CVE-2021-4170 in Calibre-Web
Summary
by MITRE • 01/17/2022
calibre-web is vulnerable to Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
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Analysis
by VulDB Data Team • 05/02/2025
The vulnerability identified as CVE-2021-4170 affects calibre-web, a web-based digital library management system that allows users to organize, search, and access their e-book collections through a web interface. This system serves as a bridge between users and their digital libraries, providing functionalities such as book browsing, metadata management, and user authentication. The vulnerability manifests in the web page generation process where input validation and sanitization mechanisms fail to properly neutralize user-provided data before incorporating it into web responses. This flaw exists within the application's rendering pipeline where user-supplied content flows directly into HTML output without adequate security controls.
The technical flaw represents a classic cross-site scripting vulnerability that falls under CWE-79 which defines improper neutralization of input during web page generation. Attackers can exploit this weakness by injecting malicious javascript code through various input vectors including book titles, author names, descriptions, or other user-editable fields within the calibre-web interface. When the system processes and displays this malicious input without proper sanitization, the injected scripts execute in the context of other users' browsers who view the affected web pages. This creates a persistent threat where malicious payloads can be stored and distributed to unsuspecting users, potentially leading to session hijacking, credential theft, or other malicious activities.
The operational impact of this vulnerability extends beyond simple data corruption or display issues. An attacker who successfully exploits this XSS flaw can compromise the entire web application's security posture by executing arbitrary code in users' browsers. This could enable attackers to steal session cookies, perform actions on behalf of authenticated users, redirect victims to malicious sites, or even establish persistent backdoors through browser-based attacks. The vulnerability affects all users who interact with the calibre-web interface, particularly those with administrative privileges who may have access to sensitive metadata and user information. The attack surface is broad since the vulnerability can be triggered through multiple input points within the web application's user interface.
Mitigation strategies should focus on implementing comprehensive input validation and output encoding mechanisms throughout the calibre-web application. The most effective approach involves applying context-specific output encoding before rendering any user-provided content into HTML pages, following the principle of least privilege for input handling. Organizations should implement Content Security Policy headers to limit script execution and prevent unauthorized code injection. Additionally, regular security audits and code reviews should be conducted to identify similar vulnerabilities in other parts of the application. The remediation process requires updating the calibre-web software to the latest version where this vulnerability has been patched, as well as implementing proper input sanitization routines that conform to OWASP Top Ten security guidelines. Security monitoring should be enhanced to detect suspicious user activities that might indicate exploitation attempts, and user access controls should be reviewed to minimize potential damage from successful attacks. This vulnerability aligns with ATT&CK technique T1059.007 which describes script injection attacks, emphasizing the importance of proper input validation and output encoding in preventing such security breaches.