Impact

This vulnerability has the potential to steal a user's cookie and gain unauthorized access to that user's account through the stolen cookie or redirect users to other malicious sites.

Proof of Concept
Step 1. Go to /admin and login.
Step 2. In Documents, go to home -> click on Sample Content -> click Document folder
Step 3. Upload file PDF content XSS payload

Patches

Apply patches
https://github.com/pimcore/pimcore/commit/757375677dc83a44c6c22f26d97452cc5cda5d7c.patch
https://github.com/pimcore/admin-ui-classic-bundle/commit/19fda2e86557c2ed4978316104de5ccdaa66d8b9.patch

Workarounds

Update to version 1.2.0 or apply patches manually
https://github.com/pimcore/pimcore/commit/757375677dc83a44c6c22f26d97452cc5cda5d7c.patch
https://github.com/pimcore/admin-ui-classic-bundle/commit/19fda2e86557c2ed4978316104de5ccdaa66d8b9.patch