Impact

A user could upload PHP files through image upload functions, which would allow them to execute code on the host system remotely. They would then have the permissions of the PHP process.

This most impacts scenarios where non-trusted users are given permission to upload images in any area of the application.

Patches

The issue was addressed in a series of patches: v0.25.3, v0.25.4 & v0.25.5.
Users should upgrade to at least v0.25.5 to avoid this patch but ideally the latest BookStack version as previous versions are un-supported.

Workarounds

Depending on BookStack version, you could use the local_secure image storage option, or use s3 or a similar compatible service.

Preventing direct execution of any php files, apart from the public/index.php file, though web-server configuration would also prevent this.

References

BookStack Beta v0.25.3
BookStack Beta v0.25.4
BookStack Beta v0.25.5

Attribution

• Thanks to inc0x0 for originally discovering and reporting this vulnerability.
• Thanks to Thiago Mayllart for discovering & reporting more ways to exploit this vulnerability & advising the creation of a CVE for this issue.

For more information

If you have any questions or comments about this advisory:

• Open an issue in the BookStack GitHub repository.
• Ask on the BookStack Discord chat.
• Follow the BookStack Security advise to contact someone privately.