In January 2001, VeriSign mistakenly issued two code-signing certificates to someone falsely claiming to be a Microsoft employee. The certificates named “Microsoft Corporation,” but they were not genuine Microsoft certificates. They could make a signed program appear to come from Microsoft and persuade a user to approve it; they did not run code automatically or exploit a flaw in Windows.
What VeriSign issued—and why it mattered
VeriSign issued two Class 3 code-signing certificates on January 29 and January 30, 2001, to a person who fraudulently claimed to work for Microsoft. Both certificates named “Microsoft Corporation.” Microsoft said VeriSign notified it in mid-March. The identity of the purchaser was unknown in Microsoft’s March 22, 2001 bulletin, which said Microsoft and VeriSign were working with law enforcement.
A code-signing certificate helps a computer identify the publisher associated with signed software. These certificates could sign programs, including ActiveX controls and Microsoft Office macros. A user encountering a signed program could see Microsoft presented as its publisher, even though Microsoft had not issued the certificates and the software did not come from Microsoft. That misleading trust signal could make a user more likely to approve harmful software.
The certificates were limited to signing programs: they could not be used to encrypt data, sign email, or log on to Windows 2000 systems. They also did not bypass normal execution controls. Users still saw a warning and had to permit the program to run.
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Why these were not compromised Microsoft certificates
The false certificates merely asserted Microsoft’s name; they were not Microsoft’s genuine certificates. Microsoft said that none of its certificates had been compromised. The incident concerned VeriSign’s identity-verification and issuance process, not theft of Microsoft signing keys or a vulnerability in Microsoft software.
Microsoft’s MS01-017 bulletin put the distinction this way: “This issue does not meet the strict definition of a security vulnerability, because there is no flaw in any of the affected Microsoft products.” Microsoft also acknowledged that the third-party error posed a grave risk to customers. Microsoft Security Bulletin MS01-017
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Why revocation did not immediately solve the problem
After discovering the certificates, VeriSign revoked them and listed them in its certificate revocation list (CRL). Revocation is useful only if software checking a certificate can find and consult the relevant list. These certificates lacked a CRL Distribution Point (CDP)—the location information a browser would ordinarily use to retrieve a CRL—so a client could not locate VeriSign’s list through the certificates themselves.
Microsoft’s update addressed that discovery gap for the legacy systems and Internet Explorer versions covered by the bulletin. It installed a local CRL containing the fraudulent certificates, added a revocation handler to consult that list when CDP information was missing or invalid, and enabled publisher-certificate CRL checking in Internet Explorer. Microsoft said that afterward, a program signed with either certificate would produce a revoked-certificate message, with the default action preventing execution.
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How to identify the two certificates
CERT/CC recorded the validity dates and serial numbers. Both certificates were issued to Microsoft Corporation by VeriSign Commercial Software Publishers CA. CERT/CC also noted that no legitimate Microsoft certificates were issued between January 29 and 30, 2001. CERT/CC vulnerability note VU#869360
| Certificate | Validity period | Serial number |
|---|---|---|
| First false certificate | January 29, 2001–January 30, 2002 | 1B51 90F7 3724 399C 9254 CD42 4637 996A |
| Second false certificate | January 30, 2001–January 31, 2002 | 750E 40FF 97F0 47ED F556 C708 4EB1 ABFD |
Which systems the historical bulletin covered
MS01-017’s affected-software summary named Windows 95, Windows 98, Windows Me, Windows NT 4.0, Windows 2000, and Windows XP Beta 2. The bulletin said the update was included in Windows XP Gold, Windows 2000 Service Pack 2, and Internet Explorer 6. It also warned that upgrading an earlier listed Windows or Internet Explorer version could require reinstalling the update. These are historical version details, not instructions for securing currently supported systems.
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