which enables an authorized attacker to execute arbitrary code over the network. The vulnerability has a CVSS v3.1 base score of 9.9, highlighting its significant risk. Exploitation could lead to a full compromise of the affected network gateway and potentially other connected resources. Defenders should prioritize patching and implementing appropriate input validation measures to mitigate this threat. This vulnerability was disclosed by Microsoft on May 22, 2026.

Attack Chain

1. Attacker authenticates to Azure with compromised or legitimate credentials.
2. Attacker crafts a malicious network configuration payload containing shell metacharacters.
3. Attacker sends a request to update the Azure Virtual Network Gateway configuration via the Azure API.
4. The Azure Virtual Network Gateway receives the configuration update request with the malicious payload.
5. Due to improper input validation, the gateway processes the malicious payload without sanitization.
6. The shell metacharacters within the payload trigger command execution on the underlying system.
7. The attacker gains arbitrary code execution within the Azure Virtual Network Gateway.
8. Attacker leverages the compromised gateway to pivot and compromise other resources on the network.

Impact

Successful exploitation of CVE-2026-40411 allows an authorized attacker to execute arbitrary code on the Azure Virtual Network Gateway. This can lead to complete compromise of the gateway, allowing the attacker to intercept and manipulate network traffic, pivot to other connected resources, and potentially exfiltrate sensitive data. Given the critical role of VPN gateways in network security, a successful attack can have widespread and severe consequences.

Recommendation

• Apply the security update provided by Microsoft to patch CVE-2026-40411 on all Azure Virtual Network Gateway instances immediately, as referenced in the advisory URL.
• Deploy the Sigma rule Detect CVE-2026-40411 Exploitation Attempt — Malicious Azure VNet Gateway Configuration to detect attempts to inject malicious code via configuration updates.
• Review and harden input validation mechanisms within Azure Virtual Network Gateway configurations.
• Monitor Azure API logs for suspicious configuration changes related to network gateways using the Detect Suspicious Azure API Activity Sigma rule.