What’s Vulnerable › Vulnerabilities
CVE-2026-27824: calibre has IP Ban Bypass via X-Forwarded-For Header Spoofing
- Severity: medium (CVSS 5.3 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.2% probability of exploitation in the next 30 days
- Public exploit or detection template: yes
- Published: 2026-02-27 · updated 2026-06-17
- Weakness: CWE-307
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.
Affected products
- kovidgoyal calibre: before 9.4.0 — fixed in 9.4.0
How to fix
Upgrade to a fixed version: kovidgoyal calibre 9.4.0. Check the vendor advisory for mitigations.
References
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