CVE-2026-27824
MEDIUMCVSS Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
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3Description
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.
Analysis
Calibre Content Server's brute-force protection can be bypassed by manipulating the X-Forwarded-For HTTP header, allowing attackers to circumvent IP-based account lockouts and conduct credential stuffing attacks. Public exploit code exists for this vulnerability, which affects Calibre versions prior to 9.4.0 and poses a significant risk to internet-exposed servers where brute-force protection is the primary authentication defense mechanism. …
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Remediation
Within 30 days: Identify affected systems and apply vendor patches as part of regular patch cycle. Monitor vendor channels for patch availability.
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