CVE-2026-73249 in calibre
Summary
by MITRE • 08/12/2026
calibre is an e-book manager. Prior to 9.12.0, the calibre Content Server endpoint POST /book-update-annotations/{library_id}/{book_id}/{fmt} in src/calibre/srv/books.py omits needs_db_write=True, causing Router.dispatch() to skip ctx.check_for_write_access() before update_annotations() passes attacker-controlled JSON to db.merge_annotations_for_book(), which allows a readonly user or an anonymous user on an unauthenticated deployment to persist unauthorized book annotation changes. This issue is fixed in version 9.12.0.
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Analysis
by VulDB Data Team • 08/12/2026
The vulnerability exists within the calibre Content Server's book annotation update functionality, specifically in the POST endpoint located at /book-update-annotations/{library_id}/{book_id}/{fmt}. This flaw represents a critical access control bypass that allows unauthorized users to modify book annotations without proper authentication or authorization privileges. The issue stems from a missing security parameter in the route definition where needs_db_write=True is omitted, which fundamentally alters the server's permission checking behavior during the request processing cycle.
The technical execution of this vulnerability begins with the Router.dispatch() method failing to invoke ctx.check_for_write_access() due to the missing write access requirement flag. This omission creates a pathway where the subsequent update_annotations() function processes attacker-controlled JSON data without proper validation of user privileges. The processed JSON data then flows directly into db.merge_annotations_for_book(), which performs the actual database modification operations. This direct data flow bypasses all standard read-only user permission checks, allowing anonymous or readonly users to manipulate book annotation records within the calibre library system.
The operational impact of this vulnerability extends beyond simple privilege escalation as it enables persistent unauthorized modifications to book metadata and annotations that can be used for malicious purposes such as data corruption, information manipulation, or serving as a foothold for further attacks within the content management system. The vulnerability affects deployments where authentication is not strictly enforced, making it particularly dangerous in open or poorly secured environments. Attackers could potentially use this weakness to alter book metadata, inject malicious annotations, or create persistent backdoors through annotation modifications that might not be immediately apparent to administrators.
This vulnerability aligns with CWE-284 Access Control Bypass and represents a classic case of insufficient authorization checks in web applications. The flaw demonstrates poor security implementation practices where the absence of proper access control flags leads to unintended privilege escalation. From an ATT&CK perspective, this issue maps to T1078 Valid Accounts and T1566 Phishing with the potential for privilege escalation through unauthorized database modifications. Organizations should immediately upgrade to calibre version 9.12.0 where this vulnerability has been addressed through proper access control enforcement mechanisms.
The fix implemented in version 9.12.0 corrects the missing needs_db_write=True parameter, which restores the intended security checks within the Router.dispatch() method. This restoration ensures that ctx.check_for_write_access() is properly invoked before any annotation modification operations are permitted, thereby enforcing the necessary authorization requirements for users attempting to update book annotations. The mitigation strategy should also include implementing proper access controls and authentication mechanisms across all calibre Content Server deployments, particularly in environments where anonymous access is enabled or where readonly user accounts exist. Organizations should conduct security reviews of their content management systems to identify similar access control bypass vulnerabilities that might exist in other components or endpoints.
The vulnerability highlights the critical importance of proper access control implementation in web applications and demonstrates how seemingly minor configuration omissions can create significant security risks. The issue represents a failure in the principle of least privilege where read-only users are granted write capabilities through improper route parameter handling. This type of vulnerability is particularly concerning because it allows attackers to make persistent changes that may not be easily detected through standard monitoring procedures, as annotation modifications might appear as legitimate user activity within normal system operations.
Organizations utilizing calibre Content Server should implement comprehensive security measures including regular vulnerability scanning, access control reviews, and privilege management audits. The fix in version 9.12.0 addresses the immediate concern but does not eliminate the need for broader security practices such as principle of least privilege enforcement, proper authentication mechanisms, and continuous monitoring of database modification activities within content management systems. This vulnerability serves as a reminder that even specialized applications like e-book managers require rigorous security implementation practices to prevent unauthorized access and data manipulation attacks.
The root cause analysis reveals that this vulnerability emerged from inadequate security architecture review during development, where the route definition did not properly consider the security implications of write operations. The missing needs_db_write=True parameter created an implicit trust model that allowed any authenticated or unauthenticated user to perform write operations on book annotations, fundamentally undermining the intended access control design. This type of oversight demonstrates the importance of security-focused code reviews and automated security testing in preventing access control bypass vulnerabilities that can have serious implications for data integrity and system security posture.
Modern security practices recommend implementing comprehensive access control matrices that explicitly define which user roles can perform specific operations on different resources within content management systems. The vulnerability in calibre Content Server illustrates the necessity of such explicit controls, particularly when dealing with database modification endpoints where unauthorized changes could have lasting impacts on system data integrity and user experience. Organizations should treat this vulnerability as a case study for implementing proper security controls in all web application development projects, ensuring that access control mechanisms are explicitly defined and enforced at every level of the application architecture.