content: draft: Add mitigation for compromised build tooling (#1251)

This threat can be mitigated in a number of ways, here I address it in
the simplest one, verifying the tooling prior to use. You can also
imagine resolving it by recording the digests in the provenance, and
propagating VSAs so that downstream verifiers can verify recursively,
but that's pretty complicated.

You can also resolve this with the attested build environments track,
but I don't think we should mention that here until it's finalized? Or
maybe we can point to it now as 'coming soon'?

fixes #1184

---------

Signed-off-by: Tom Hennen <[email protected]>
Signed-off-by: Tom Hennen <[email protected]>
Co-authored-by: Marcela Melara <[email protected]>
Co-authored-by: Trishank Karthik Kuppusamy <[email protected]>

docs/spec/draft/threats.md

@@ -873,14 +873,24 @@ libDep, resulting in MyPackage also having the security vulnerability.
 process, which alters the build process and injects unintended behavior into the
 output artifact.
 
+*Mitigation:* This can be partially mitigated by treating build tooling,
+including OS images, as any other artifact to be verified prior to use.
+The threats described in this document apply recursively to build tooling
+as do the mitigations and examples.  A future
+[Build Environment track](future-directions#build-environment-track) may
+provide more comprehensive guidance on how to address more specfiic
+aspects of this threat.
+
 *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**
+MyPackage artifact.  Solution: [apply SLSA recursively] to all build tools
+prior to the build.  The build platform verifies the disk image,
+or the individual components on the disk image, against the associated
+provenance or VSAs prior to running a build.  Depending on where the initial
+compromise took place (i.e. before/during vs *after* the build of the build tool itself), the modified `/usr/bin/tar` will fail this verification.
 
 </details>
 
@@ -1044,6 +1054,7 @@ collision resistance.
 
 <!-- Links -->
 
+[apply SLSA recursively]: verifying-artifacts.md#step-3-optional-check-dependencies-recursively
 [authentic]: requirements.md#provenance-authentic
 [exists]: requirements.md#provenance-exists
 [isolated]: requirements.md#isolated