Send arbitrary PUT requests when user clicks on a link

How it was found

Dear teams,

Summary

Mermaid allows users to set class name of a block. This ability becomes vulnerable in Gitlab issues because of issue.js#L90:

    return $(document).on(
      'click',
      '.js-issuable-actions a.btn-close, .js-issuable-actions a.btn-reopen',
      e => {
...
       const $button = $(e.currentTarget);
...
        const url = $button.attr('href');
        return axios
          .put(url)
          .then(({ data }) => {
...

Steps to reproduce

1. Create any issue
2. Enter the following payload as the description of the issue:

```mermaid
graph TD;
 A[Click to send a PUT request];
 class A js-issuable-actions;
 class A btn-close;
 click A "./put-destination" "click to PUT"

After saving the issue, if you click on the block Click to send a PUT request, a PUT request will be sent to ./put-destination

Impact

Since attacker can control ./put-destination, he can theoretically can perform any PUT requests on behalf of the current user. For example, attacker can use the following url to update the description of issue #2:

/api/v4/projects/16210710/issues/2?description=a

Examples

An example is available here: https://gitlab.com/yvvdwf/xss/-/issues/1 (it is private, pls let me know if you cannot access it)

Output of checks

This bug happens on GitLab.com

Impact

When received click of user, attacker may perform arbitrary PUT requests of the behalf of the user