Courses Software supply chain securityBuild integrity & SLSA

The SLSA framework & levels

What each level buys, and what it costs.

Advanced14 min · lesson 7 of 18

SLSA (Supply-chain Levels for Software Artifacts, said "salsa") is an OpenSSF framework that answers "how tamper-resistant is this build?" with a ladder of levels rather than a yes/no. It exists because reviewing source is not enough — SolarWinds proved the build itself can be subverted — so SLSA defines increasing guarantees about the integrity of the build process and the provenance it produces. It is the common language for "how much can I trust where this artifact came from."

The SLSA build levels (v1.0)

the ladder

L1: provenance exists

the build documents how it ran

L2: signed provenance

from a hosted build service

L3: hardened builder

isolated, non-falsifiable provenance

Each level raises the bar on how hard it is to forge the provenance. Most teams target L2→L3; higher means an attacker cannot fake "this came from our build".

What each level buys

Level 1 is simply: the build produces provenance describing how the artifact was made — useful for inventory, but the provenance could be forged. Level 2 requires that a hosted build service generates and signs that provenance, so it is authentic and tamper-evident, not something a developer hand-wrote. Level 3 hardens the builder itself — isolated, ephemeral, with no way for the build steps to influence the provenance — so even a malicious build cannot forge a legitimate-looking origin. The higher you climb, the more the phrase "this was built by our pipeline from this source" becomes something you can verify rather than assume.

What each level costs, and where to aim

The levels also cost more as you climb: L1 is a documentation habit, L2 needs a build platform that signs provenance (GitHub Actions, GitLab, and others increasingly generate it), and L3 requires genuinely isolated, hardened builders (dedicated reusable workflows, or a builder like the SLSA generators). Most organizations get large gains reaching L2 broadly — authentic, signed provenance on everything — and reserve L3 for their most critical artifacts. SLSA is a roadmap, not an all-or-nothing badge; pick the level that matches each artifact’s blast radius.

A level is a claim you must still verify

Producing SLSA provenance at some level is only half the value — the provenance has to be verified by whoever consumes the artifact, checking it meets the level and identity you require before you run it. Unverified provenance, however high the level, changes nothing; it is the verification step (covered later, at admission) that turns the SLSA claim into an enforced control.

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