# EPUBCheck RSC-031: remote resource over HTTP, how to fix it

Source: https://epubcc.com/epubcheck/rsc-031/ (updated 2026-11-04)

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. It is a warning: some reading apps block insecure connections, so the resource may not load.

- 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.
- Remote audio, video or fonts copied from an old project or from a server that has since moved to https.

## How to fix it

1. Find the URL named in the message.
2. Change it to https if the server supports it.
3. If the resource is essential, consider storing it inside the EPUB instead, so the book does not depend on a server or a network connection, then validate again.

## When it's not this

If the remote resource is not allowed to be remote at all, the error is RSC-006.

## Sources

- EPUB 3.3, §3.6 Resource locations: https://www.w3.org/TR/epub-33/#sec-resource-locations (read 2026-11-04)

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