Calibre
CVE-2026-27824
MEDIUM
Vulnerability Assessment
| Weakness |
CWE
OWASP Top 10:2025
A07:2025 - Authentication Failures
ASVS 5.0
V6 - Authentication Brute-force prevention, rate limiting
|
| Impact | CVSS 5.3 (MEDIUM). Attack vector: Network. Complexity: Low. Privileges required: None. User interaction: None. Exploitable via remote/network. Public exploit/POC available. |
| Exploit ScenarioDerived | A remote attacker could exploit this vulnerability to compromise the affected system. |
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Alert meAnalysisAIv1 – Mar 12, 2026
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. No patch is currently available for affected versions.
Earlier versions of this analysis (1)
Calibre versions up to 9.4.0 is affected by improper restriction of excessive authentication attempts (CVSS 5.3).
RemediationAI
Monitor vendor advisories for a patch. Restrict network access to the affected service where possible.
Affects your stack
Find out whether the stacks and servers you run are on an affected version.
Checking this CVE against your own stacks and servers is part of the Basic plan and up.
Technical ContextAI · show details
This vulnerability (CWE-307: Improper Restriction of Excessive Authentication Attempts) affects Calibre. 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-forc
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- Exploit and PoC references, threat intelligence from 22 sources
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Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Primary rating from GitHub Advisory.
CVSS VectorGitHub Advisory
Lifecycle Timeline
2DescriptionGitHub Advisory
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.
Executive ViewAI
Organizations using calibre should be aware of moderate risk from CVE-2026-27824 rated CVSS 5.3. Exploitation could compromise the security of affected deployments. Proof-of-concept code is publicly available. Organizations should apply vendor updates when available and monitor for exploitation.
Apply vendor patch. If patching is delayed: restrict network access and enable monitoring.
Threat Intelligence Overview
NVD
CVE.org
EPSS
GitHub
How this decision was reached
The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.
What every action level means
SSVC AssessmentENISA
Priority Score
Attack Technique
Authentication bypass attacks exploit flaws in the verification mechanisms that control access to systems and applications.
Affected Products (CPE)GitHub Advisory
Remote code execution in Calibre 9.2.1 and earlier allows authenticated users to write arbitrary files via a path traver
Calibre versions 9.2.1 and below allow authenticated users to write arbitrary files with any extension to any writable l
Remote code execution in Calibre prior to version 9.2.0 through a path traversal flaw in the CHM reader allows local att
Calibre 9.1.0 and earlier contains a path traversal vulnerability in EPUB conversion that allows malicious EPUB files to
calibre is an e-book manager. [CVSS 7.8 HIGH]
gui2/viewer/bookmarkmanager.py in Calibre 3.18 calls cPickle.load on imported bookmark data, which allows remote attacke
Path traversal in Calibre <= 7.14.0 allow unauthenticated attackers to achieve arbitrary file read. Rated high severity
link_to_local_path in ebooks/conversion/plugins/html_input.py in calibre before 6.19.0 can, by default, add resources ou
calibre before 5.32.0 contains a regular expression that is vulnerable to ReDoS (Regular Expression Denial of Service) i
Unsanitized user-input in Calibre <= 7.15.0 allow users with permissions to perform full-text searches to achieve SQL in
HTTP response header injection in Calibre Content Server prior to version 9.4.0 permits authenticated users to inject ar
Unsanitized user-input in Calibre <= 7.15.0 allow attackers to perform reflected cross-site scripting. Rated medium seve
Exploit-DB