Networking / Beginners

Understanding Code Signing

Digital certificates are assigned through CAs. A CA is a vendor that associates a public key with the person applying for the certificate. One of the largest organizations to provide such code signing certificates is VeriSign ( An Authenticode certificate is used for software publishing and timestamp services. It can be attached to the file a programmer is distributing and allows users to identify that it is a valid, unadulterated file.

Digital certificates can be applied to a number of different file types. For example, using such tools as Microsoft Visual Studio's CryptoAPI tools and VeriSign code signing certificates, developers can sign such files as the following:

  • .EXE An executable program
  • .CAB Cabinet files commonly used for the installation and setup of applications; contain numerous files that are compressed in the cabinet file
  • .CAT Digital thumbprints used to guarantee the integrity of files
  • .OCX ActiveX controls
  • .DLL Dynamic link library files, containing executable functions
  • .STL Contains a certificate trust list

When a person downloads a file with a digital certificate, the status of that certificate is checked through the CA. If the certificate is not valid, the user will be warned. If it is found to be valid, a message will appear stating that the file has a valid certificate.The message will contain additional information and will show to whom the certificate belongs. When the user agrees to install the software, it will begin the installation.

The Benefits of Code Signing

Digital signatures can be used to guarantee the integrity of files and that the package being installed is authentic and unmodified.This signature is attached to the file being downloaded, and identifies who is distributing the files and shows that they have not been modified since being created.The certificate helps to keep malicious users from impersonating someone else.

This is the primary benefit of code signing. It provides users with the identity of the software's creator. It allows them to know who manufactured the program and provides them with the option of deciding whether to trust that person or company. When the browser is about to download the component, a warning message is displayed, allowing them to choose whether it is to be installed or loaded into memory.This puts the option of running it in the user's hands.

Problems with the Code Signing Process

A major problem with code signing is that you must rely on a third party for checking authenticity. If a programmer provided fake information to a CA or stole the identity of another individual or company, they could then effectively distribute a malicious program over the Internet.The deciding factor here would be the CA's ability to check the information provided when the programmer applied for the certificate.

Another problem occurs when valid information is provided to the CA, but the certificate is attached to software that contains bad or malicious code. An example of such a problem with code signing is seen in the example of Internet Exploder, an ActiveX control that was programmed by Fred McLain.This programmer obtained an Authenticode certificate through VeriSign. When users running Windows 95 with Advanced Power Management ran the code for Internet Exploder, it would perform a clean shutdown of their systems.The certificate for this control was later revoked. Certificate Revocation Lists (CRLs), which store a listing of revoked certificates, can also be problematic.Web browsers and Internet applications rarely check certificate revocation lists, so it is possible for a program to be used even though its certificate has been revoked. If a certificate was revoked, but its status was not checked, the software could appear to be okay even though it has been compromised.

These problems with code signing do not necessarily apply to any given CA. Certificates can also be issued within an intranet using software such as Microsoft Certificate Server. Using this server software, users can create a CA to issue their own digital certificates for use on a network.This allows technically savvy individuals to self-sign their code with their own CA and gives the appearance that the code is valid and secure.Therefore, users should always verify the validity of the CA before accepting any files.The value of any digital certificate depends entirely on how much trust there is in the CA that issued it. By ensuring that the CA is a valid and reputable one, administrators can avoid installing a hacker's code onto their system.

An additional drawback to code signing for applications distributed over the Internet is that users must guess and choose whom they trust and whom they do not.The browser displays a message informing them of who the creator is, a brief message about the dangers of downloading any kind of data, and then leave it up to the user whether to install it or not.The browser is unable to verify code.

As a whole, code signing is a secure and beneficial process, but as with anything dealing with computers, there are vulnerabilities that may be exploited by hackers. An example of this was seen in 2003, when a vulnerability was identified in Authenticode verification that could result in a hacker installing malicious software or executing code remotely.The vulnerability affected a wide number of Windows OSes, including Windows NT,Windows 2000,Windows XP, and Windows 2003 Server. Under certain low memory conditions on the computer, a user could open HTML e-mail or visit a Web site that downloads and installs an ActiveX control without prompting the user for permission. Because a dialog box isn't displayed, the user isn't asked whether they want to install the control, and has no way of verifying its publisher or whether it's been tampered with. As such, a malicious program could be installed that allows a hacker to run code remotely with the same privileges as the user who's logged in. Although a security patch is available that fixes this problem, it shows that Authenticode isn't immune to vulnerabilities that could be exploited.

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