Code signing for the Windows platform

Signing methods

The primary code signing mechanism of Windows is Authenticode. It is used to sign executables and libraries as well as installers, packages, device drivers, and PowerShell scripts. Additionally, some programs use code signing to validate add-ins on installation. They use specific package formats, such as ClickOnce (Microsoft Office) or Open Packaging Convention (Microsoft Visual Studio), and therefore require specific tools for code signing. Every program has its own rules for displaying and enforcing code signing.

A new member of the code signing family is NuGet, a platform for distribution of program libraries to software developers. NuGet is currently introducing optional code signing with its own tool set. Code signing will be verified by the NuGet Gallery (nuget.org) and displayed prominently online as well as in Visual Studio.

Applications

Windows will mark downloaded software as insecure and evaluate the code signing properties when it is first executed. It requires a signature that is cryptographically valid, based on a certificate issued by a trusted Certificate Authority, and it checks various properties of the certificate and the signature.

  • A missing or invalid signature will lead to a security warning, encouraging the user to abort the execution.
  • A valid signature will either prompt the user with the publisher name, or silently execute the program, depending on the certificate’s SmartScreen reputation.

When a program is already installed, signatures of individual program files may still be verified

  • When a program tries to make changes to the system, User Account Control (UAC) will prompt the user for permission. The software publisher is displayed in the UAC prompt.
  • When an administrator deploys a code signing policy, e.g. via Windows Code Integrity Policy or third party software. See Whitelisting and code-signing policies for more information.

Mark-of-the-Web: downloaded files and code signing

The Mark-of-the-Web (MotW) is an indication that a file has been downloaded from the internet. It is applied by browsers and other tools that download files to local disks. This indicates to Windows that the program might not be secure. When the program is started, the signature and certificate validation process is executed. All major browsers for Windows apply the MotW correctly: (Internet Explorer, Edge, Chrome, Firefox and Opera). Notably, 7zip does not propagate the MotW when unpacking. You can see (and remove) this indication in Windows Explorer by opening the program file’s Properties dialog just below the file attributes.

File types

Most downloaded programs are installation programs or packages. Popular file types include Microsoft Installer (MSI), Cabinet Files (CAB), AppX Packages and Bundles, and App-V packages. Installers are also often distributed in the form of self-extracting programs (e.g. Setup.exe). For device drivers, Windows strictly requires Authenticode signing, either by the publisher or by the Windows Hardware Quality Lab (WHQL). PowerShell scripts and modules may also require code signing, depending on the security policy and the source of the file.

Signing individual program files (Deep signing)

Installation programs and packages install the actual program files on the computer’s disk. Installed program files should be signed too, but the reason for this is less obvious, since Windows will usually not check their signatures. Here are some reasons for signing every single program file:

  • It’s just the right thing to do. By signing all executables and libraries you present yourself as a publisher that has the customer’s security in mind.
  • After installation, users and their tools have no way of checking the integrity of program files unless they are signed. A virus infection could go undetected.
  • Enterprises sometimes repackage software. This allows them to customize the configuration; for Microsoft Application Virtualization (App-V) this is a required step. Repackaging removes program files from the safety of a signed container while transferring them to another, thus putting their integrity temporarily at risk.
  • Administrators can create and deploy policies that allow program execution based on various signature-based criteria. This is called whitelisting, and is considered to be much safer than relying only on anti-malware tools.

Whitelisting and code signing policies

In the ongoing fight against malware and other security risks, enterprises are increasingly using whitelisting mechanisms. These mechanisms only allow the loading and execution of program files signed by a trusted publisher. If the software publisher does not apply signatures to all of its executables and libraries, whitelisting becomes ineffective. There are ways to work around missing signatures, such as hash-based policies, but they are hard to configure and essentially insecure: It is quite hard to ensure the integrity of program files that are received without signatures, both initially and with every update.

Windows 10 features two flavors of Code Integrity Policy mechanisms

  • Windows Defender Application Control (WDAC), formerly known as Device Guard Configurable Code Integrity
  • Hypervisor Code Integrity (HVCI)

While WDAC is integrated in the Windows kernel, HVCI works in its own environment, completely separated from the Windows OS and thereby safe even from dangerous rootkits. HVCI can only be used with compatible hardware. Both mechanisms are supported by Intelligent Security Graph (ISG), a cloud-based reputation system. They can be configured automatically by managed installers, such as the System Center Configuration Manager (SCCM).

Older versions of Windows provide different mechanisms for whitelisting: Windows 7 introduced AppLocker, and Windows XP featured Software Restriction Policies. Also, many anti-malware vendors provide their own whitelisting tools in their endpoint protection and management suites.

Redistributable files

The most obvious case for signing individual program files can be made for redistributable files, such as libraries and tools that will be distributed with other programs. Since they are included with installation packages of other programs, they are vulnerable in the entire software development process. From software developers downloading these files from unsafe sources or over unsafe connections, to malware-infected developer PCs, there are more risks of modification or infection than can be managed.

Independent software vendors, and also enterprises with in-house development departments, are increasingly aware of the risk associated with third-party libraries and tools. Once more, the only viable way of ensuring integrity is code signing by the original publisher.

Tools used for code singing

SignTool.exe (Windows SDK)

The primary tool for code signing is SignTool.exe from the Windows SDK, which can be called via the command line. When using it, you have to provide the following information:

  • The file you want to sign
  • The certificate
  • The algorithm for calculating the hash digest
  • The time stamp server (optional)

The digest algorithm defaults to SHA-1. However, you need to use SHA-2 for any practical purpose. The most common choice is SHA-256 (SHA-2 algorithm with 256-bit digests).

The certificate can be a PFX file or a certificate from a Windows certificate store.

  • A PFX file must contain the password. If it contains a password protected private key, the password must be specified using an additional parameter. Alternatively, the private key can be provided by a CSP (see below).
  • A certificate can be stored in a certificate store. In this case, you must provide the certificate name or thumbprint. (A certificate from the Windows certificate store can be chosen automatically, but this is fragile and therefore not recommended.)
  • You can also use a hardware security module through the Windows certificate store.

SHA-1 vs SHA-2

SHA-1 is no longer considered secure by the crypto community or Microsoft. Hackers may be able to forge signatures based on this outdated algorithm. Therefore, current versions of Windows will only accept signatures based on SHA-1 digests for files that were signed and time-stamped before 2016. For a signature to be considered valid, the certificate itself as well as any intermediate certificates in the certificate chain must be signed using SHA-2 too. (Note that Windows Explorer will still report SHA-1 signatures as valid in the file properties dialog, but for all purposes they will be rejected by Windows.)

For some time, it was recommended that files should be dual-signed with both SHA-1 and SHA-2 signatures for backwards compatibility, but this seems no longer necessary: SHA-2 has been in Windows starting with Windows 8 and Server 2012. Also, Windows 7 and Server 2008 have been updated in 2015 to support SHA-2.

HSMs

SignTool.exe (or Authenticode in general) cannot use HSMs directly, but it uses the Windows CryptoAPI to access certificates in Windows certificate stores. CryptoAPI uses an extensible architecture for storing certificates: A Cryptographic Service Provider (CSP) can implement physical storage of certificates as keys as well as implement cryptographic algorithms, like creating signatures. HSMs usually bring their own installable CSPs. You can think of this CSP as a device driver for the HSM. So if you use SignTool.exe referencing a certificate via its HSM’s CSP, SignTool.exe will effectively transfer the digest to the HSM, and the HSM will create the signature without ever exposing the private key.

The current implementation of Authenticode and SignTool.exe has two weaknesses:

  • It can only use CryptoAPI, but not the more recent Cryptographic API: Next Generation (CNG).
  • SHA-2 is only supported for Windows CSPs, not for third party CSPs. Some HSM vendors work around this limitation, but officially SHA-2 signatures are not available using HSMs.

Buyer’s advice

  • Whether a HSM supports SHA-2 code signing is an important consideration when buying a HSM. You should ask specifically for support of SHA-256 in Microsoft Authenticode. We also recommend that you consider the complexity of creating certificates via CSRs and accessing them before choosing a HSM.
  • Some complex key management systems have a FIPS-certified HSM at their core as part of a larger physical or virtual system. In this case, the FIPS certification might not encompass the entire system. You should ask specifically if FIPS certification is provided for the entire use case of generating keys and CSRs, as well as creating signatures.
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