Windows SIP Provider Modification for Defense Evasion

Attackers may target the Windows cryptographic system by modifying registered Subject Interface Package (SIP) providers to bypass signature validation checks and inject malicious code. This allows unsigned or maliciously signed files to be treated as trusted by the operating system. Modifications to these providers can be an attempt to subvert trust controls and execute unauthorized code, leading to potential system compromise. This technique can be employed in various stages of an attack, often after initial access and privilege escalation, to maintain persistence and evade detection by security software relying on signature verification. Defenders should monitor registry modifications related to SIP providers, specifically looking for changes to the DLL paths and data values under the Cryptography\OID and Providers\Trust registry keys.

Attack Chain

1. An attacker gains initial access to the system, potentially through phishing or exploiting a vulnerability.
2. The attacker escalates privileges to gain the necessary permissions to modify the registry.
3. The attacker modifies the registry keys associated with SIP providers, specifically targeting the CryptSIPDllPutSignedDataMsg and FinalPolicy values.
4. The attacker changes the Dll value within these keys to point to a malicious DLL.
5. The modified DLL is loaded by the system when signature verification is performed.
6. The malicious DLL executes its payload, potentially injecting code into a trusted process or disabling security features.
7. The attacker uses the modified system to execute unsigned or maliciously signed code without triggering signature validation errors.
8. The attacker achieves persistence and maintains control over the compromised system.

Impact

Successful modification of SIP providers allows attackers to bypass signature validation checks, enabling the execution of unsigned or maliciously signed code. This can lead to the compromise of critical system processes, the installation of malware, and the exfiltration of sensitive data. The impact can range from localized system instability to widespread network compromise, depending on the scope and objective of the attack. Since SIP providers are integral to the Windows trust system, tampering with them undermines the security posture of the entire operating environment.

Recommendation

• Deploy the Sigma rule "Detect SIP Provider Modification via Registry Change" to detect suspicious changes to SIP provider-related registry keys (registry_set).
• Investigate any registry modifications to the CryptSIPDllPutSignedDataMsg and FinalPolicy keys, especially changes to the Dll value, as highlighted in the overview (registry_set).
• Examine the process responsible for the registry modification, cross-referencing against known benign processes like msiexec.exe and regsvr32.exe (process_creation).
• Enable Sysmon registry event logging to capture registry modifications related to SIP providers (Sysmon).