1 files changed, 88 insertions, 0 deletions
diff --git a/src/unsafe-pr-checkout-helper.ts b/src/unsafe-pr-checkout-helper.ts
new file mode 100644
index 0000000..efc0ef6
--- /dev/null
+++ b/src/unsafe-pr-checkout-helper.ts
@@ -0,0 +1,88 @@
+import * as github from '@actions/github'
+import {fromPayload} from './ref-helper'
+
+const PR_REF_PATTERN = /^refs\/pull\/[0-9]+\/(?:head|merge)$/
+
+export interface IUnsafePrCheckoutInput {
+  qualifiedRepository: string
+  ref: string
+  commit: string | undefined
+  allowUnsafePrCheckout: boolean
+}
+
+export function assertSafePrCheckout(input: IUnsafePrCheckoutInput): void {
+  if (input.allowUnsafePrCheckout) {
+    return
+  }
+
+  const eventName = github.context.eventName
+  if (eventName !== 'pull_request_target' && eventName !== 'workflow_run') {
+    return
+  }
+
+  const baseRepoId = fromPayload('repository.id')
+  if (typeof baseRepoId !== 'number') {
+    return
+  }
+
+  let prHeadRepoId: unknown
+  let prHeadRepoFullName: unknown
+  const prShas: string[] = []
+
+  if (eventName === 'pull_request_target') {
+    prHeadRepoId = fromPayload('pull_request.head.repo.id')
+    prHeadRepoFullName = fromPayload('pull_request.head.repo.full_name')
+    pushIfSha(prShas, fromPayload('pull_request.head.sha'))
+    pushIfSha(prShas, fromPayload('pull_request.merge_commit_sha'))
+  } else {
+    const wrEvent = fromPayload('workflow_run.event')
+    if (typeof wrEvent !== 'string' || !wrEvent.startsWith('pull_request')) {
+      return
+    }
+    prHeadRepoId = fromPayload('workflow_run.head_repository.id')
+    prHeadRepoFullName = fromPayload('workflow_run.head_repository.full_name')
+    pushIfSha(prShas, fromPayload('workflow_run.head_commit.id'))
+    // For `pull_request_target`-triggered workflow_run, `head_sha` is the base
+    // default branch SHA (not the PR head)
+    if (wrEvent !== 'pull_request_target') {
+      pushIfSha(prShas, fromPayload('workflow_run.head_sha'))
+    }
+  }
+
+  // (A) Fork PR?
+  if (typeof prHeadRepoId !== 'number' || prHeadRepoId === baseRepoId) {
+    return
+  }
+
+  // (B) We cannot check for all fork PR refs so check to see
+  // if the resolved input points to the fork PR sha we have in the payload
+  const repositoryMatchesPrHead =
+    typeof prHeadRepoFullName === 'string' &&
+    input.qualifiedRepository.toLowerCase() === prHeadRepoFullName.toLowerCase()
+  const refMatchesPullPattern = PR_REF_PATTERN.test(input.ref)
+  const commitMatchesPrHeadSha =
+    !!input.commit && prShas.includes(input.commit.toLowerCase())
+
+  if (
+    !repositoryMatchesPrHead &&
+    !refMatchesPullPattern &&
+    !commitMatchesPrHeadSha
+  ) {
+    return
+  }
+
+  throw new Error(
+    `Refusing to check out fork pull request code from a '${eventName}' workflow. ` +
+      `This workflow runs with the base repository's GITHUB_TOKEN, secrets, default-branch ` +
+      `cache scope, and runner access. Fetching and executing a fork's code in that trusted ` +
+      `context commonly leads to "pwn request" vulnerabilities. To opt in after reviewing ` +
+      `the risks at https://gh.io/securely-using-pull_request_target, set ` +
+      `'allow-unsafe-pr-checkout: true' on the actions/checkout step.`
+  )
+}
+
+function pushIfSha(target: string[], value: unknown): void {
+  if (typeof value === 'string' && value.length > 0) {
+    target.push(value.toLowerCase())
+  }
+}