Introduction to the Linux Foundation’s SLSA project
Supply Chain Security is a hot topic these days. And more and more, we as developers are dealing with this daily. But what does this mean for us, and how is this influencing our job? I want to give an overview of common attacks against the Software Supply Chain from the developer’s view and will introduce the Open Source project SLSA from the Linux Foundation.
a) Who is the project SLSA
Various experts from the security field started this project to share their knowledge, leading to the mentioned project. There is no company or governmental organisation behind this project. It is a pure Open Source project under the umbrella of the Linux Foundation.
b) What is the goal of this project?
The SLSA project is open-source and will not provide its own source code. It is, therefore, not a classic open-source project to publish a specific solution. Instead, it is a documentation project to process knowledge about supply chain security in software development and make it freely accessible. The structure contains examples of where the events or attacks mentioned have been successfully taken and which countermeasures should be taken. We’ll look at the security levels provided a little later. The aim here is to allow the reader to prepare for these threats with specific steps slowly.
After all, based on your options and the current environment, you have to decide which next steps and measures make sense to implement.
c) What is the current status of the project?
The project is currently (beginning of 2022) still in the alpha phase. There is a lot of documentation, but some places still provide references to future content. So it’s always worth taking a look inside. Since this is an open-source project, you can, of course, also contribute yourself. Here you have the opportunity to make your expertise available to others.
SLSA security levels?
a) Why do we need these security levels?
When you look around the SLSA project website for the first time, the description of the security levels quickly catches your eye. But what are these levels all about? With these levels, you want to achieve two things. First, it should help to enable a self-assessment. This first consideration serves to classify the own existing environment and to identify the primary weak points. The second goal of this level is to help you plan the following purposes that should be implemented to increase your protection significantly. These recommendations are based on evaluating many projects and environments and on the security experts’ knowledge. This enables you to use the available means and opportunities to increase your security.
b) Level 0
The first level mentioned is marked with the number zero. This is the state in which no action has yet been taken. The documentation that describes the entire build process must be available and up-to-date to reach this level. This enables an initial analysis of the environment to be secured. It can also be seen as a way of taking stock.
c) Level 1
The level with the number one requires first actions on the environment. The point here is to list all the components used to create a binary file. Not only are the dependencies used listed, but all other metadata is recorded if possible. There is the term SBOM, Software Bills Of Materials in the industry. However, the term can be broadened considerably here. Within JFrog Artifactory, there is “build info”, which means a superset of SBOM. All sorts of metadata are recorded here, such as the operating system and version of the build machine, environment variables and their contents, etc. I have created my video for this.
In this video, I’ll go over the details of SBOM and Build Info.
Within the SLSA project, the term SBOM or Build Info is not explicitly mentioned. Instead, the requirement is described more generally, and it is also clearly pointed out that this level does not yet help against the compromise itself. It is only the basis for the follow-up and the comprehensive risk assessment. Based on this level, one can start with vulnerability management.
d) Level 2
Level two describes code versioning software in combination with a hosted build service. With this combination, you want to trust the build service and thus trust the origin of the generated files. Here you must take a critical look at where and who will offer these services. The fundamental consideration is based on the assumption that a specialized provider has a better grip on the issue of security within the build infrastructure than you can guarantee yourself. However, the manipulation of protection of the build service can be increased in various ways.
e) Level 3
With level three, auditors are now included in the considerations. Here, trained security specialists are used to check the existing build environment for possible vulnerabilities. As part of an accreditation process, the auditors determine whether specific standards are met to ensure the verifiability of the source and the integrity of the provenance (origin). This level offers better protection than all previous levels to ward off well-known threats. Preventing cross-build contamination is an example.
f) Level 4
In the SLSA project, level four is the highest currently achieved. To reach this level, the following conditions are set. The first requirement describes the necessity that two people constantly carry out all changes. This does not only refer to the changes made to the source code. The term is broader here and also includes all changes to the system and the configurations. This ensures that no intentional or unintentional compromise can occur at any point. Such a process has prevented most bugs, vulnerabilities and compromise attempts.
The second requirement relates to the technical environment. It is assumed here that it is a hermetic and reproducible creation process. However, a distinction should be made between the requirement for hermetic builds and reproducible builds. The former guarantees that all required dependencies are known and checked for vulnerabilities. Here, of course, it plays a role from which sources these dependencies were obtained. On the other hand, reproducible builds help detect changes and conduct subsequent analyses. However, the reproducibility of a build is not a mandatory part of the fourth level.
Even if many supply chain threads are addressed with the SLSA project, there are some limitations that I would like to list here.
1) The list of all dependencies in a project can simply become too large to be able to fully record and evaluate them. That can depend on different things. The point that shows this most clearly is based on the possibility of some dependency management systems not only specifying explicit dependency versions. In some scenarios, you can also define a version range. This now results in an exponential increase in all possible version combinations.
2) The number of all components to be backed up can exceed the available capacities. The teams then have to decide which elements are subjected to safety analysis.
3) The SLSA security levels are not transitive. This means that this component itself consists of components that have a lower security level. So here, you have to pay very close attention to the details.
4) Even if automation is the solution in many areas, there may be areas where this will not be possible. Again, this refers in many areas to closed-source projects. Here, some analyzes are simply not feasible. It must then be decided how this dependency will be classified on a case-by-case basis.
What are the requirements?
All the requirements mentioned here are a little more detailed in reality. An overview of the requirements was provided to guide what exactly is meant by each point. Here, you can see which condition belongs to which level and how this is understood. Then the details are discussed, such as what difference it makes if you only use a building service or if you also use the “Build as Code” strategy. All the details are too extensive here, so I refer to the section on the project’s website.
Supply Chain Threads
To better understand the project and its limitations, you should look at the individual types of attacks on a typical software development environment. A distinction is generally made between source, image and dependency threads. The SLSA project also delimits precisely what it includes and what it does not.
The following overview comes directly from the project and will be further developed if necessary. At this point, one must not forget that this project is only in the alpha phase (early 2022). However, these types of attacks mentioned are timeless and apply to almost every development environment.
Let’s get to the source threads. This type of attack aims to compromise the source code itself.
b.1) Bypass Code Review
The attack called “Bypass Code Review” describes a scenario in which an attempt is made to bypass the control mechanisms and directly add compromised source code to the project. Here public paths are taken to reach the destination. Such attacks were carried out on the Linux kernel, for example, via the mailing list. The best and probably simplest way to thwart such attacks is to have all source code changes checked independently by at least two people. However, at this point, one must also find ways to prevent mutual “covering up”.
b.2) Compromised Source Control System
If importing compromised source code does not work, you can try to attack the CVS itself. Attempts are being made here to make changes to the source code on the server, bypassing the official channels. A prominent example is a successful attack on the source code of PHP. The self-managed system was attacked, and the changes were imported directly as a commit. Only securing the system helps against this type of attack. To manage such a system yourself, you must have the necessary resources and knowledge. In this case, it is often easier and cheaper to use a managed system with Github.
Next, we come to the build threads.
b.3) Modified Source Code after Source Control
The source code cannot only be changed where it is stored. When a Build process is started, the source code is fetched from the specified source. The source text on the build page can also be modified before the translation process starts. One attack in which this approach was used is the “Webmin” project. Here, additional and modified source texts were injected into the translation process. The agents responsible for carrying out the individual build steps must be secured to prevent this attack. Here, too, one has to weigh up how much of this work is carried out by oneself since how safe such an environment depends primarily on one’s own abilities.
b.4) Compromised Build Platform
Since the built environment is part of the value chain, this part must also be secured classically. Here, too, it is again the case that one’s own abilities in safety must be assessed. A well-known representative of this type of attack is the Solarwinds Hack. Here, the build route was modified to compromise the newly created binary with each run.
b.5) Using Bad Dependencies – Dependency Thread
At this point, I would like to insert the dependency threads briefly. Dependencies are specifically modified and then offered. The security of the repositories plays a central role here. I’ll come back to that in a moment. However, this attack pattern refers to external locations where dependencies are fetched to process them. Particular attention should be paid to assessing the integrity of caches and mirrors of known official bodies. Often the “official” mirror servers of publicly accessible repositories do not have the same financial resources as the original. This causes attacks to be placed there.
You may be offered modified versions of known commonly used dependencies when you dock at such a place.
b.6) Bypassed CI/CD
Let’s get back to the build threads. Sometimes it is easier to bypass the CI/CD environment at an appropriate point. Here, the topics within a development path are selected where a transfer between the build server and the associated repositories is not sufficiently secured. The attack involves offering a recipient such as a repository a modified binary to make it look like it is from the CI environment. Malicious code is injected parallel to its own build infrastructure.
b.7) Compromised package repo
A component repository is also part of the infrastructure and can fall victim to an attack. As with all other elements, protection is necessary here. Attackers try to gain access to the repositories to replace the binaries of known and frequently used dependencies with modified versions. Depending on your ability to harden such a system against attacks, you have to decide whether an externally managed alternative will increase the security standard of your value chain.
b.8) Using a bad package
We come to the last attack variant mentioned here. In this scenario, known dependencies are taken as a template and then their own modified versions are offered in a public repository. Here, an attempt is made to give the own variant a name close to the original name and depicts typical spelling errors. This dependency can still be resolved if a misspelling of the original name is used in a version definition. Only in this case a modified version of it will be loaded.
The only way to protect yourself against this attack is to carefully check how each individual dependency has been defined in your own projects.
Project Persia Overview
We have seen which general types of attacks on a classic software development environment are possible. These are always generic attack patterns that can be used independently of the current products. Others also saw this as an idea to develop a general strategy to counter this. The language here is from the open-source project Persia, which is currently (early 2022) in the alpha phase. We are close to officially accepting it as an incubator in the CD Foundation. The project website is https://pyrsia.io.
The basic idea of this project is that there will be a network of trust for open-source projects. A P2P network consisting of build and distribution nodes should exist where the sources are fetched, built and offered decentrally. In addition to the classic advantages such as reliability and better utilisation of the network resources of a P2P network, increased security should also be achieved with the binaries themselves. The types of attacks on source code management software are excluded. But all subsequent attack patterns can be repelled in this way.
Unfortunately, it is not possible to fully describe the project. But at this point, I would like to draw attention to my video about this project. (https://pyrsia.io/)