============================================================================ == ____________________________________________________________________________ __ SECURITY ALERT Windows File Protection Old Security Catalog Vulnerability December 26, 2002 [Full Disclosure, secureat_private and others] August 26, 2002 [Private Disclosure, Microsoft Press and others] Jason Coombs <jasoncat_private> http://www.science.org/jasonc FORENSICS.ORG Security Coordinator <secalertat_private> http://www.forensics.org/secalert ____________________________________________________________________________ __ Abstract Old Security Catalogs (.CAT files) containing valid digital signatures are left in place under %WinDir%\System32\CatRoot when new files and their associated Security Catalogs aredeployed. Windows uses its CryptCATAdminCalcHashFromFileHandle API function (found in mscat32.dll under Windows 2000 and in wintrust.dll under Windows XP/.NET Server) to compute a file's SIP hash code (see below) and then its CryptCATAdminEnumCatalogFromHash API function to automatically locate the digitally-signed .CAT files, if any, that contain the file's SIP hash code. If a file's SIP hash code cannot be found inside any digitally-signed .CAT file and the file itself contains no digital signature (see below) then the file is considered to be "unsigned". Windows File Protection gives the same priority and preference to authentic hash codes of old binaries (and other protected files) as it does to authentic hash codes of newer, updated binaries. An attacker can therefore place old authentic files containing known security vulnerabilities in place of newer files from hotfixes and service packs and WFP will automatically trust and certify the authenticity of the older files. Only manual verification of full-file hashes against known good hashes (i.e. authentic hashes) of newer OS files will properly reveal the malicious replacement. A related SECURITY ALERT issued today "Windows File Protection Arbitrary Certificate Chain Vulnerability" explains that WFP is designed to trust any certificate chain rooted at any one of Windows' Trusted Root Certification Authorities, making it easy for an attacker to replace authentic OS binaries with digitally-signed malicious code that WFP will, by design, automatically authenticate and trust. ____________________________________________________________________________ __ Discussion 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). To enable multiple trust authorities to certify the same files independently without altering the hash code computed by WHQL for its Windows Hardware Compatibility signature, WFP relies on a proprietary Subject Interface Package (SIP) [3] object file hashing mechanism that applies hash algorithms such as FEDERAL INFORMATION PROCESSING STANDARDS PUBLICATION 180-1 (SHA-1) [4] to a subset of the bits contained in any Portable Executable (PE) file rather than to the entire file (full-file hashing). In particular the "Certificates Table" data directory entry [5] in the executable’s IMAGE_DATA_DIRECTORY table located at the end of its PE header IMAGE_OPTIONAL_HEADER structure is excluded from the hashed bits by the SIP object file hash preprocessor module which is visible during debugging sessions as "WINTRUST!SIPObjectPE_". Every PE file can thus have digital signature(s) attached at-will in a production system without invalidating the file's SIP hash code, which would impact the perception of any code that computes a full-file hash for the purpose of identifying the executable portion of the PE file as authentic, trustworthy, and/or trusted code. WFP uses SIP hashes to avoid this impact, the variable full-file hash, caused by the potential to apply digital signatures directly to PE files. To simplify the process of code signing, so that every file need not be signed individually and updated signatures can be deployed at run-time (e.g. when certificates expire or private keys become compromised) without replacing files that might be in use (and thus locked for writing) Windows File Protection uses Security Catalogs (.CAT files) [6] that are digitally-signed. Each .CAT file contains a list of authentic SIP hashes of trusted files that Windows File Protection (via SFC.EXE and SIGVERIF.EXE as well as automatic protection feature) considers to be valid SIP hashes. Every file that, when hashed with the help of "WINTRUST!SIPObjectPE_", results in a SIP hash code that is contained in any .CAT file is considered trustworthy by Windows File Protection, even if updates to the file (based on filename and version) have been deployed and the newest version of the authentic code in fact contains a different SIP hash from the one that Windows File Protection encounters. Windows uses its CryptCATAdminCalcHashFromFileHandle API function [7] (found in mscat32.dll under Windows 2000 and in wintrust.dll under Windows XP/.NET Server) to compute a file's SIP hash code and then its CryptCATAdminEnumCatalogFromHash API function to automatically locate the digitally-signed .CAT files, if any, that contain the file's SIP hash. If a file's SIP hash code cannot be found inside any digitally-signed .CAT file and the file itself contains no digital signature then the file is considered to be "unsigned". Windows File Protection gives the same priority and preference to authentic hash codes of old binaries (and other protected files) as it does to authentic hash codes of newer, updated binaries. An attacker can therefore place old authentic files containing known security vulnerabilities in place of newer files from hotfixes and service packs and WFP will automatically trust and certify the authenticity of the older files. A related SECURITY ALERT issued today [8] "Windows File Protection Arbitrary Certificate Chain Vulnerability" explains that WFP is designed to trust any certificate chain rooted at any one of Windows' Trusted Root Certification Authorities, making it easy for an attacker to replace authentic OS binaries with digitally-signed malicious code that WFP will, by design, automatically authenticate and trust. 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 SIP hash can still be found in an old Security Catalog file. 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) Delete the Security Catalogs (.CAT files) provided by your vendors. Produce your own instead, and sign them with 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 Note the following instructions provided by Microsoft for producing Security Catalogs using the MakeCat utility: http://msdn.microsoft.com/library/en-us/security/Security/makecat.asp http://msdn.microsoft.com/library/en-us/security/Security/using_makecat.asp Note the following caveat that requires certain Root CA certificates to remain trusted in order to avoid breaking WFP entirely. Q293781 Trusted Root Certificates That Are Required By Windows 2000 http://support.microsoft.com/support/kb/articles/293/7/81.ASP (Adequate Protection) 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: Q324036 HOW TO: Use Software Restriction Policies in the Windows .NET Server Family http://support.microsoft.com/support/kb/articles/324/0/36.ASP Q310791 Description of the Software Restriction Policies in Windows XP http://support.microsoft.com/default.aspx?scid=KB;en-us;310791 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): Q296241 Windows File Protection May Not Start http://support.microsoft.com/default.aspx?scid=kb;EN-US;296241 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: Q293819 How to Remove a Root Certificate from the Trusted Root Store http://support.microsoft.com/default.aspx?scid=kb;EN-US;293819 (Common Sense) 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: Tamper-Resistant Apps Cryptographic Hash Algorithms Let You Detect Malicious Code in ASP.NET by Jason Coombs http://www.msdn.microsoft.com/msdnmag/issues/02/09/ASPNETHashAlgorithms/defa ult.aspx ____________________________________________________________________________ __ 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 let mutually-exclusive Security Catalog files exist in production systems simply because it works, somewhat, today, when nobody tampers with a Windows box on purpose. Shipping a hotfix or service pack with a new Security Catalog file without a mechanism to remove the out-of-date .CAT file(s) when the new ones are installed defeats a core purpose of Windows File Protection entirely: simplifying the process of distributing authentic hashes. Even Windows File Protection is unable to determine which of the SIP hashes is the most-current "authentic hash" of a given file. A redesign of this whole process is necessary, with enhancements to the Security Catalog file format. This author recommends inclusion of full-file hashes and filenames, file sizes, and file version numbers accompanying each and every authentic SIP hash so that end-users can, if they wish, utilize standard full-file hashing tools on a platform of their choice to verify the authenticity of the code they plan to deploy (and trust) to a production Windows box. 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. ____________________________________________________________________________ __ Responsible Disclosure The Internet Draft known as draft-christey-wysopal-vuln-disclosure-00.txt formerly located at the following URL has expired and has been removed from publication: http://www.ietf.org/internet-drafts/draft-christey-wysopal-vuln-disclosure-0 0.txt Neither its authors nor any other party chose to advance a responsible disclosure standard through any IETF working group due to lack of interest. Therefore the following observations take priority as de facto "best practices" for information security and encryption research and responsible communication of security- and cryptography-related vulnerability findings: A. Full disclosure made directly to those who care enough about security to read security alert and advisory documents like this one is an effective way to communicate vulnerability details to those who most urgently need them and who are most likely to act upon them. B. There are always mitigating factors; and there may be imperfections in this author's forensic analysis of the information security vulnerability described in this communication due to time constraints and the need to disseminate the information that resulted from the author's information security and encryption research in a timely manner so that it can be peer-reviewed, confirmed widely through experiments conducted independently by other researchers, and custom-tailored to the needs and circumstances of those who are affected by the discovery. This author relies on the information security community at large to identify and document every permutation of legitimate mitigating factors. C. THE MOST IMPORTANT MITIGATING FACTOR IS ALWAYS AN INFORMED POPULATION THAT IS READY, WILLING, AND ABLE TO ACT WHEN ACTION IS REQUIRED TO ENSURE THE SECURITY AND INTEGRITY OF INFORMATION SYSTEMS AND PROTECT STAKE-HOLDERS WHO WOULD OTHERWISE BE BOTH AT-RISK AND UNINFORMED; IT IS IRRESPONSIBLE FOR A SECURITY RESEARCHER TO TRUST SOMEBODY ELSE TO DISSEMINATE IMPORTANT INFORMATION ABOUT NEW VULNERABILITIES AND IT IS FURTHER IRRESPONSIBLE FOR A PERSON WHO KNOWS OF A SECURITY VULNERABILITY TO KEEP IT SECRET FOR A PROLONGED PERIOD OF TIME IN THE IRRATIONAL AND NARCISSISTIC HOPE THAT NOBODY ELSE IS SMART ENOUGH TO FIND THE SAME VULNERABILITY. D. A small, highly-skilled and diligent distributed group of self-coordinating, self-organizing infosec experts who know each other and communicate freely is far more capable of responding to security incidents and moving forward any and all preventative measures necessary to minimize the security risk and imminent dangers of any infosec threat than are dozens of people and organizations compromised by politics and fear. To ensure continued high-quality, timely, and accurate vulnerability disclosure requires peer-reviewed communication free from restrictive and oppressive forces. Those who pose a threat to information security have this freedom to communicate because they take it or they make it even though it requires them to take personal risk. For information security professionals and the United States Government to deny themselves, their employees, and citizens this same freedom as a defense against attack is not only counter-productive it is also insane. E. The Digital Millennium Copyright Act (DMCA) makes it a crime in the United States (including Hawaii, Alaska, the U.S. Virgin Islands, Guam, and so forth) to circumvent computer security devices and algorithms that have the effect of protecting copyrighted works from unauthorized access, copying, modification, tampering, or reverse engineering unless there is a legitimate information security research purpose and the researcher's methodology meets certain requirements. If this author was, or henceforth is, physically present in a jurisdiction regulated by the DMCA when any copy of this communication was/is authored, or any portion of its communicated information security and/or encryption research was/is physically conducted by this author, this author hereby invokes the information security exemptions contained in the DMCA section 1201 and other sections, subsections, and paragraphs. The information security analysis performed by the author of this communication was conducted on equipment owned by the author or to which the author was granted authorized access. Each copyrighted work relied upon by the author while performing information security testing in connection with this communication was duly licensed to the author, the author's employer, and/or the entity who authorized the author to conduct information security and/or encryption research using same, to the best of the author's knowledge. The author hereby disclaims any and all liability, both civil and criminal, for benefits the author received from copyright violations potentially committed by others and the author further asserts freedom from criminal prosecution as an individual by virtue of the fact that author may have been acting in the capacity of employee, board member, or other representative of (an) employer(s) rather than acting in the capacity of individual when the author prepared this communication for distribution and conducted the aforementioned information security analysis, penetration, circumvention, reverse engineering, and/or encryption research. F. The DMCA section 1201 "Circumvention of copyright protection systems" also includes provisions for "PERMISSIBLE ACTS OF ENCRYPTION RESEARCH". There should be no concern on the part of any security researcher or cryptographer that communicating the results of an ethical information security analysis might result in arrest and prosecution for violation of the DMCA or other laws. THE DMCA DOES NOT TRUMP THE FIRST AMENDMENT. The author of this document hereby declares this communication to be protected speech as defined by prevailing Constitutional law interpretation of Amendment I of the United States Constitution; this speech is protected because of its political nature, because the author was/is forced by the existence of laws and the existence of irrational legal- and/or peer-pressure to fear prosecution or hardship resulting from this communication, because it represents the author's exercise of a freedom to assemble insofar as this communication is a call for an assembly of the author's peers for the purpose of analysis of the aforementioned security vulnerability, an assembly that may in fact occur in meatspace as well as cyberspace, and because it petitions the U.S. Government to relieve the present atmosphere of uncertainty it has created and/or allowed to be created with respect to the freedom of information security researchers to carry out unauthorized penetrations and circumventions of information security, copyright, and/or digital access control systems whenever the circumventions or access control penetrations occur without explicit permission from every copyright-holder, patent-holder, or other interested party whose rights and reasonable expectations of law enforcement protection might have been violated or infringed. The U.S. Government portends in the language of its legislation that a person must not be criminally liable for penetration testing her own door lock, but fails to distinguish between the act as a protected exemption that violates no law and subjects the owner of the door (and of the lock) to no possible civil or criminal liability, and the subsequent detailed communication of the act inclusive of instructions that are executable by a digital machine that enable other owners of doors (and locks) of a similar design to benefit from this security and/or cryptography research. Therefore, while this author is certain of impunity in all U.S. civil and criminal courts for information security research and encryption research actions taken by this author prior to this communication, this author has reason to fear that the content of this communication, should it be deemed to be sufficiently-detailed so as to be usable as a tool of digital system penetration and/or circumvention, could create devastating legal problems for this author sufficient to destroy the rest of this author's life and significantly damage the lives of this author's dependents. This author participates in the action of authorship and takes credit for this communication openly in spite of the extreme risk it may represent, confident that unjust abusive prosecution, violations of this author's various Constitutional rights, and abusive civil lawsuits that are criminal acts on behalf of the plaintiff in many jurisdictions, and which may in fact be criminal acts in the jurisdiction local to this author, will not be tolerated by a just society. G. With respect to registered Trademarks and copyrighted materials referenced or contained wholly in this communication the author claims fair use under Title 17 of United States Code with respect to alleged copyright infringement; and the author claims the privilege of media communication made in the public interest (freedom of the press) with respect to alleged violations of United States Federal law, State and municipal laws, and/or international Treaties that seek to regulate this communication or control subsequent access to it. H. Like an IETF Working Group or an open source or free software/GNU development effort, anyone who wishes to do so and who has something of value to contribute can contact infosec peers and solicit the forensic analysis help of any other security coordinator, infosec, or forensics expert without fear of prosecution for criminal conspiracy. In practice, contacting a vendor expecting forensic analysis assistance is futile; vendors will take a new vulnerability report and conduct their own forensic analysis but won't reveal additional aspects of a vulnerability to you because you are untrustworthy. The vendor has no incentive to spread vulnerability information and you have no "need to know" more than the vendor chooses to tell you about the scope of the vulnerability you discovered. Entrusting vendors with exclusive possession of vulnerability details is counterproductive to the desired end-result of secure information systems and properly hardened security policies; the analysis capabilities of security researchers who are not restricted by employment contracts, confidentiality agreements, and other impairments are superior in every respect and in every instance thus far examined by this author. I. This entire communication is Copyright (C) 2002-9999 by its author and/or the author's employer(s), renewed annually through the creation of derived works and potential copyright registration with the Library of Congress, and all rights are reserved. You are hereby granted a limited right to access this communication and distribute copies of this communication for the limited purposes of information security, encryption research, or fair use reproduction/citation. All other reproduction and access rights are reserved by the Copyright holder. J. Notice is hereby served that patent rights may exist benefiting the author and/or the author's employer(s) in any or all work, methods, and discoveries communicated herein. Should such patent rights ever be claimed by a third-party in any jurisdiction covered by U.S. Federal law or international treaty to which the United States is a party, author and/or author's employer hereby claim right of priority. Failure to file an application for patent within the statutory window of opportunity for exercising a right of priority shall in no way diminish the author's right to file, or cause to be filed, a Statutory Invention Registration (SIR) patent with the United States Patent and Trademark Office (USPTO); the potential prosecution of which can be inferred by public distribution of this communication and this notice. Any patentable matter, method, process, algorithm, or other intellectual property deserving of patent protection expressed in this communication is now prior art, subject to the aforementioned right of priority and right to prosecute application for SIR. ____________________________________________________________________________ __ References [1] Driver Signing for Windows http://www.microsoft.com/hwdev/driver/digitsign.asp [2] Driver Signing / File Protection http://www.microsoft.com/hwdev/driver/drvsign.asp [3] Subject Interface Package (SIP) Functions and documentation CryptSIPAddProvider, CryptSIPRemoveProvider, SIP_ADD_NEWPROVIDER http://msdn.microsoft.com/library/en-us/security/security/cryptography_funct ions.asp [4] Secure Hash Standard (SHA-1) http://csrc.nist.gov/publications/fips/fips180-1/fip180-1.txt [5] Microsoft Portable Executable and Common Object File Format Specification v6.0 Appendix: Calculating Image Message Digests Fields Not To Include In Digests http://www.microsoft.com/hwdev/hardware/PECOFF.asp [6] Security Catalog (.CAT File) MakeCat Utility http://msdn.microsoft.com/library/en-us/security/Security/makecat.asp http://msdn.microsoft.com/library/en-us/security/Security/using_makecat.asp [7] CryptCATAdminCalcHashFromFileHandle API Function http://msdn.microsoft.com/library/en-us/security/Security/cryptcatadmincalch ashfromfilehandle.asp [8] Windows File Protection Arbitrary Certificate Chain Vulnerability http://www.forensics.org/secalert/WFP_Arbitrary_Certificate_Chain_Vulnerabil ity.txt ____________________________________________________________________________ __ ============================================================================ ==
This archive was generated by hypermail 2b30 : Thu Dec 26 2002 - 16:46:20 PST