nextcloud/server

[Bug]: Improper input validation in PublicPreviewController triggers internal server error

Open

#59229 opened on Mar 26, 2026

View on GitHub
 (1 comment) (0 reactions) (0 assignees)PHP (34,953 stars) (4,865 forks)batch import
0. Needs triage32-feedbackbugfeature: previews and thumbnailsfeature: sharinggood first issue

Description

⚠️ This issue respects the following points: ⚠️

Bug description

An incomplete input validation in PublicPreviewController can trigger an internal server error.

Steps to reproduce

Case A

  1. Create a public link for a folder
  2. Send GET https://server33.internal/index.php/apps/files_sharing/publicpreview/{token}
  3. 💥

https://github.com/nextcloud/server/blob/7e9e1269a059ddfc7807f977707a1800e3a303e4/apps/files_sharing/lib/Controller/PublicPreviewController.php#L123-L130

  • Default for $file is an empty string.
  • $file = $node->get(''); is still an Folder instance
  • getPreview expectes File

Case B

  1. Create a public link for a folder
  2. Send GET https://server33.internal/index.php/apps/files_sharing/publicpreview/{token}?file=notexist.png&mimeFallback=1
  3. 💥

https://github.com/nextcloud/server/blob/7e9e1269a059ddfc7807f977707a1800e3a303e4/apps/files_sharing/lib/Controller/PublicPreviewController.php#L122-L142

  • get and getPreview both throw NotFoundException.
  • However the branch with mimetype fallback only works if the preview not exists, not if the node not exists.

Expected behavior

No internal server error

Contributor guide

Tech stack
php
Domain
backendapi
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Basic PHPFamiliarity with Nextcloud public sharingUnderstanding of exception handling
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75
Research direction
Investigate the PublicPreviewController.php file, particularly lines 122-142. The bug occurs when an empty $file parameter or a non existent file path is passed. The code should check if the node is a File instance before calling getPreview, and handle NotFoundException appropriately. Consider modifying the logic to return a proper error response instead of triggering an exception. Also, review the existing tests for public previews.