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Update README regarding pull_request_target #321

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Update README regarding pull_request_target #321

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Update README regarding `pull_request_target`
ylemkimon Aug 9, 2020
4c4d2b5
Update README.md
ylemkimon Aug 9, 2020
f16edde
Update README.md
ylemkimon Sep 14, 2020
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17 changes: 17 additions & 0 deletions README.md
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Original file line number Diff line number Diff line change
@@ -118,6 +118,7 @@ Refer [here](https://github.com/actions/checkout/blob/v1/README.md) for previous
- [Checkout multiple repos (private)](#Checkout-multiple-repos-private)
- [Checkout pull request HEAD commit instead of merge commit](#Checkout-pull-request-HEAD-commit-instead-of-merge-commit)
- [Checkout pull request on closed event](#Checkout-pull-request-on-closed-event)
- [Checkout pull request on `pull_request_target`](#Checkout-pull-request-on-pull_request_target)
- [Push a commit using the built-in token](#Push-a-commit-using-the-built-in-token)

## Fetch all history for all tags and branches
@@ -214,6 +215,22 @@ jobs:
- uses: actions/checkout@v2
```

## Checkout pull request on `pull_request_target`

```yaml
on:
- pull_request_target
jobs:
build:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v2
with:
ref: refs/pull/${{ github.event.pull_request.number }}/merge
```

**WARNING! NEVER** run code from pull requests of public repositories! The token of `pull_request_target` event has write access.

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@ylemkimon ylemkimon Sep 14, 2020 •
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@matfax The pull_request_target runs in the context of the base repository of the pull request, i.e., its GITHUB_TOKEN has read/write access even for pull requests from forked repos.

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I've tested various things now. Even though GITHUB_TOKEN doesn't seem to be available all the time, the github.token from the context has indeed write permissions as you stated. actions/checkout sets this context token as repository token, so it's available anyways once checkout is used. I find it rather odd that GitHub introduced pull_request_target as some kind of safe alternative but how are you supposed to check anything in a pull request if you can't even check out its code? Imho, they should change the permissions of the token if used in pull_request_target to read only the repository + write access to the pull request (except merge). The use case of checking forked pull requests is integrated and documented insufficiently.


## Push a commit using the built-in token

```yaml