# EPUBCheck RSC-031: Remote resource references should use…

Source: https://epublab.com/epubcheck/rsc-031/ (updated 2026-11-03)

EPUBCheck RSC-031 is a warning message: “Remote resource references should use HTTPS, but found "…".” A remote resource is referenced with http instead of https.

- Official message (EPUBCheck 5.4.0): Remote resource references should use HTTPS, but found "%1$s".
- Default severity: Warning

## Typical causes

- Old links or remote media that were never updated to https.

## How to fix it

- Change the URL to https if the server supports it.

Message text and severity: EPUBCheck (W3C, BSD-3-Clause). Explanation: EPUBLab.