CVE-2026-27824 calibre has IP Ban Bypass via X-Forwarded-For Header Spoofing

Medium 5.3 Published 27 Feb 2026 Updated 2 Mar 2026

Attackers can bypass IP-based brute-force protection in calibre Content Server by spoofing the X-Forwarded-For header, facilitating credential stuffing attacks.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-27824?

CVE-2026-27824 is a medium-severity vulnerability in kovidgoyal calibre, published on 27 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

calibre is a cross-platform e-book manager for viewing, converting, editing, and cataloging e-books. Prior to version 9.4.0, the calibre Content Server's brute-force protection mechanism uses a ban key derived from both `remote_addr` and the `X-Forwarded-For` header. Since the `X-Forwarded-For` header is read directly from the HTTP request without any validation or trusted-proxy configuration, an attacker can bypass IP-based bans by simply changing or adding this header, rendering the brute-force protection completely ineffective. This is particularly dangerous for calibre servers exposed to the internet, where brute-force protection is the primary defense against credential stuffing and password guessing attacks. Version 9.4.0 contains a fix for the issue.

Weakness: CWE-346, CWE-307

Which versions are affected by CVE-2026-27824?

It affects 1 product.

Products include Calibre-Ebook Calibre.

How do I fix CVE-2026-27824?

Upgrade to a fixed version.

Is CVE-2026-27824 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-27824 exists.

Sources

Also known as DEBIAN-CVE-2026-27824, GHSA-vhxc-r7v8-2xrw, UBUNTU-CVE-2026-27824.