checkout/src/unsafe-pr-checkout-helper.ts

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())
  }
}