Merge 0865c4bfce into f9e715a95f

* block checking out fork pr for some events

* address copilot and reviewer feedback

* run prettier formatting

* build

* update urls

* update readme

* update description and url again

* edit url one more time

README.md

@@ -63,6 +63,11 @@ Please refer to the [release page](https://github.com/actions/checkout/releases/
     # Otherwise, uses the default branch.
     ref: ''
 
+    # The commit SHA to checkout. Used when ref is not specified or is ambiguous. This
+    # can be used as a replacement for ref, or alongside it to checkout a specific
+    # commit of the ref.
+    commit: ''
+
     # Personal access token (PAT) used to fetch the repository. The PAT is configured
     # with the local git config, which enables your scripts to run authenticated git
     # commands. The post-job step removes the PAT.
@@ -160,6 +165,15 @@ Please refer to the [release page](https://github.com/actions/checkout/releases/
     # running from unless specified. Example URLs are https://github.com or
     # https://my-ghes-server.example.com
     github-server-url: ''
+
+    # Required to check out fork pull request code from a workflow triggered by
+    # `pull_request_target` or `workflow_run`. These workflows run 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. Set to `true` only after reviewing the
+    # risks at https://gh.io/securely-using-pull_request_target.
+    # Default: false
+    allow-unsafe-pr-checkout: ''
 ```
 <!-- end usage -->
 

__test__/git-auth-helper.test.ts

@@ -1173,7 +1173,8 @@ async function setup(testName: string): Promise<void> {
     sshUser: '',
     workflowOrganizationId: 123456,
     setSafeDirectory: true,
-    githubServerUrl: githubServerUrl
+    githubServerUrl: githubServerUrl,
+    allowUnsafePrCheckout: false
   }
 }
 

__test__/input-helper.test.ts

@@ -91,6 +91,7 @@ describe('input-helper tests', () => {
     expect(settings.repositoryOwner).toBe('some-owner')
     expect(settings.repositoryPath).toBe(gitHubWorkspace)
     expect(settings.setSafeDirectory).toBe(true)
+    expect(settings.allowUnsafePrCheckout).toBe(false)
   })
 
   it('qualifies ref', async () => {
@@ -154,4 +155,30 @@ describe('input-helper tests', () => {
     const settings: IGitSourceSettings = await inputHelper.getInputs()
     expect(settings.workflowOrganizationId).toBe(123456)
   })
+
+  it('accepts ref and commit', async () => {
+    inputs.ref = 'refs/pull/123/merge'
+    inputs.commit = '0123456789012345678901234567890123456789'
+    const settings: IGitSourceSettings = await inputHelper.getInputs()
+    expect(settings).toBeTruthy()
+    expect(settings.ref).toBeTruthy()
+    expect(settings.ref).toStrictEqual('refs/pull/123/merge')
+    expect(settings.commit).toBeTruthy()
+    expect(settings.commit).toStrictEqual(
+      '0123456789012345678901234567890123456789'
+    )
+  })
+
+  it('ref fallbacks to commit if ref is empty but commit is specified', async () => {
+    inputs.ref = ''
+    inputs.commit = '0123456789012345678901234567890123456789'
+    const settings: IGitSourceSettings = await inputHelper.getInputs()
+    expect(settings).toBeTruthy()
+    expect(settings.ref).toBeFalsy()
+    expect(settings.ref).toStrictEqual('')
+    expect(settings.commit).toBeTruthy()
+    expect(settings.commit).toStrictEqual(
+      '0123456789012345678901234567890123456789'
+    )
+  })
 })

__test__/unsafe-pr-checkout-helper.test.ts

@@ -0,0 +1,267 @@
+import * as github from '@actions/github'
+import {assertSafePrCheckout} from '../lib/unsafe-pr-checkout-helper'
+
+// Shallow clone original @actions/github context
+const originalContext = {...github.context}
+const originalEventName = github.context.eventName
+const originalPayload = github.context.payload
+
+const BASE_REPO_ID = 100
+const FORK_REPO_ID = 200
+const PR_HEAD_SHA = '1111111111111111111111111111111111111111'
+const PR_MERGE_SHA = '2222222222222222222222222222222222222222'
+const SAFE_BASE_SHA = '3333333333333333333333333333333333333333'
+const WORKFLOW_RUN_HEAD_COMMIT_SHA = '4444444444444444444444444444444444444444'
+const BASE_QUALIFIED_REPO = 'some-owner/some-repo'
+const FORK_QUALIFIED_REPO = 'another-repo/fork'
+
+function setContext(eventName: string, payload: object): void {
+  ;(github.context as {eventName: string}).eventName = eventName
+  ;(github.context as {payload: object}).payload = payload
+}
+
+function forkPullRequestTargetPayload(): object {
+  return {
+    repository: {id: BASE_REPO_ID},
+    pull_request: {
+      head: {
+        sha: PR_HEAD_SHA,
+        repo: {id: FORK_REPO_ID, full_name: FORK_QUALIFIED_REPO}
+      },
+      merge_commit_sha: PR_MERGE_SHA
+    }
+  }
+}
+
+function sameRepoPullRequestTargetPayload(): object {
+  return {
+    repository: {id: BASE_REPO_ID},
+    pull_request: {
+      head: {
+        sha: PR_HEAD_SHA,
+        repo: {id: BASE_REPO_ID, full_name: BASE_QUALIFIED_REPO}
+      },
+      merge_commit_sha: PR_MERGE_SHA
+    }
+  }
+}
+
+function forkWorkflowRunPayload(): object {
+  return {
+    repository: {id: BASE_REPO_ID},
+    workflow_run: {
+      event: 'pull_request',
+      head_commit: {id: WORKFLOW_RUN_HEAD_COMMIT_SHA},
+      head_repository: {id: FORK_REPO_ID, full_name: FORK_QUALIFIED_REPO}
+    }
+  }
+}
+
+describe('unsafe-pr-checkout-helper', () => {
+  beforeAll(() => {
+    jest.spyOn(github.context, 'repo', 'get').mockReturnValue({
+      owner: 'some-owner',
+      repo: 'some-repo'
+    })
+  })
+
+  afterEach(() => {
+    ;(github.context as {eventName: string}).eventName = originalEventName
+    ;(github.context as {payload: object}).payload = originalPayload
+  })
+
+  afterAll(() => {
+    ;(github.context as {eventName: string}).eventName =
+      originalContext.eventName
+    ;(github.context as {payload: object}).payload = originalContext.payload
+    jest.restoreAllMocks()
+  })
+
+  it('allows pull_request events untouched', () => {
+    setContext('pull_request', forkPullRequestTargetPayload())
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: 'attacker/fork',
+        ref: 'refs/pull/1/merge',
+        commit: '',
+        allowUnsafePrCheckout: false
+      })
+    ).not.toThrow()
+  })
+
+  it('allows pull_request_target default checkout (base branch)', () => {
+    setContext('pull_request_target', forkPullRequestTargetPayload())
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: BASE_QUALIFIED_REPO,
+        ref: 'refs/heads/main',
+        commit: SAFE_BASE_SHA,
+        allowUnsafePrCheckout: false
+      })
+    ).not.toThrow()
+  })
+
+  it('allows same-repo pull_request_target checkout of PR head', () => {
+    setContext('pull_request_target', sameRepoPullRequestTargetPayload())
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: BASE_QUALIFIED_REPO,
+        ref: '',
+        commit: PR_HEAD_SHA,
+        allowUnsafePrCheckout: false
+      })
+    ).not.toThrow()
+  })
+
+  it('refuses pull_request_target fork PR head SHA checkout', () => {
+    setContext('pull_request_target', forkPullRequestTargetPayload())
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: BASE_QUALIFIED_REPO,
+        ref: '',
+        commit: PR_HEAD_SHA,
+        allowUnsafePrCheckout: false
+      })
+    ).toThrow(/Refusing to check out fork pull request code/)
+  })
+
+  it('refuses pull_request_target fork PR merge_commit_sha checkout', () => {
+    setContext('pull_request_target', forkPullRequestTargetPayload())
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: BASE_QUALIFIED_REPO,
+        ref: '',
+        commit: PR_MERGE_SHA,
+        allowUnsafePrCheckout: false
+      })
+    ).toThrow(/allow-unsafe-pr-checkout/)
+  })
+
+  it('refuses pull_request_target fork PR ref pattern (head)', () => {
+    setContext('pull_request_target', forkPullRequestTargetPayload())
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: BASE_QUALIFIED_REPO,
+        ref: 'refs/pull/42/head',
+        commit: '',
+        allowUnsafePrCheckout: false
+      })
+    ).toThrow()
+  })
+
+  it('refuses pull_request_target fork PR ref pattern (merge)', () => {
+    setContext('pull_request_target', forkPullRequestTargetPayload())
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: BASE_QUALIFIED_REPO,
+        ref: 'refs/pull/42/merge',
+        commit: '',
+        allowUnsafePrCheckout: false
+      })
+    ).toThrow()
+  })
+
+  it('refuses pull_request_target when repository points at the fork', () => {
+    setContext('pull_request_target', forkPullRequestTargetPayload())
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: FORK_QUALIFIED_REPO,
+        ref: 'refs/heads/main',
+        commit: '',
+        allowUnsafePrCheckout: false
+      })
+    ).toThrow()
+  })
+
+  it('allows pull_request_target checkout of an unrelated third-party repo', () => {
+    setContext('pull_request_target', forkPullRequestTargetPayload())
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: 'some-other/unrelated',
+        ref: 'refs/heads/main',
+        commit: '',
+        allowUnsafePrCheckout: false
+      })
+    ).not.toThrow()
+  })
+
+  it('refuses pull_request_target ignoring repository case differences', () => {
+    setContext('pull_request_target', forkPullRequestTargetPayload())
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: FORK_QUALIFIED_REPO.toUpperCase(),
+        ref: '',
+        commit: '',
+        allowUnsafePrCheckout: false
+      })
+    ).toThrow()
+  })
+
+  it('refuses pull_request_target ignoring commit SHA case differences', () => {
+    setContext('pull_request_target', forkPullRequestTargetPayload())
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: BASE_QUALIFIED_REPO,
+        ref: '',
+        commit: PR_HEAD_SHA.toUpperCase(),
+        allowUnsafePrCheckout: false
+      })
+    ).toThrow()
+  })
+
+  it('allows pull_request_target fork PR checkout when opted in', () => {
+    setContext('pull_request_target', forkPullRequestTargetPayload())
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: BASE_QUALIFIED_REPO,
+        ref: 'refs/pull/42/merge',
+        commit: '',
+        allowUnsafePrCheckout: true
+      })
+    ).not.toThrow()
+  })
+
+  it('refuses workflow_run fork PR head_commit.id checkout', () => {
+    setContext('workflow_run', forkWorkflowRunPayload())
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: BASE_QUALIFIED_REPO,
+        ref: '',
+        commit: WORKFLOW_RUN_HEAD_COMMIT_SHA,
+        allowUnsafePrCheckout: false
+      })
+    ).toThrow()
+  })
+
+  it('refuses workflow_run with pull_request_target underlying event', () => {
+    const payload = forkWorkflowRunPayload() as {
+      workflow_run: {event: string}
+    }
+    payload.workflow_run.event = 'pull_request_target'
+    setContext('workflow_run', payload)
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: BASE_QUALIFIED_REPO,
+        ref: '',
+        commit: WORKFLOW_RUN_HEAD_COMMIT_SHA,
+        allowUnsafePrCheckout: false
+      })
+    ).toThrow()
+  })
+
+  it('allows workflow_run same-repo PR (head_repository.id matches base)', () => {
+    const payload = forkWorkflowRunPayload() as {
+      workflow_run: {head_repository: {id: number}}
+    }
+    payload.workflow_run.head_repository.id = BASE_REPO_ID
+    setContext('workflow_run', payload)
+    expect(() =>
+      assertSafePrCheckout({
+        qualifiedRepository: BASE_QUALIFIED_REPO,
+        ref: '',
+        commit: WORKFLOW_RUN_HEAD_COMMIT_SHA,
+        allowUnsafePrCheckout: false
+      })
+    ).not.toThrow()
+  })
+})

action.yml

@@ -9,6 +9,11 @@ inputs:
       The branch, tag or SHA to checkout. When checking out the repository that
       triggered a workflow, this defaults to the reference or SHA for that
       event.  Otherwise, uses the default branch.
+  commit:
+    description: >
+      The commit SHA to checkout. Used when ref is not specified or is ambiguous.
+      This can be used as a replacement for ref, or alongside it to checkout a
+      specific commit of the ref.
   token:
     description: >
       Personal access token (PAT) used to fetch the repository. The PAT is configured
@@ -98,6 +103,15 @@ inputs:
   github-server-url:
     description: The base URL for the GitHub instance that you are trying to clone from, will use environment defaults to fetch from the same instance that the workflow is running from unless specified. Example URLs are https://github.com or https://my-ghes-server.example.com
     required: false
+  allow-unsafe-pr-checkout:
+    description: >
+      Required to check out fork pull request code from a workflow triggered by
+      `pull_request_target` or `workflow_run`. These workflows run 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. Set to `true`
+      only after reviewing the risks at https://gh.io/securely-using-pull_request_target.
+    default: false
 outputs:
   ref:
     description: 'The branch, tag or SHA that was checked out'

dist/index.js

@@ -2023,6 +2023,7 @@ const core = __importStar(__nccwpck_require__(2186));
 const fsHelper = __importStar(__nccwpck_require__(7219));
 const github = __importStar(__nccwpck_require__(5438));
 const path = __importStar(__nccwpck_require__(1017));
+const unsafePrCheckoutHelper = __importStar(__nccwpck_require__(843));
 const workflowContextHelper = __importStar(__nccwpck_require__(9568));
 function getInputs() {
     return __awaiter(this, void 0, void 0, function* () {
@@ -2057,7 +2058,11 @@ function getInputs() {
         const isWorkflowRepository = qualifiedRepository.toUpperCase() ===
             `${github.context.repo.owner}/${github.context.repo.repo}`.toUpperCase();
         // Source branch, source version
-        result.ref = core.getInput('ref');
+        result.commit = core.getInput('commit');
+        if (result.commit && !result.commit.match(/^[0-9a-fA-F]{40}$/)) {
+            throw new Error(`The commit SHA '${result.commit}' is not a valid SHA.`);
+        }
+        result.ref = core.getInput('ref') || result.commit;
         if (!result.ref) {
             if (isWorkflowRepository) {
                 result.ref = github.context.ref;
@@ -2144,6 +2149,17 @@ function getInputs() {
         // Determine the GitHub URL that the repository is being hosted from
         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;
     });
 }
@@ -2284,6 +2300,7 @@ exports.getRefSpecForAllHistory = getRefSpecForAllHistory;
 exports.getRefSpec = getRefSpec;
 exports.testRef = testRef;
 exports.checkCommitInfo = checkCommitInfo;
+exports.fromPayload = fromPayload;
 const core = __importStar(__nccwpck_require__(2186));
 const github = __importStar(__nccwpck_require__(5438));
 const url_helper_1 = __nccwpck_require__(9437);
@@ -2732,6 +2749,105 @@ if (!exports.IsPost) {
 }
 
 
+/***/ }),
+
+/***/ 843:
+/***/ (function(__unused_webpack_module, exports, __nccwpck_require__) {
+
+"use strict";
+
+var __createBinding = (this && this.__createBinding) || (Object.create ? (function(o, m, k, k2) {
+    if (k2 === undefined) k2 = k;
+    var desc = Object.getOwnPropertyDescriptor(m, k);
+    if (!desc || ("get" in desc ? !m.__esModule : desc.writable || desc.configurable)) {
+      desc = { enumerable: true, get: function() { return m[k]; } };
+    }
+    Object.defineProperty(o, k2, desc);
+}) : (function(o, m, k, k2) {
+    if (k2 === undefined) k2 = k;
+    o[k2] = m[k];
+}));
+var __setModuleDefault = (this && this.__setModuleDefault) || (Object.create ? (function(o, v) {
+    Object.defineProperty(o, "default", { enumerable: true, value: v });
+}) : function(o, v) {
+    o["default"] = v;
+});
+var __importStar = (this && this.__importStar) || function (mod) {
+    if (mod && mod.__esModule) return mod;
+    var result = {};
+    if (mod != null) for (var k in mod) if (k !== "default" && Object.prototype.hasOwnProperty.call(mod, k)) __createBinding(result, mod, k);
+    __setModuleDefault(result, mod);
+    return result;
+};
+Object.defineProperty(exports, "__esModule", ({ value: true }));
+exports.assertSafePrCheckout = assertSafePrCheckout;
+const github = __importStar(__nccwpck_require__(5438));
+const ref_helper_1 = __nccwpck_require__(8601);
+const PR_REF_PATTERN = /^refs\/pull\/[0-9]+\/(?:head|merge)$/;
+function assertSafePrCheckout(input) {
+    if (input.allowUnsafePrCheckout) {
+        return;
+    }
+    const eventName = github.context.eventName;
+    if (eventName !== 'pull_request_target' && eventName !== 'workflow_run') {
+        return;
+    }
+    const baseRepoId = (0, ref_helper_1.fromPayload)('repository.id');
+    if (typeof baseRepoId !== 'number') {
+        return;
+    }
+    let prHeadRepoId;
+    let prHeadRepoFullName;
+    const prShas = [];
+    if (eventName === 'pull_request_target') {
+        prHeadRepoId = (0, ref_helper_1.fromPayload)('pull_request.head.repo.id');
+        prHeadRepoFullName = (0, ref_helper_1.fromPayload)('pull_request.head.repo.full_name');
+        pushIfSha(prShas, (0, ref_helper_1.fromPayload)('pull_request.head.sha'));
+        pushIfSha(prShas, (0, ref_helper_1.fromPayload)('pull_request.merge_commit_sha'));
+    }
+    else {
+        const wrEvent = (0, ref_helper_1.fromPayload)('workflow_run.event');
+        if (typeof wrEvent !== 'string' || !wrEvent.startsWith('pull_request')) {
+            return;
+        }
+        prHeadRepoId = (0, ref_helper_1.fromPayload)('workflow_run.head_repository.id');
+        prHeadRepoFullName = (0, ref_helper_1.fromPayload)('workflow_run.head_repository.full_name');
+        pushIfSha(prShas, (0, ref_helper_1.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, (0, ref_helper_1.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, value) {
+    if (typeof value === 'string' && value.length > 0) {
+        target.push(value.toLowerCase());
+    }
+}
+
+
 /***/ }),
 
 /***/ 9437:

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'
 
@@ -57,7 +58,12 @@ export async function getInputs(): Promise<IGitSourceSettings> {
     `${github.context.repo.owner}/${github.context.repo.repo}`.toUpperCase()
 
   // Source branch, source version
-  result.ref = core.getInput('ref')
+  result.commit = core.getInput('commit')
+  if (result.commit && !result.commit.match(/^[0-9a-fA-F]{40}$/)) {
+    throw new Error(`The commit SHA '${result.commit}' is not a valid SHA.`)
+  }
+
+  result.ref = core.getInput('ref') || result.commit
   if (!result.ref) {
     if (isWorkflowRepository) {
       result.ref = github.context.ref
@@ -161,5 +167,18 @@ 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 +1,88 @@
+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())
+  }
+}