A change board approved an image by sha256 digest but the deployment names a mutable tag - what is the risk?

## Two different kinds of name A container image can be named two ways. A **digest** reference (`payments@sha256:1a2b...`) is a cryptographic hash of the image manifest: it names one specific set of bytes, and a client that pulls it recomputes the hash and rejects anything that does not match. Nobody can change what a digest points at, because the digest *is* the content. A **tag** reference (`payments:release-2026.04`) is a mutable pointer stored in the registry: a row that says "this name currently maps to that digest". Whoever can push to the repository can rewrite that row. That difference is the whole question. Pinning to a digest is an integrity control - it makes the identity of the artifact non-negotiable. Naming a tag is a lookup that happens later, under conditions you do not control. ## Where the gap opens Review and execution are two separate moments, and they can be hours or weeks apart. The change board looks at an artifact identified by digest: the scan results, the test evidence and the sign-off are all about those bytes. The deployment, meanwhile, says `:release-2026.04`. At the moment somebody actually pulls, the registry is asked what that name means *now* - not what it meant at review time. So the approval covers a digest, the runtime resolves a tag, and no mechanism compares them. An insider on the release team, or an attacker who has taken over a release account, pushes a modified build and repoints the tag. In a payment-settlement service that means the code moving money is provably not the code the board approved - and the damage is not only the malicious behaviour, it is that the audit trail now says something false. ## Who can actually move the tag The honest blast radius is the union of every identity holding push rights to that repository: release engineers, other teams sharing the repo, every CI job holding a push token, service accounts nobody has rotated, and registry administrators. That set is nearly always much wider than the set of people who reviewed the change, and it is rarely enumerated until an incident forces it. ## Why nothing detects it The usual change-detection tooling watches *your* files. A tag repoint touches nothing you own: no commit, no pull request, no deployment diff, no configuration change. From the outside the system looks completely stable. Worse, the swap does not need a deployment to take effect - a pod restart, a scale-out event, a node replacement or an eviction is enough, because each of those triggers a pull. The change can therefore land at an arbitrary time long after the push, which also makes it hard to correlate with anything. ## What the fix looks like The rule is simple: **the reference that is actually pulled must carry the digest**. Practically that means: - The deployment configuration in source control names `payments@sha256:...`, not a tag. The approved digest and the deployed digest are then the same string, diffable in review. - A tag may still exist for humans, for release notes and for tooling - it just must not be the thing the runtime resolves. - Anything that resolves a tag on the way to deployment must persist the digest it resolved, not throw it away. Two supporting controls make this durable rather than a convention. First, restrict who can push to release repositories so the set of identities able to move a tag is small and audited. Second, where the registry supports it, forbid re-aiming existing tags, so an overwrite is rejected at the source. ## What the pin still does not give you Be precise about the scope of the guarantee, because overclaiming here is a common interview failure. A digest pin says *these exact bytes*. It says nothing about whether those bytes are good: a reviewed-and-approved image can still be vulnerable, backdoored by whoever built it, or built from source nobody looked at. A digest is an integrity claim about identity, not a quality claim and not a statement about origin. It closes exactly one hole - substitution after approval - and that is the hole this question is about.