RSC-031 is reported as warning by default. The EPUB is still valid, but something is likely to cause problems in some reading apps. EPUBCheck exits with 0 unless you pass --failonwarnings.

EPUBCheck · Resources and references

`RSC-031`

Updated: Nov 3, 2026

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.

The official message

Remote resource references should use HTTPS, but found "%1$s".

Default severity: Warning. The EPUB is still valid, but something is likely to cause problems in some reading apps. EPUBCheck exits with 0 unless you pass --failonwarnings.
EPUBCheck version: 5.4.0

%1$s, %2$s… are filled in with the file, value or element EPUBCheck found.

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.

Terms used here

Other RSC messages

RSC-030 File URLs are not allowed in EPUB, but found "…".
RSC-032 Fallback must be provided for foreign resources, but found none for resource "…" of type "…".
RSC-029 Data URL is not allowed in this context.
RSC-033 Relative URL strings must not have a query component, but found one in "…".
RSC-028 XML documents must be encoded in UTF-8, but … was detected.
RSC-027 XML document is encoded in UTF-16. It should be encoded in UTF-8 instead.

Message text and severity from EPUBCheck 5.4.0 (W3C, BSD-3-Clause). Explanation by EPUBLab.