Подмена поставщика доверия и SIP
In user mode, Windows Authenticode (Citation: Microsoft Authenticode) digital signatures are used to verify a file's origin and integrity, variables that may be used to establish trust in signed code (ex: a driver with a valid Microsoft signature may be handled as safe). The signature validation process is handled via the WinVerifyTrust application programming interface (API) function, (Citation: Microsoft WinVerifyTrust) which accepts an inquiry and coordinates with the appropriate trust provider, which is responsible for validating parameters of a signature. (Citation: SpectorOps Subverting Trust Sept 2017)
Because of the varying executable file types and corresponding signature formats, Microsoft created software components called Subject Interface Packages (SIPs) (Citation: EduardosBlog SIPs July 2008) to provide a layer of abstraction between API functions and files. SIPs are responsible for enabling API functions to create, retrieve, calculate, and verify signatures. Unique SIPs exist for most file formats (Executable, PowerShell, Installer, etc., with catalog signing providing a catch-all (Citation: Microsoft Catalog Files and Signatures April 2017)) and are identified by globally unique identifiers (GUIDs). (Citation: SpectorOps Subverting Trust Sept 2017)
Similar to Code Signing, adversaries may abuse this architecture to subvert trust controls and bypass security policies that allow only legitimately signed code to execute on a system. Adversaries may hijack SIP and trust provider components to mislead operating system and whitelisting tools to classify malicious (or any) code as signed by: (Citation: SpectorOps Subverting Trust Sept 2017)
* Modifying the Dll and FuncName Registry values in HKLM\SOFTWARE[\WOW6432Node\]Microsoft\Cryptography\OID\EncodingType 0\CryptSIPDllGetSignedDataMsg\{SIP_GUID} that point to the dynamic link library (DLL) providing a SIP’s CryptSIPDllGetSignedDataMsg function, which retrieves an encoded digital certificate from a signed file. By pointing to a maliciously-crafted DLL with an exported function that always returns a known good signature value (ex: a Microsoft signature for Portable Executables) rather than the file’s real signature, an adversary can apply an acceptable signature value to all files using that SIP (Citation: GitHub SIP POC Sept 2017) (although a hash mismatch will likely occur, invalidating the signature, since the hash returned by the function will not match the value computed from the file).
* Modifying the Dll and FuncName Registry values in HKLM\SOFTWARE\[WOW6432Node\]Microsoft\Cryptography\OID\EncodingType 0\CryptSIPDllVerifyIndirectData\{SIP_GUID} that point to the DLL providing a SIP’s CryptSIPDllVerifyIndirectData function, which validates a file’s computed hash against the signed hash value. By pointing to a maliciously-crafted DLL with an exported function that always returns TRUE (indicating that the validation was successful), an adversary can successfully validate any file (with a legitimate signature) using that SIP (Citation: GitHub SIP POC Sept 2017) (with or without hijacking the previously mentioned CryptSIPDllGetSignedDataMsg function). This Registry value could also be redirected to a suitable exported function from an already present DLL, avoiding the requirement to drop and execute a new file on disk.
* Modifying the DLL and Function Registry values in HKLM\SOFTWARE\[WOW6432Node\]Microsoft\Cryptography\Providers\Trust\FinalPolicy\{trust provider GUID} that point to the DLL providing a trust provider’s FinalPolicy function, which is where the decoded and parsed signature is checked and the majority of trust decisions are made. Similar to hijacking SIP’s CryptSIPDllVerifyIndirectData function, this value can be redirected to a suitable exported function from an already present DLL or a maliciously-crafted DLL (though the implementation of a trust provider is complex).
* **Note:** The above hijacks are also possible without modifying the Registry via DLL Search Order Hijacking.
Hijacking SIP or trust provider components can also enable persistent code execution, since these malicious components may be invoked by any application that performs code signing or signature validation. (Citation: SpectorOps Subverting Trust Sept 2017)
Контрмеры |
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| Контрмера | Описание |
|---|---|
| Execution Prevention |
Prevent the execution of unauthorized or malicious code on systems by implementing application control, script blocking, and other execution prevention mechanisms. This ensures that only trusted and authorized code is executed, reducing the risk of malware and unauthorized actions. This mitigation can be implemented through the following measures: Application Control: - Use Case: Use tools like AppLocker or Windows Defender Application Control (WDAC) to create whitelists of authorized applications and block unauthorized ones. On Linux, use tools like SELinux or AppArmor to define mandatory access control policies for application execution. - Implementation: Allow only digitally signed or pre-approved applications to execute on servers and endpoints. (e.g., `New-AppLockerPolicy -PolicyType Enforced -FilePath "C:\Policies\AppLocker.xml"`) Script Blocking: - Use Case: Use script control mechanisms to block unauthorized execution of scripts, such as PowerShell or JavaScript. Web Browsers: Use browser extensions or settings to block JavaScript execution from untrusted sources. - Implementation: Configure PowerShell to enforce Constrained Language Mode for non-administrator users. (e.g., `Set-ExecutionPolicy AllSigned`) Executable Blocking: - Use Case: Prevent execution of binaries from suspicious locations, such as `%TEMP%` or `%APPDATA%` directories. - Implementation: Block execution of `.exe`, `.bat`, or `.ps1` files from user-writable directories. Dynamic Analysis Prevention: - Use Case: Use behavior-based execution prevention tools to identify and block malicious activity in real time. - Implemenation: Employ EDR solutions that analyze runtime behavior and block suspicious code execution. |
| Restrict Registry Permissions |
Restricting registry permissions involves configuring access control settings for sensitive registry keys and hives to ensure that only authorized users or processes can make modifications. By limiting access, organizations can prevent unauthorized changes that adversaries might use for persistence, privilege escalation, or defense evasion. This mitigation can be implemented through the following measures: Review and Adjust Permissions on Critical Keys - Regularly review permissions on keys such as `Run`, `RunOnce`, and `Services` to ensure only authorized users have write access. - Use tools like `icacls` or `PowerShell` to automate permission adjustments. Enable Registry Auditing - Enable auditing on sensitive keys to log access attempts. - Use Event Viewer or SIEM solutions to analyze logs and detect suspicious activity. - Example Audit Policy: `auditpol /set /subcategory:"Registry" /success:enable /failure:enable` Protect Credential-Related Hives - Limit access to hives like `SAM`,`SECURITY`, and `SYSTEM` to prevent credential dumping or other unauthorized access. - Use LSA Protection to add an additional security layer for credential storage. Restrict Registry Editor Usage - Use Group Policy to restrict access to regedit.exe for non-administrative users. - Block execution of registry editing tools on endpoints where they are unnecessary. Deploy Baseline Configuration Tools - Use tools like Microsoft Security Compliance Toolkit or CIS Benchmarks to apply and maintain secure registry configurations. *Tools for Implementation* Registry Permission Tools: - Registry Editor (regedit): Built-in tool to manage registry permissions. - PowerShell: Automate permissions and manage keys. `Set-ItemProperty -Path "HKLM:\Software\Microsoft\Windows\CurrentVersion\Run" -Name "KeyName" -Value "Value"` - icacls: Command-line tool to modify ACLs. Monitoring Tools: - Sysmon: Monitor and log registry events. - Event Viewer: View registry access logs. Policy Management Tools: - Group Policy Management Console (GPMC): Enforce registry permissions via GPOs. - Microsoft Endpoint Manager: Deploy configuration baselines for registry permissions. |
| SIP and Trust Provider Hijacking Mitigation |
Ensure proper permissions are set for Registry hives to prevent users from modifying keys related to SIP and trust provider components. Also ensure that these values contain their full path to prevent DLL Search Order Hijacking. (Citation: SpectorOps Subverting Trust Sept 2017) Consider removing unnecessary and/or stale SIPs. (Citation: SpectorOps Subverting Trust Sept 2017) Restrict storage and execution of SIP DLLs to protected directories, such as C:\Windows, rather than user directories. Enable whitelisting solutions such as AppLocker and/or Device Guard to block the loading of malicious SIP DLLs. Components may still be able to be hijacked to suitable functions already present on disk if malicious modifications to Registry keys are not prevented. |
| Restrict File and Directory Permissions |
Restricting file and directory permissions involves setting access controls at the file system level to limit which users, groups, or processes can read, write, or execute files. By configuring permissions appropriately, organizations can reduce the attack surface for adversaries seeking to access sensitive data, plant malicious code, or tamper with system files. Enforce Least Privilege Permissions: - Remove unnecessary write permissions on sensitive files and directories. - Use file ownership and groups to control access for specific roles. Example (Windows): Right-click the shared folder → Properties → Security tab → Adjust permissions for NTFS ACLs. Harden File Shares: - Disable anonymous access to shared folders. - Enforce NTFS permissions for shared folders on Windows. Example: Set permissions to restrict write access to critical files, such as system executables (e.g., `/bin` or `/sbin` on Linux). Use tools like `chown` and `chmod` to assign file ownership and limit access. On Linux, apply: `chmod 750 /etc/sensitive.conf` `chown root:admin /etc/sensitive.conf` File Integrity Monitoring (FIM): - Use tools like Tripwire, Wazuh, or OSSEC to monitor changes to critical file permissions. Audit File System Access: - Enable auditing to track permission changes or unauthorized access attempts. - Use auditd (Linux) or Event Viewer (Windows) to log activities. Restrict Startup Directories: - Configure permissions to prevent unauthorized writes to directories like `C:\ProgramData\Microsoft\Windows\Start Menu`. Example: Restrict write access to critical directories like `/etc/`, `/usr/local/`, and Windows directories such as `C:\Windows\System32`. - On Windows, use icacls to modify permissions: `icacls "C:\Windows\System32" /inheritance:r /grant:r SYSTEM:(OI)(CI)F` - On Linux, monitor permissions using tools like `lsattr` or `auditd`. |
Обнаружение
Periodically baseline registered SIPs and trust providers (Registry entries and files on disk), specifically looking for new, modified, or non-Microsoft entries. (Citation: SpectorOps Subverting Trust Sept 2017) Enable CryptoAPI v2 (CAPI) event logging (Citation: Entrust Enable CAPI2 Aug 2017) to monitor and analyze error events related to failed trust validation (Event ID 81, though this event can be subverted by hijacked trust provider components) as well as any other provided information events (ex: successful validations). Code Integrity event logging may also provide valuable indicators of malicious SIP or trust provider loads, since protected processes that attempt to load a maliciously-crafted trust validation component will likely fail (Event ID 3033). (Citation: SpectorOps Subverting Trust Sept 2017) Utilize Sysmon detection rules and/or enable the Registry (Global Object Access Auditing) (Citation: Microsoft Registry Auditing Aug 2016) setting in the Advanced Security Audit policy to apply a global system access control list (SACL) and event auditing on modifications to Registry values (sub)keys related to SIPs and trust providers: (Citation: Microsoft Audit Registry July 2012) * HKLM\SOFTWARE\Microsoft\Cryptography\OID * HKLM\SOFTWARE\WOW6432Node\Microsoft\Cryptography\OID * HKLM\SOFTWARE\Microsoft\Cryptography\Providers\Trust * HKLM\SOFTWARE\WOW6432Node\Microsoft\Cryptography\Providers\Trust **Note:** As part of this technique, adversaries may attempt to manually edit these Registry keys (ex: Regedit) or utilize the legitimate registration process using Regsvr32. (Citation: SpectorOps Subverting Trust Sept 2017) Analyze Autoruns data for oddities and anomalies, specifically malicious files attempting persistent execution by hiding within auto-starting locations. Autoruns will hide entries signed by Microsoft or Windows by default, so ensure “Hide Microsoft Entries” and “Hide Windows Entries” are both deselected. (Citation: SpectorOps Subverting Trust Sept 2017)
Ссылки
- Microsoft. (2012, July 2). Audit Registry. Retrieved January 31, 2018.
- Microsoft. (2016, August 31). Registry (Global Object Access Auditing). Retrieved January 31, 2018.
- Entrust Datacard. (2017, August 16). How do I enable CAPI 2.0 logging in Windows Vista, Windows 7 and Windows 2008 Server?. Retrieved January 31, 2018.
- Graeber, M. (2017, September 14). PoCSubjectInterfacePackage. Retrieved January 31, 2018.
- Hudek, T. (2017, April 20). Catalog Files and Digital Signatures. Retrieved January 31, 2018.
- Navarro, E. (2008, July 11). SIP’s (Subject Interface Package) and Authenticode. Retrieved January 31, 2018.
- Graeber, M. (2017, September). Subverting Trust in Windows. Retrieved January 31, 2018.
- Microsoft. (n.d.). WinVerifyTrust function. Retrieved January 31, 2018.
- Microsoft. (n.d.). Authenticode. Retrieved January 31, 2018.
Связанные риски
Каталоги
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