Bug 2443417 (CVE-2026-27824)

Summary:	CVE-2026-27824 calibre: Calibre: Brute-force protection bypass via X-Forwarded-For header manipulation
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security DevOps Team <prodsec-dev>
Status:	NEW ---	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	Keywords:	Security
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Hardware:	All
OS:	Linux
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Doc Text:	A flaw was found in calibre, an e-book manager. A remote attacker can bypass the brute-force protection mechanism in the calibre Content Server by manipulating the X-Forwarded-For header. This vulnerability allows attackers to circumvent IP-based bans, making the server susceptible to credential stuffing and password guessing attacks, potentially leading to unauthorized access.	Story Points:	---
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oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
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Bug Depends On:	2443782
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Description OSIDB Bzimport 2026-02-27 20:01:55 UTC

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.