Contao is an Open Source CMS. From version 5.0.0 until 5.3.50 and 5.7.12, ImagesController joins the user-controlled {path} parameter to the configured image target directory with Path::join() but does not use Path::isBasePath() to verify that the canonical path remains inside that directory. An unauthenticated request containing encoded parent-directory segments can therefore return files under the project directory through BinaryFileResponse when their names use an extension allowed by contao.image.valid_extensions. The route can also reveal whether arbitrary paths exist, and debug responses can disclose absolute filesystem paths, but paths below the upload directory were not shown to be readable. This issue is fixed in versions 5.3.50 and 5.7.12.

Affected: Packagist: contao/core-bundle

`ImagesController` joins the user controlled `{path}` onto the configured image target directory with `Path::join()`, which canonicalises `..` segments, and never calls `Path::isBasePath()` to confirm the result stayed inside that directory. An unauthenticated GET with percent encoded dot segments therefore escapes the directory and the file is returned by `BinaryFileResponse`. Reads are bounded to the extensions in `contao.image.valid_extensions`, and paths below the upload directory fail for an unrelated reason, so this is reported as a missing boundary check rather than as a general arbitrary file read. Both Apache with the shipped `public/.htaccess` and nginx with the documented configuration are affected.

### Impact

Any unauthenticated visitor can read files that are deliberately kept outside the document root, anywhere under the project directory, as long as the name ends in one of `contao.image.valid_extensions` (`jpg`, `jpeg`, `gif`, `png`, `tif`, `tiff`, `bmp`, `svg`, `svgz`, `webp`). The route also acts as an existence oracle for arbitrary paths, and on a debug enabled instance the 404 message discloses the absolute filesystem path.

**Honesty caveat, stated because it lowers the severity.** The reporter could not use this to read protected member folders: every path below the upload directory returns 500 for the reason given under Details, so the practically reachable set is image typed files outside `files/`.