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Journey — A Pull Request From a Fork

The same substrate, a different credential posture, and the phase where silence does the damage.

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Outside contributor ~300 per quarter Goal — Get a small fix tested and reviewed without being treated as an attacker Trigger — Opens a pull request from their own fork Done when — Tests ran, and the maintainer can see the result beside the diff 1 · Open PR 2 · Await run classification ◆ moment of truth 3 · Build runs 4 · Iterate 5 · Merged ◆ moment of truth What they do Opens the PR Waits, then asks Watches the log Pushes a change Maintainer merges What the platform does Classifies untrusted Runs without secrets Cache read-only Egress allow-list only Re-runs on new head Re-runs as trusted How it feels Welcome Fine Suspect Where it hurts Blocked, no reason given Slower: no cache write What the platform gains No secret reaches a fork No cache poisoning path Trusted re-run before merge Journey — A Pull Request From a Fork The trough is silence, not slowness. Telling a contributor why their build is restricted costs nothing and is the whole fix. v 1.0 · owner Platform Engineering · date 2026-09

The finding

  • The trough is Await run, and its cause is silence rather than slowness. A contributor told why their build is restricted is not frustrated; one told nothing assumes they are unwelcome.
  • The restriction that costs them most is the read-only cache, which lengthens their build. That cost is accepted deliberately: a cache writable by a fork is a code-execution path into every trusted build that reads it afterwards.

What the platform gains

  • No secret, publishing identity or cache-write credential ever reaches code the company has not reviewed.
  • Egress is restricted to the mirror and source control, so a hostile build cannot exfiltrate what it did manage to read.
  • The run re-executes as trusted at merge, so the artefact that ships was built under full credentials.

Risks

  • Maintainers under deadline pressure will want a one-click "run with secrets" for a fork. That control is the single most dangerous feature this platform could ship and is deliberately absent.