Generally, a software update system is an application (or part of an application) running on a client system that obtains and installs software. These systems typically update the applications installed on client systems to introduce new features, enhancements, and security fixes.
Three major classes of software update systems are:
- Application updaters which are used by applications to update themselves. For example, Firefox updates itself through its own application updater.
- Library package managers such as those offered by many programming languages for installing additional libraries. These are systems such as Python's pip/easy_install + PyPI, Perl's CPAN, Ruby's RubyGems, and PHP's Composer.
- System package managers used by operating systems to update and install all of the software on a client system. Debian's APT, Red Hat's YUM, and openSUSE's YaST are examples of these.
There are literally thousands of different software update systems in common use today. (In fact the average Windows user has about two dozen different software updaters on their machine!)
We are building a library that can be universally (and in most cases transparently) used to secure software update systems.
On the surface, the update procedure followed by a software update system can be regarded as straightforward. Obtaining and installing an update just means:
- Knowing when an update exists.
- Downloading the update.
- Applying the changes introduced by the update.
The problem with this view is that it is only straightforward when there are no malicious parties involved throughout the update procedure. If an attacker is trying to interfere with these seemingly simple steps, there is plenty that they can do.
TUF is designed to perform the first two steps of the above update procedure, while guarding against the majority of attacks that malicious actors have at their disposal; especially those attacks that are overlooked by security-conscious developers.
Let's assume you take the approach that most systems do (at least, the ones that even try to be secure). You download both the file you want and a cryptographic signature of the file. You already know which key you trust to make the signature. You check that the signature is correct and was made by this trusted key. All seems well, right? Wrong. You are still at risk in many ways, including:
- An attacker keeps giving you the same file, so you never realize there is an update.
- An attacker gives you an older, insecure version of a file that you already have, so you download that one and blindly use it thinking it's newer.
- An attacker gives you a newer version of a file you have but it's not the newest one. It's newer to you, but it may be insecure and exploitable by the attacker.
- An attacker compromises the key used to sign these files and now you download a malicious file that is properly signed.
These are just some of the attacks software update systems are vulnerable to when only using signed files. See Security for a full list of attacks and updater weaknesses TUF is designed to prevent.
The following papers provide detailed information on securing software updater systems, TUF's design and implementation details, attacks on package managers, and package management security:
- Mercury: Bandwidth-Effective Prevention of Rollback Attacks Against Community Repositories
- Diplomat: Using Delegations to Protect Community Repositories
- Survivable Key Compromise in Software Update Systems
- A Look In the Mirror: Attacks on Package Managers
- Package Management Security
In order to securely download and verify target files, TUF requires a few extra files to exist on a repository. These are called metadata files. TUF metadata files contain additional information, including information about which keys are trusted, the cryptographic hashes of files, signatures on the metadata, metadata version numbers, and the date after which the metadata should be considered expired.
When a software update system using TUF wants to check for updates, it asks TUF to do the work. That is, your software update system never has to deal with this additional metadata or understand what's going on underneath. If TUF reports back that there are updates available, your software update system can then ask TUF to download these files. TUF downloads them and checks them against the TUF metadata that it also downloads from the repository. If the downloaded target files are trustworthy, TUF hands them over to your software update system. See Metadata for more information and examples.