Windows File Protection Arbitrary Certificate Chain Vulnerability
26 Dec. 2002
Summary
Windows File Protection will trust any digital signature whose certificate chain is rooted at any one of the Trusted Root Certification Authorities. Versions of Windows (and Internet Explorer) ship with various preconfigured Trusted Root Certification Authorities that are automatically trusted not just as potential Root CA's for SSL certificate chains but also as valid Root CA's for code signing certificates.
Many Root CA's issue SSL certificates that have improper Key Usage and Enhanced Key Usage Object Identifiers (OIDs), and missing or invalid Basic Constraints, making many SSL certificates identical in function to more privileged certificates.
In the case of missing Basic Constraints, Windows is known to trust a certificate as though it were a legitimate Intermediate Certification Authority even with recent patch (Q329115) applied to resolve MS02-050 "Certificate Validation Flaw Could Enable Identity Spoofing" where the Basic Constraints field, if present, was ignored completely.
A related SECURITY ALERT issued today "Windows File Protection Old Security Catalog Vulnerability" explains that WFP is designed to trust equally every version of published code that it has ever trusted by way of its installed Security Catalogs (.CAT files), making it easy for an attacker to replace new code that contains bug fixes and patches for security vulnerabilities with old code that is known to be vulnerable to various exploits.
Windows File Protection (a.k.a. Windows Driver Signing) [1] verifies digital signatures applied to operating system binaries, device drivers, and other OS files, as well as files published by third-parties [2] that are certified by Windows Hardware Quality Labs (WHQL) (a.k.a. Microsoft Windows Hardware Compatibility). The vulnerability disclosed in this communication is simply that any digitally-signed replacement file of malicious origin will take priority over any authentic WFP/WHQL-signed file.
Anyone can now obtain anonymous code signing and SSL certificates automatically and free of charge from the following CA:
whose Root Certificate is:
CN = UTN-USERFirst-Network Applications
OU = http://www.usertrust.com
O = The USERTRUST Network
L = Salt Lake City
S = UT
C = US
And use this anonymous freessl.com/usertrust.com/geotrust.com certificate to digitally sign malicious code (e.g. using SIGNCODE.EXE) that Windows File Protection (WFP) will automatically trust by virtue of the fact that the certificate's Root CA (usertrust.com) is one of the Root Certificates trusted by default in standard Windows/IE installations. It should be noted, however, that every Root CA that issues certificates that can be used for code signing (all CA's of which this author is aware do sell code signing certificates in addition to SSL certificates) enables any attacker in possession of a valid code signing certificate signed by any Root CA to apply a digital signature to malicious code and deploy it without detection to any Windows box that relies on WFP for malicious code/Trojan detection.
A related SECURITY ALERT issued today [3] "Windows File Protection Old Security Catalog Vulnerability" explains that WFP is designed to trust equally every version of published code that it has ever trusted by way of its installed Security Catalogs (.CAT files), making it easy for an attacker to replace new code that contains bug fixes and patches for security vulnerabilities with old code that is known to be vulnerable to various exploits.
Therefore, only manual forensic verification of full-file hashes with comparison against a list of known good hashes (i.e. authentic hashes) will properly reveal the malicious replacement when an attacker applies a verifiable digital signature to an old Windows binary whose hash code can still be found in an old Security Catalog file, or when an attacker is able to place malicious code that contains a digital signature embedded in the PE file format "Certificates Table" data directory entry [4]. The following "Action Required" is thus inadequate to defend against misplaced trust when the attack uses digitally-signed malicious code or digitally-signed old, but authentic, vulnerable code published (and digitally-signed) in the past by a legitimate software vendor.
Action Required: Current Best Practice (Jason's)
Delete your default Root CA Certificates. All of them.
Ignore Windows File Protection. If you must use it, run SIGVERIF.EXE and manually examine the log file (Click the Advanced button to configure scan parameters and logging) to determine who the publisher is of each trusted file that appears to have a valid digital signature. Be aware that it's possible for an attacker to acquire a certificate from a trusted Root CA or Intermediate CA that has the same common name (CN) as an authentic Microsoft certificate, such as "Microsoft Windows 2000 Publisher" in which case your analysis of the log file created by SIGVERIF.EXE will be useless unless you also know the filename of the Security Catalog file inside which each file's hash code should be found. The only way to get this information easily is to compare SIGVERIF.EXE log files between Windows boxes, because the Security Catalog files themselves do not contain filenames of the files they are meant to authenticate, .CAT files contain only hashes.
Script the verification of trust for your executable code files using CHKTRUST.EXE instead of WFP, since CHKTRUST.EXE relies on the WinTrust API instead of WFP. WinTrust will only trust software publisher certificates (SPCs) that are selected explicitly and configured for automatic trust by way of the following Registry keys:
Produce your own digital signatures instead using a code signing certificate that you issued to yourself from your own Trusted Root Certification Authority certificate store. Details for producing your own Security Catalogs and managing your own Public Key Infrastructure (PKI) for the purpose of preventing unwanted code execution will be available at the following URL which is controlled by this author: http://www.countermand.org
Adequate Protection (Jason's)
First, upgrade to Windows XP or Windows .NET Server 2003 from whatever prehistoric version of Windows you're using now so that you can enable Software Restriction Policies per the following instructions. Add a new hash code rule for every system binary and other executable file you wish to allow to run on your box; this establishes a fixed trust policy based on the authentic hashes of code that you choose to trust rather than a variable trust policy based on anything that Windows thinks is legitimate based on the appearance that it has a valid digital signature. This fixed/static trust policy is superior to the dynamic one provided through the use of digital signatures, because whether or not something is digitally-signed or meant to be trusted (today, as opposed to in the past) is determined automatically by Windows, inclusive of its known flaws in analyzing certificate chains, when signatures (and PKI) are used -- these fancy cryptographic schemes are not necessary in order to countermand execution of unwanted code, and they actually interfere with your ability to prevent unwanted code when there are problems with the implementation or design of these variable trust-based PKI systems:
And then... (it will take you a long time to explicitly authorize each executable module and DLL, which is why deploying your own Security Catalogs with your own PKI-based Root CA and code signing certificate is the Best Practice today.)
Disable Windows File Protection completely by deleting all Root CA certificates from every trusted certificate store per the following instructions, which you must apply in reverse (that is, the following Knowledge Base Article shows you how to recover from a failed Windows File Protection condition due to missing Root Certificates -- if WFP is already working, kill it by following these instructions in reverse):
Note that you should NOT follow the instructions found in Q293819, as they remove only the current user's Root CA certificates rather than every certificate deployed to your box:
Common Sense (Jason's)
Remember to make a record of the authentic hashes of the files you've chosen to trust explicitly so that you can audit your system later and compare your hashes against those of a peer or another Windows box that you also control. Command-line utilities to compute full-file hashes are available on every OS platform, and you can build your own easily with Microsoft .NET per the following article written by this author and published in MSDN Magazine:
Preventive per Contra Response to Vendor Response:
The following Bellum Omnium Contra Omnes per Contra response is offered in advance to the anticipated vendor response to this SECURITY ALERT as a preventive measure in the interest of diminishing the Mean Time Before Fix (MTBF). Yes, this behavior is by design; placing documentation to this effect in the manual and crying RTFM is foul. There is no reason to automatically trust any Root CA when it comes to code signing. Windows File Protection should never have been designed to trust digital signatures on software based on certificate chains, WFP should trust only specific certificates the way that WinTrust operates.
When the vendor redesigns this feature, it would be sensible to also deploy a better system for managing certificates and trust relationships with End Entities, Root CA's, and any site that needs SSL for the purpose of encryption but doesn't care about the authentication features that it never provided properly in the first place for users of Windows. There is a large demand, and a legitimate demand, for anonymous SSL certificates like those distributed by http://freessl.com/geotrust.com/usertrust.com -- however, bad code deployed in the wild today like Windows File Protection, and flawed security policies that rely on such bad code, make the availability of free, anonymous SSL certificates/code signing certificates an urgent and immediate information security threat. Rather than shut down Certification Authorities with bad security practices, this author suggests that vendors who produce bad code should issue patches immediately to remediate the vulnerability from the source rather than attempt to prune competition for security-related products and services from the digital marketplace.
Certificates should be managed by each node/end-user in a manner similar to the way that cookies are now managed in Internet Explorer. Each end-user/administrator of each node on the network should be able to easily define a security policy and the default setting should be to block and deny all. Each capability possible with respect to each certificate (SSL/code signing/e-mail signatures, etc.) should be a separate security policy setting that the end-user or an authorized administrator must explicitly allow on a per-certificate basis.
This author will gladly code these revisions for vendor if vendor will release the relevant source code under the terms of an open source license.
