block checking out fork pr for some events

src/git-source-settings.ts

@@ -118,4 +118,10 @@ export interface IGitSourceSettings {
    * User override on the GitHub Server/Host URL that hosts the repository to be cloned
    */
   githubServerUrl: string | undefined
+
+  /**
+   * Opt-in to allow checking out fork pull request code from a workflow
+   * triggered by pull_request_target or workflow_run.
+   */
+  allowUnsafePrCheckout: boolean
 }

src/input-helper.ts

@@ -2,6 +2,7 @@ import * as core from '@actions/core'
 import * as fsHelper from './fs-helper'
 import * as github from '@actions/github'
 import * as path from 'path'
+import * as unsafePrCheckoutHelper from './unsafe-pr-checkout-helper'
 import * as workflowContextHelper from './workflow-context-helper'
 import {IGitSourceSettings} from './git-source-settings'
 
@@ -161,5 +162,18 @@ export async function getInputs(): Promise<IGitSourceSettings> {
   result.githubServerUrl = core.getInput('github-server-url')
   core.debug(`GitHub Host URL = ${result.githubServerUrl}`)
 
+  // Allow unsafe PR checkout (opt-in for pull_request_target / workflow_run fork PRs)
+  result.allowUnsafePrCheckout =
+    (core.getInput('allow-unsafe-pr-checkout') || 'false').toUpperCase() ===
+    'TRUE'
+  core.debug(`allow unsafe PR checkout = ${result.allowUnsafePrCheckout}`)
+
+  unsafePrCheckoutHelper.assertSafePrCheckout({
+    qualifiedRepository,
+    ref: result.ref,
+    commit: result.commit,
+    allowUnsafePrCheckout: result.allowUnsafePrCheckout
+  })
+
   return result
 }

src/ref-helper.ts

@@ -292,7 +292,7 @@ export async function checkCommitInfo(
   }
 }
 
-function fromPayload(path: string): any {
+export function fromPayload(path: string): any {
   return select(github.context.payload, path)
 }
 

src/unsafe-pr-checkout-helper.ts

@@ -0,0 +1,80 @@
+import * as github from '@actions/github'
+import {fromPayload} from './ref-helper'
+
+const PR_REF_PATTERN = /^refs\/pull\/[0-9]+\/(?:head|merge)$/
+
+export interface IUnsafePrCheckoutInput {
+  qualifiedRepository: string
+  ref: string
+  commit: string
+  allowUnsafePrCheckout: boolean
+}
+
+export function assertSafePrCheckout(input: IUnsafePrCheckoutInput): void {
+  if (input.allowUnsafePrCheckout) {
+    return
+  }
+
+  const eventName = github.context.eventName
+  if (eventName !== 'pull_request_target' && eventName !== 'workflow_run') {
+    return
+  }
+
+  const baseRepoId = fromPayload('repository.id')
+  if (typeof baseRepoId !== 'number') {
+    return
+  }
+
+  let prHeadRepoId: unknown
+  const prShas: string[] = []
+
+  if (eventName === 'pull_request_target') {
+    prHeadRepoId = fromPayload('pull_request.head.repo.id')
+    pushIfSha(prShas, fromPayload('pull_request.head.sha'))
+    pushIfSha(prShas, fromPayload('pull_request.merge_commit_sha'))
+  } else {
+    const wrEvent = fromPayload('workflow_run.event')
+    if (typeof wrEvent !== 'string' || !wrEvent.startsWith('pull_request')) {
+      return
+    }
+    prHeadRepoId = fromPayload('workflow_run.head_repository.id')
+    pushIfSha(prShas, fromPayload('workflow_run.head_commit.id'))
+  }
+
+  // (A) Fork PR?
+  if (typeof prHeadRepoId !== 'number' || prHeadRepoId === baseRepoId) {
+    return
+  }
+
+  // (B) We cannot check for all fork PR refs so check to see
+  // if the resolved input points to the fork PR sha we have in the payload
+  const baseQualifiedRepository = `${github.context.repo.owner}/${github.context.repo.repo}`
+  const repositoryDiffersFromBase =
+    input.qualifiedRepository.toLowerCase() !==
+    baseQualifiedRepository.toLowerCase()
+  const refMatchesPullPattern = PR_REF_PATTERN.test(input.ref)
+  const commitMatchesPrHeadSha =
+    !!input.commit && prShas.includes(input.commit.toLowerCase())
+
+  if (
+    !repositoryDiffersFromBase &&
+    !refMatchesPullPattern &&
+    !commitMatchesPrHeadSha
+  ) {
+    return
+  }
+
+  throw new Error(
+    `Refusing to check out fork pull request code from a '${eventName}' workflow. ` +
+      `This workflow runs with the base repository's GITHUB_TOKEN, secrets, default-branch ` +
+      `cache scope, and runner access. Fetching fork's code in that trusted context is a ` +
+      `"pwn request" supply-chain attack pattern. To opt in after reviewing the risk, 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())
+  }
+}