CVE-2026-116020 in BookStack
Summary
by MITRE • 10/24/2026
BookStack before 26.09.2 contains an incorrect authorization vulnerability in AttachmentController that allows authenticated users to access other users' draft page attachments by ID. Attackers with attachment permissions can enumerate sequential attachment IDs to download, rename, relink, or replace files on another user's unpublished draft pages.
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Analysis
by VulDB Data Team • 10/24/2026
The identified security flaw resides within the AttachmentController component of BookStack versions prior to 26.09.2 and represents a classic case of insecure direct object reference leading to broken access control. This vulnerability stems from an incorrect authorization check that fails to validate whether the authenticated user requesting a specific attachment actually owns or has legitimate permissions for that resource. Instead of verifying ownership against the session context, the application relies solely on the sequential nature of the attachment identifiers provided in the request parameters. Consequently, any authenticated user who possesses basic attachment creation and management privileges can bypass intended isolation boundaries between different users' workspaces.
The technical mechanism behind this exploitation involves the predictable generation of unique identifiers for attachments within the database. Because these IDs are typically assigned sequentially or via a simple incrementing counter without sufficient randomization or cryptographic uniqueness, an attacker can easily enumerate valid attachment references by iterating through integer values. By submitting HTTP requests with varying ID parameters to endpoints associated with downloading, renaming, relinking, or replacing files, the application processes each request based on the existence of the object rather than the relationship between the user and that object. This lack of server-side ownership verification allows an adversary to interact directly with resources belonging to other users who are currently working on unpublished draft pages.
The operational impact of this vulnerability is significant for organizations relying on BookStack for collaborative documentation where sensitive or proprietary information may be stored in drafts before publication. An attacker can exploit this flaw to exfiltrate confidential data by downloading attachments associated with another user's private drafts, potentially leading to a breach of confidentiality and intellectual property theft. Furthermore, the ability to rename, relink, or replace files introduces risks related to integrity and availability. For instance, an adversary could replace a legitimate file with malicious content that might be executed if processed later, or delete critical attachments entirely, thereby disrupting workflow and causing data loss for other team members who depend on those documents.
This vulnerability aligns closely with CWE-639, which describes authorization bypass through prediction of resource identifiers, as well as the broader category of CWE-284 regarding improper access control. In terms of offensive security frameworks, this behavior maps to MITRE ATT&CK technique T1078, specifically valid accounts and potentially T1530 for data exfiltration from cloud storage or object stores if the attachments are stored externally. The exploitation does not require complex payload construction but rather systematic enumeration, making it a low-effort attack vector that can be automated easily by script kiddies or advanced persistent threats alike.
To mitigate this risk, organizations must immediately upgrade BookStack to version 26.09.2 or later where the authorization logic has been corrected to enforce strict ownership checks on all attachment-related operations. Until an upgrade is feasible, administrators should consider restricting user permissions to limit who can create and manage attachments if possible, although this may impact legitimate workflow requirements. Additionally, implementing web application firewall rules that detect rapid sequential access patterns to resource IDs could provide a temporary layer of defense against automated enumeration attacks. Regular security audits focusing on object-level authorization checks are recommended to prevent similar vulnerabilities in other parts of the application where direct object references might be used without proper validation.