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fix: add missing Checkout-GhAwPr.ps1 script #788

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Merged
PureWeen merged 1 commit into from
Apr 28, 2026
Merged

fix: add missing Checkout-GhAwPr.ps1 script #788

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59 changes: 59 additions & 0 deletions .github/scripts/Checkout-GhAwPr.ps1
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Original file line number Diff line number Diff line change
@@ -0,0 +1,59 @@
#!/usr/bin/env pwsh
# Checkout-GhAwPr.ps1 — Security-checked PR checkout for gh-aw workflow_dispatch.
#
# Verifies the PR author has write access to the repo, checks out the PR branch,
# then restores .github/ from the base branch (main) to prevent prompt injection
# via modified workflow files in the PR.

$ErrorActionPreference = 'Stop'

$prNumber = $env:PR_NUMBER
if (-not $prNumber) {
Write-Error "PR_NUMBER environment variable is required"
exit 1
}

Write-Host "Checking out PR #$prNumber..."

# Get PR info
$prJson = gh pr view $prNumber --json headRefName,headRepository,headRepositoryOwner,author,baseRefName --repo $env:GITHUB_REPOSITORY 2>&1
if ($LASTEXITCODE -ne 0) {
Write-Error "Failed to get PR #$prNumber info: $prJson"
exit 1
}
$pr = $prJson | ConvertFrom-Json

$branch = $pr.headRefName
$baseBranch = $pr.baseRefName
$author = $pr.author.login

Write-Host "PR #$prNumber by $author, branch: $branch, base: $baseBranch"

# Check author has write access (skip for bots)
if ($author -notmatch '\[bot\]$') {
$permJson = gh api "repos/$($env:GITHUB_REPOSITORY)/collaborators/$author/permission" --jq '.permission' 2>&1
if ($LASTEXITCODE -ne 0) {
Write-Warning "Could not verify author permissions: $permJson"
Comment on lines +35 to +36
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@github-actions github-actions Bot Apr 28, 2026

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🔴 CRITICAL · 3/3 reviewers · Permission check fail-open — API failure silently bypasses access gate

The collaborators API returns HTTP 404 for non-collaborators (the exact users this gate should block). When $LASTEXITCODE -ne 0, the script logs a Write-Warning and continues — the checkout proceeds without any authorization check. This also triggers on rate limits, insufficient token scope, or network errors.

Scenario: A fork PR author with read-only access triggers workflow_dispatch. The API returns 404, the warning is logged and ignored, and the untrusted PR is checked out and reviewed.

Fix: Fail closed — treat API failure as denied:

if ($LASTEXITCODE -ne 0) {
    Write-Error "Could not verify author permissions (non-collaborator or API error): $permJson"
    exit 1
}

} else {
$perm = $permJson.Trim()
if ($perm -notin @('admin', 'maintain', 'write')) {
Write-Error "Author '$author' has '$perm' permission — write access required for workflow_dispatch review"
exit 1
}
Write-Host "Author '$author' has '$perm' access — OK"
}
}

# Fetch and checkout the PR branch
git fetch origin "pull/$prNumber/head:pr-$prNumber" 2>&1 | Write-Host
git checkout "pr-$prNumber" 2>&1 | Write-Host

# Save the PR HEAD SHA
$prSha = git rev-parse HEAD
Write-Host "PR HEAD: $prSha"

# Restore .github/ from the base branch to prevent workflow tampering
Write-Host "Restoring .github/ from $baseBranch..."
git checkout "origin/$baseBranch" -- .github/ 2>&1 | Write-Host
Comment on lines +48 to +57
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🔴 CRITICAL · 3/3 reviewers · git exit codes never checked — security contract voided on failure

$ErrorActionPreference = 'Stop' only governs PowerShell cmdlets, not native executables. None of the three git commands check $LASTEXITCODE. If git fetch fails (deleted PR, network error), the script continues — git checkout fails silently, and the .github/ restore (line 57) runs against whatever tree is already in the workspace. The script exits 0 regardless, voiding its core security purpose.

Fix: Check $LASTEXITCODE after each git command:

git fetch origin "pull/$prNumber/head:pr-$prNumber" 2>&1 | Write-Host
if ($LASTEXITCODE -ne 0) { Write-Error "git fetch failed"; exit 1 }

git checkout "pr-$prNumber" 2>&1 | Write-Host
if ($LASTEXITCODE -ne 0) { Write-Error "git checkout failed"; exit 1 }

# ... same for line 57's .github/ restore
git checkout "origin/$baseBranch" -- .github/ 2>&1 | Write-Host
if ($LASTEXITCODE -ne 0) { Write-Error ".github/ restore failed"; exit 1 }

Alternatively, use $PSNativeCommandErrorActionPreference = $true (PowerShell 7.3+).

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🟡 MODERATE · 3/3 reviewers · origin/$baseBranch may not exist — .github/ restore can silently fail

The script only fetches pull/$prNumber/head. It never fetches the base branch. In shallow clones or when the PR targets a non-default branch (e.g., release/v2), origin/$baseBranch won't exist locally. Combined with the missing exit-code checks above, this failure is silent and leaves the PR's untrusted .github/ on disk.

Fix: Fetch the base ref before restoring (prefer immutable SHA via baseRefOid):

# Add baseRefOid to the gh pr view --json fields
$baseSha = $pr.baseRefOid
git fetch origin $baseSha 2>&1 | Write-Host
if ($LASTEXITCODE -ne 0) { Write-Error "Failed to fetch base ref"; exit 1 }
git checkout $baseSha -- .github/ 2>&1 | Write-Host
if ($LASTEXITCODE -ne 0) { Write-Error "Failed to restore .github/"; exit 1 }


Write-Host "Checkout complete — PR #$prNumber on branch $branch, .github/ from $baseBranch"
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