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editorial: Address some of the TODOs in threats and mitigation section
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This patch focuses on merely editorial changes such as moving sections
around and changing 'out of scope' to 'not currently addressed'.

Signed-off-by: Arnaud J Le Hors <lehors@us.ibm.com>
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lehors committed Aug 27, 2024
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307 changes: 148 additions & 159 deletions docs/spec/draft/threats.md
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Expand Up @@ -15,9 +15,6 @@ The examples on this page are meant to:
- Help implementers better understand what they are protecting against so that
they can better design and implement controls.

**TODO:** Update the ordering to match the diagram. We're currently in the
middle of refactoring, with a jumble of new and old.

**TODO:** Expand this threat model to also cover "unknowns". Not sure if that is
a "threat" or a "risk". Example: If libFoo is compromised, how do you know if
you are compromised? At a first level, if you don't even know whether you
Expand All @@ -27,12 +24,6 @@ that somehow. This isn't specific to dependencies - it applies to the entire
diagram.
([discussion](https://github.com/slsa-framework/slsa/pull/1046/files/ebf34a8f9e874b219f152bad62673eae0b3ba2c3#r1585440922))

**TODO:** Revisit and possibly update any item that says "out of scope", since
we want to really expand SLSA's scope to include all of these threats. A more
nuanced answer would be that the current version does not define a specific
mitigation, but it's in scope for the project overall. We can also list specific
mitigations even if they're not listed as a level requirement.

<article class="threats">

## Overview
Expand Down Expand Up @@ -264,24 +255,24 @@ does not accept this because the version X is not considered reviewed.

*Threat:* Two trusted persons collude to author and approve a bad change.

*Mitigation:* **Outside the scope of SLSA.** We use "two trusted persons" as a
proxy for "intent of the software producer".
*Mitigation:* This threat is not currently addressed by SLSA. We use "two
trusted persons" as a proxy for "intent of the software producer".

</details>
<details><summary>Trick reviewer into approving bad code</summary>

*Threat:* Construct a change that looks benign but is actually malicious, a.k.a.
"bugdoor."

*Mitigation:* **Outside the scope of SLSA.**
*Mitigation:* This threat is not currently addressed by SLSA.

</details>
<details><summary>Reviewer blindly approves changes</summary>

*Threat:* Reviewer approves changes without actually reviewing, a.k.a. "rubber
stamping."

*Mitigation:* **Outside the scope of SLSA.**
*Mitigation:* This threat is not currently addressed by SLSA.

</details>

Expand Down Expand Up @@ -339,13 +330,22 @@ change to MyPackage without review.
*Threat:* Exploit a vulnerability in the implementation of the source code
management system to bypass controls.

*Mitigation:* **Outside the scope of SLSA.**
*Mitigation:* This threat is not currently addressed by SLSA.

</details>

### (D) External build parameters
## Build threats

**TODO:** Move under "Build threats".
A build integrity threat is a potential for an adversary to introduce behavior
to an artifact without changing its source code, or to build from a
source, dependency, and/or process that is not intended by the software
producer.

The SLSA Build track mitigates these threats when the consumer
[verifies artifacts](verifying-artifacts.md) against expectations, confirming
that the artifact they received was built in the expected manner.

### (D) External build parameters

An adversary builds from a version of the source code that does not match the
official source control repository, or changes the build parameters to inject
Expand Down Expand Up @@ -433,141 +433,6 @@ the source repo does not match the expected value.

</details>

## Dependency threats

**TODO:** Move after Usage Threats.

A dependency threat is a potential for an adversary to introduce unintended
behavior in one artifact by compromising some other artifact that the former
depends on at build time. (Runtime dependencies are excluded from the model, as
[noted below](#runtime-dep).)

Unlike other threat categories, dependency threats develop recursively through
the supply chain and can only be exploited indirectly. For example, if
application *A* includes library *B* as part of its build process, then a build
or source threat to *B* is also a dependency threat to *A*. Furthermore, if
library *B* uses build tool *C*, then a source or build threat to *C* is also a
dependency threat to both *A* and *B*.

This version of SLSA does not explicitly address dependency threats, but we
expect that a future version will. In the meantime, you can [apply SLSA
recursively] to your dependencies in order to reduce the risk of dependency
threats.

- **TODO:** Should we distinguish 1P vs 3P boundaries in the diagram, or
otherwise visualize 1P/3P?
- **TODO:** Expand to cover typosquatting, dependency confusion, and other
"dependency" threats.
- **TODO:** The word "compromised" is a bit too restrictive. If the publisher
intends to do harm, either because they tricked you into using a dependency
(typosquatting or dependency confusion), or because they were good and now
do something bad, that's not really "compromised" per se.
- **TODO:** Should we expand this to cover "transitive SLSA verification"?
- **TODO:** Update the Terminology page to show "build time" vs "runtime",
since the latter term results in confusion. Also consider the term "deploy
time" as an alternative.

[apply SLSA recursively]: verifying-artifacts.md#step-3-optional-check-dependencies-recursively

### Build dependency

An adversary compromises the target artifact through one of its build
dependencies. Any artifact that is present in the build environment and has the
ability to influence the output is considered a build dependency.

<details id="included-dep"><summary>Include a vulnerable dependency (library, base image, bundled file, etc.)</summary>

*Threat:* Statically link, bundle, or otherwise include an artifact that is
compromised or has some vulnerability, causing the output artifact to have the
same vulnerability.

*Example:* The C++ program MyPackage statically links libDep at build time. A
contributor accidentally introduces a security vulnerability into libDep. The
next time MyPackage is built, it picks up and includes the vulnerable version of
libDep, resulting in MyPackage also having the security vulnerability.

*Mitigation:* **TODO**

</details>
<details id="build-tool"><summary>Use a compromised build tool (compiler, utility, interpreter, OS package, etc.)</summary>

*Threat:* Use a compromised tool or other software artifact during the build
process, which alters the build process and injects unintended behavior into the
output artifact.

*Example:* MyPackage is a tarball containing an ELF executable, created by
running `/usr/bin/tar` during its build process. An adversary compromises the
`tar` OS package such that `/usr/bin/tar` injects a backdoor into every ELF
executable it writes. The next time MyPackage is built, the build picks up the
vulnerable `tar` package, which injects the backdoor into the resulting
MyPackage artifact.

*Mitigation:* **TODO**

</details>

Reminder: dependencies that look like [runtime dependencies](#runtime-dep)
actually become build dependencies if they get loaded at build time.

<details id="runtime-dep-at-build-time"><summary>Use a compromised runtime dependency during the build (for tests, dynamic linking, etc.)</summary>

*Threat:* During the build process, use a compromised runtime dependency (such
as during testing or dynamic linking), which alters the build process and
injects unwanted behavior into the output.

**NOTE:** This is technically the same case as [Use a compromised build
tool](#build-tool). We call it out to remind the reader that
[runtime dependencies](#runtime-dep) can become build dependencies if they are
loaded during the build.

*Example:* MyPackage has a runtime dependency on package Dep, meaning that Dep
is not included in MyPackage but required to be installed on the user's machine
at the time MyPackage is run. However, Dep is also loaded during the build
process of MyPackage as part of a test. An adversary compromises Dep such that,
when run during a build, it injects a backdoor into the output artifact. The
next time MyPackage is built, it picks up and loads Dep during the build
process. The malicious code then injects the backdoor into the new MyPackage
artifact.

*Mitigation:* In addition to all the mitigations for build tools, you can often
avoid runtime dependencies becoming build dependencies by isolating tests to a
separate environment that does not have write access to the output artifact.

</details>

### Related threats

The following threats are related to "dependencies" but are not modeled as
"dependency threats".

<details id="runtime-dep"><summary>Use a compromised dependency at runtime <span>(modeled separately)</span></summary>

*Threat:* Load a compromised artifact at runtime, thereby compromising the user
or environment where the software ran.

*Example:* MyPackage lists package Dep as a runtime dependency. Adversary
publishes a compromised version of Dep that runs malicious code on the user's
machine when Dep is loaded at runtime. An end user installs MyPackage, which in
turn installs the compromised version of Dep. When the user runs MyPackage, it
loads and executes the malicious code from Dep.

*Mitigation:* N/A - This threat is out of scope of SLSA. SLSA's threat model
does not explicitly model runtime dependencies. Instead, each runtime dependency
is considered a distinct artifact with its own threats.

</details>

## Build threats

A build integrity threat is a potential for an adversary to introduce behavior
to an artifact without changing its source code, or to build from a
source, dependency, and/or process that is not intended by the software
producer.

The SLSA Build track mitigates these threats when the consumer
[verifies artifacts](verifying-artifacts.md) against expectations, confirming
that the artifact they received was built in the expected manner.

### (E) Build process

An adversary introduces an unauthorized change to a build output through
Expand Down Expand Up @@ -808,16 +673,140 @@ from the public registry instead of the internal one.
*Threat:* Register a package name that is similar looking to a popular package
and get users to use your malicious package instead of the benign one.

*Mitigation:* **Mostly outside the scope of SLSA.** That said, the requirement
to make the source available can be a mild deterrent, can aid investigation or
ad-hoc analysis, and can complement source-based typosquatting solutions.
*Mitigation:* This threat is not currently addressed by SLSA. That said, the
requirement to make the source available can be a mild deterrent, can aid
investigation or ad-hoc analysis, and can complement source-based typosquatting
solutions.

</details>

### (I) Usage

**TODO:** What should we put here?


## Dependency threats

A dependency threat is a potential for an adversary to introduce unintended
behavior in one artifact by compromising some other artifact that the former
depends on at build time. (Runtime dependencies are excluded from the model, as
[noted below](#runtime-dep).)

Unlike other threat categories, dependency threats develop recursively through
the supply chain and can only be exploited indirectly. For example, if
application *A* includes library *B* as part of its build process, then a build
or source threat to *B* is also a dependency threat to *A*. Furthermore, if
library *B* uses build tool *C*, then a source or build threat to *C* is also a
dependency threat to both *A* and *B*.

This version of SLSA does not explicitly address dependency threats, but we
expect that a future version will. In the meantime, you can [apply SLSA
recursively] to your dependencies in order to reduce the risk of dependency
threats.

- **TODO:** Should we distinguish 1P vs 3P boundaries in the diagram, or
otherwise visualize 1P/3P?
- **TODO:** Expand to cover typosquatting, dependency confusion, and other
"dependency" threats.
- **TODO:** The word "compromised" is a bit too restrictive. If the publisher
intends to do harm, either because they tricked you into using a dependency
(typosquatting or dependency confusion), or because they were good and now
do something bad, that's not really "compromised" per se.
- **TODO:** Should we expand this to cover "transitive SLSA verification"?
- **TODO:** Update the Terminology page to show "build time" vs "runtime",
since the latter term results in confusion. Also consider the term "deploy
time" as an alternative.

[apply SLSA recursively]: verifying-artifacts.md#step-3-optional-check-dependencies-recursively

### Build dependency

An adversary compromises the target artifact through one of its build
dependencies. Any artifact that is present in the build environment and has the
ability to influence the output is considered a build dependency.

<details id="included-dep"><summary>Include a vulnerable dependency (library, base image, bundled file, etc.)</summary>

*Threat:* Statically link, bundle, or otherwise include an artifact that is
compromised or has some vulnerability, causing the output artifact to have the
same vulnerability.

*Example:* The C++ program MyPackage statically links libDep at build time. A
contributor accidentally introduces a security vulnerability into libDep. The
next time MyPackage is built, it picks up and includes the vulnerable version of
libDep, resulting in MyPackage also having the security vulnerability.

*Mitigation:* **TODO**

</details>
<details id="build-tool"><summary>Use a compromised build tool (compiler, utility, interpreter, OS package, etc.)</summary>

*Threat:* Use a compromised tool or other software artifact during the build
process, which alters the build process and injects unintended behavior into the
output artifact.

*Example:* MyPackage is a tarball containing an ELF executable, created by
running `/usr/bin/tar` during its build process. An adversary compromises the
`tar` OS package such that `/usr/bin/tar` injects a backdoor into every ELF
executable it writes. The next time MyPackage is built, the build picks up the
vulnerable `tar` package, which injects the backdoor into the resulting
MyPackage artifact.

*Mitigation:* **TODO**

</details>

Reminder: dependencies that look like [runtime dependencies](#runtime-dep)
actually become build dependencies if they get loaded at build time.

<details id="runtime-dep-at-build-time"><summary>Use a compromised runtime dependency during the build (for tests, dynamic linking, etc.)</summary>

*Threat:* During the build process, use a compromised runtime dependency (such
as during testing or dynamic linking), which alters the build process and
injects unwanted behavior into the output.

**NOTE:** This is technically the same case as [Use a compromised build
tool](#build-tool). We call it out to remind the reader that
[runtime dependencies](#runtime-dep) can become build dependencies if they are
loaded during the build.

*Example:* MyPackage has a runtime dependency on package Dep, meaning that Dep
is not included in MyPackage but required to be installed on the user's machine
at the time MyPackage is run. However, Dep is also loaded during the build
process of MyPackage as part of a test. An adversary compromises Dep such that,
when run during a build, it injects a backdoor into the output artifact. The
next time MyPackage is built, it picks up and loads Dep during the build
process. The malicious code then injects the backdoor into the new MyPackage
artifact.

*Mitigation:* In addition to all the mitigations for build tools, you can often
avoid runtime dependencies becoming build dependencies by isolating tests to a
separate environment that does not have write access to the output artifact.

</details>

### Related threats

The following threats are related to "dependencies" but are not modeled as
"dependency threats".

<details id="runtime-dep"><summary>Use a compromised dependency at runtime <span>(modeled separately)</span></summary>

*Threat:* Load a compromised artifact at runtime, thereby compromising the user
or environment where the software ran.

*Example:* MyPackage lists package Dep as a runtime dependency. Adversary
publishes a compromised version of Dep that runs malicious code on the user's
machine when Dep is loaded at runtime. An end user installs MyPackage, which in
turn installs the compromised version of Dep. When the user runs MyPackage, it
loads and executes the malicious code from Dep.

*Mitigation:* N/A - This threat is not currently addressed by SLSA. SLSA's
threat model does not explicitly model runtime dependencies. Instead, each
runtime dependency is considered a distinct artifact with its own threats.

</details>

## Availability threats

**TODO:** Merge into the list above rather than having a separate section.
Expand Down Expand Up @@ -849,23 +838,23 @@ retention and availability of the source code.

*Threat:* Unable to perform a build with the intended dependencies.

*Mitigation:* **Outside the scope of SLSA.** That said, some solutions to
support hermetic and reproducible builds may also reduce the impact of this
threat.
*Mitigation:* This threat is not currently addressed by SLSA. That said, some
solutions to support hermetic and reproducible builds may also reduce the
impact of this threat.

</details>
<details><summary>De-list artifact</summary>

*Threat:* The package registry stops serving the artifact.

*Mitigation:* N/A - This threat is out of scope of SLSA v1.0.
*Mitigation:* N/A - This threat is not currently addressed by SLSA.

</details>
<details><summary>De-list provenance</summary>

*Threat:* The package registry stops serving the provenance.

*Mitigation:* N/A - This threat is out of scope of SLSA v1.0.
*Mitigation:* N/A - This threat is not currently addressed by SLSA.

</details>

Expand Down

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