Harden pr-merge's run-cache restore in the token-holding jobs #9455

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1. github-actions

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   needs-triageNot yet classified. Floor until a type and one priority tier are set.Not yet classified. Floor until a type and one priority tier are set.

   on Oct 23, 2026
2. github-actions commented on Oct 23, 2026

   

   github-actionsBot

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   Proposed work class: none

   Thanks for the report. Automated triage handed this issue to a maintainer.

   _{Automated triage. A maintainer reviews it.}
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   needs-triageNot yet classified. Floor until a type and one priority tier are set.Not yet classified. Floor until a type and one priority tier are set.

   on Oct 23, 2026
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5. melodic-automation-lanes

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   status: needs-decisionAwaiting a human or maintainer judgment call.Awaiting a human or maintainer judgment call.

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6. kyle-sexton commented on Oct 23, 2026

   

   kyle-sexton

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   The claude-security-review lane on #9450 (#9450 (comment)) suggests an integrity check on the carried run cache: an HMAC, or a check that a main-branch pr-merge run saved it. It also suggests reading the cache back through the artifact path. Today the carried sections can only count a FAIL (carry-run-cache.mjs:9-11; Reader._run_state reads a PASS's run fresh), so this protects the refuse direction and the extraction path, not the pass direction.
7. kyle-sexton commented on Oct 26, 2026

   

   kyle-sexton

   on Oct 26, 2026

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   Owner question (security), not a decision.

   Owner question (security): The body's two unverified risks were checked this session. The extraction risk is verified; the writer chain holds up to one link not read (item 4). Rated HIGH: a chain ends in arbitrary code in a job that holds the lanes App merge token and id-token: write. This record decides nothing else.

   Evidence:

   1. The pinned restore extracts outside path. actions/cache/restore@55cc8345… is v6.1.0 and depends on "@actions/cache": "^6.1.0" (https://raw.githubusercontent.com/actions/cache/55cc8345863c7cc4c66a329aec7e433d2d1c52a9/package.json). Its bundled dist/restore-only/index.js:94080-94082 extracts with -xf <archive> -P -C <GITHUB_WORKSPACE> (:94128-94131), and :94413-94418 lists the archive only in debug mode, then extracts it with no entry filter. With -P, an archive entry keeps its own path, so an entry such as .base/.github/scripts/pr-merge/carry-run-cache.mjs lands in the checkout, not under path.
   2. The restore runs where that matters. In .github/workflows/pr-merge.yml, the merge job holds id-token: write (:456), restores by bare prefix restore-keys: pr-merge-run-cache- (:756-760), and the next step runs node .base/.github/scripts/pr-merge/carry-run-cache.mjs (:768). The enable-auto-merge job does the same (:1444, :1553-1565). Both restores come after the lane token step (:605, :1522). The file's own comment already says "Any job in the default branch's cache scope can write that prefix" (:746).
   3. Head code runs in a job that can write the default-branch scope. GitHub's docs say "only these workflow triggers can create or overwrite caches in the default branch's scope", listing push, workflow_dispatch, repository_dispatch, delete, registry_package, page_build and schedule, and that cache-mode "controls the cache access granted to the job's token" (https://docs.github.com/en/actions/reference/workflows-and-actions/dependency-caching, read 2026-10-26). pr-unblock.yml runs on workflow_dispatch (:35) and calls pr-run-activity-read.yml (:318 and five more call sites), whose run job runs PR head code (pr-run-activity-read.yml:15-16) on a runner the file calls hostile (:216). pr-refine.yml does the same (:25, :126, :200). No job in those files sets cache-mode; the only one on main is work-resolve-issue.yml:132 (cache-mode: read).
   4. Not read this session (UNVERIFIED): whether head code in that run job can obtain the job's cache-write token and save an entry. The published cache-poisoning research the body cites (Cacheract) claims it can; its source was not read here.

   The carried JSON's content filter (carry-run-cache.mjs) does not cover this: a crafted archive replaces the script before it runs.

   Options, each a .github/workflows/** edit (.github/actions/resolve-config/patch-rules.json:64 refuses github-workflows for lane patches, so a host session builds it):

   • A. Body option 2: move the carry to the artifact-read pattern perf(source-control): carry untied FAIL head refs between pr-merge runs #9416 uses, so neither token-holding job restores a cache. Removes the shared cache scope from the trust path.
   • B. Body option 1: restore into an empty directory in a job with no token, list the archive, refuse any entry but the one file, then hand the file over.
   • C. Set cache-mode: none (documented on the page above) on every job that runs PR head code, so none can write the default-branch scope. Narrows the writers but keeps the prefix restore.
   • Recommended: A, as the body proposes. Basis: it removes the cause in both jobs and reuses an existing pattern (DRY); C is a cheap extra layer, not a substitute, since any later head-code job without the key reopens the path.
8. kyle-sexton

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   status: needs-decisionAwaiting a human or maintainer judgment call.Awaiting a human or maintainer judgment call.

   needs-humanHuman-in-the-loop required; autonomous sessions must not resolve items carrying this.Human-in-the-loop required; autonomous sessions must not resolve items carrying this.

   on Oct 26, 2026
9. kyle-sexton commented on Oct 26, 2026

   

   kyle-sexton

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   Released for re-triage under the owner's 2026-10-26 rule: a person is involved only for "1. A change in original intent/vision 2. Something comes up during implementation ... that alters intent/vision or the primary design 3. If there is a HIGH/CRITICAL security issue". None applies here, so the hold label came off and intake triage runs again now.

   To hold this issue again, add needs-human (or status: needs-decision) and name which of the three applies.
10. github-actions

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    needs-triageNot yet classified. Floor until a type and one priority tier are set.Not yet classified. Floor until a type and one priority tier are set.

    needs-humanHuman-in-the-loop required; autonomous sessions must not resolve items carrying this.Human-in-the-loop required; autonomous sessions must not resolve items carrying this.

    and removed

    needs-triageNot yet classified. Floor until a type and one priority tier are set.Not yet classified. Floor until a type and one priority tier are set.

    on Oct 26, 2026
11. melodic-automation-lanes commented on Oct 26, 2026

    

    melodic-automation-lanesBot

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    The triage verdict left this issue at status: needs-decision:

    • no Size: line
    • no Complexity: line
    • no Uncertainty: line
    • no Flags: line
    • the triage run escalated
    • held by needs-human
    • no lanes App verdict comment shows whether an earlier verdict was flagged
12. melodic-automation-lanes

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    status: needs-decisionAwaiting a human or maintainer judgment call.Awaiting a human or maintainer judgment call.

    on Oct 26, 2026
13. kyle-sexton commented on Oct 26, 2026

    

    kyle-sexton

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    Decision recorded. The owner answered in the Refinement session (2026-10-26), choosing option A for this HIGH finding: "A: artifacts (Recommended)" (carry the run data the way #9416 does, so neither token-holding job restores a cache).

    This is a HIGH security finding (rated in the evidence below). Decided: option A, the hardening option, which removes the cause and loosens nothing.

    Evidence:

    1. The pinned restore extracts outside path. actions/cache/restore@55cc8345… is v6.1.0 and depends on "@actions/cache": "^6.1.0" (https://raw.githubusercontent.com/actions/cache/55cc8345863c7cc4c66a329aec7e433d2d1c52a9/package.json). Its bundled dist/restore-only/index.js:94080-94082 extracts with -xf <archive> -P -C <GITHUB_WORKSPACE> (:94128-94131), and :94413-94418 lists the archive only in debug mode, then extracts it with no entry filter. With -P, an archive entry keeps its own path, so an entry such as .base/.github/scripts/pr-merge/carry-run-cache.mjs lands in the checkout, not under path.
    2. The restore runs where that matters. In .github/workflows/pr-merge.yml, the merge job holds id-token: write (:456), restores by bare prefix restore-keys: pr-merge-run-cache- (:756-760), and the next step runs node .base/.github/scripts/pr-merge/carry-run-cache.mjs (:768). The enable-auto-merge job does the same (:1444, :1553-1565). Both restores come after the lane token step (:605, :1522). The file's own comment already says "Any job in the default branch's cache scope can write that prefix" (:746).
    3. Head code runs in a job that can write the default-branch scope. GitHub's docs say "only these workflow triggers can create or overwrite caches in the default branch's scope", listing push, workflow_dispatch, repository_dispatch, delete, registry_package, page_build and schedule, and that cache-mode "controls the cache access granted to the job's token" (https://docs.github.com/en/actions/reference/workflows-and-actions/dependency-caching, read 2026-10-26). pr-unblock.yml runs on workflow_dispatch (:35) and calls pr-run-activity-read.yml (:318 and five more call sites), whose run job runs PR head code (pr-run-activity-read.yml:15-16) on a runner the file calls hostile (:216). pr-refine.yml does the same (:25, :126, :200). No job in those files sets cache-mode; the only one on main is work-resolve-issue.yml:132 (cache-mode: read).
    4. Not read this session (UNVERIFIED): whether head code in that run job can obtain the job's cache-write token and save an entry. The published cache-poisoning research the body cites (Cacheract) claims it can; its source was not read here.

    The carried JSON's content filter (carry-run-cache.mjs) does not cover this: a crafted archive replaces the script before it runs.

    Options, each a .github/workflows/** edit (.github/actions/resolve-config/patch-rules.json:64 refuses github-workflows for lane patches, so a host session builds it):

    • A. Body option 2: move the carry to the artifact-read pattern perf(source-control): carry untied FAIL head refs between pr-merge runs #9416 uses, so neither token-holding job restores a cache. Removes the shared cache scope from the trust path.
    • B. Body option 1: restore into an empty directory in a job with no token, list the archive, refuse any entry but the one file, then hand the file over.
    • C. Set cache-mode: none (documented on the page above) on every job that runs PR head code, so none can write the default-branch scope. Narrows the writers but keeps the prefix restore.
      Decided: A. It removes the shared cache scope from the trust path of both token-holding jobs and reuses the artifact-read pattern perf(source-control): carry untied FAIL head refs between pr-merge runs #9416 already uses (DRY). C may be added as a second layer; it is not a substitute.

    Build: host session

    Acceptance criteria:

    • Neither the merge nor the enable-auto-merge job in .github/workflows/pr-merge.yml runs actions/cache/restore; the carried run data reaches them through the artifact-read pattern perf(source-control): carry untied FAIL head refs between pr-merge runs #9416 uses.
    • No job that holds id-token: write or a lanes App token restores a cache by a bare restore-keys prefix.
    • carry-run-cache.mjs reads only the carried file, from a path no extracted archive can write.
    • A test or workflow check fails if a token-holding job in pr-merge.yml adds a cache restore.
14. kyle-sexton

    added

    status: confirmedA maintainer confirmed the item is real and in scope; ready to be worked.A maintainer confirmed the item is real and in scope; ready to be worked.

    and removed

    status: needs-decisionAwaiting a human or maintainer judgment call.Awaiting a human or maintainer judgment call.

    needs-humanHuman-in-the-loop required; autonomous sessions must not resolve items carrying this.Human-in-the-loop required; autonomous sessions must not resolve items carrying this.

    on Oct 26, 2026
15. kyle-sexton

    added 3 commits that reference this issue on Oct 26, 2026

    fix(pr-merge): read the queue's carried verdict reads from pr-merge's…

    ...

    e9e1f67

    fix(pr-merge): refuse a trusted run while a tag named main exists

    ...

    01e128b

    perf(pr-merge): read the tag named main and main's tip once per process

    ...

    834ab73
16. kyle-sexton

    closed this as completedin #10292on Oct 26, 2026