CVE-2026-105205 in SiYuan
Summary
by MITRE • 10/04/2026
SiYuan before 3.8.5 contains an information disclosure vulnerability that allows publish-mode readers to learn backlink block IDs and reference counts from password-protected and publish-disabled documents by querying a published document. Attackers can send POST requests to /api/block/getDocInfo or getDocsInfo for a published document ID to obtain refIDs and refCount of hidden referencing blocks, bypassing the publish confidentiality boundary.
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Analysis
by VulDB Data Team • 10/04/2026
The vulnerability identified in SiYuan versions prior to 3.8.5 represents a critical failure in access control logic within the application's API layer, specifically affecting documents that are configured with restricted visibility settings such as password protection or disabled publishing status. This flaw allows an attacker who has obtained the document ID of a published resource to bypass intended confidentiality boundaries and extract sensitive metadata about hidden referencing blocks. The core technical issue lies in the insufficient validation performed by the server-side endpoints /api/block/getDocInfo and /api/block/getDocsInfo when processing POST requests for specific document identifiers. Instead of verifying whether the requesting user possesses the necessary permissions to view backlink information or reference counts, these endpoints return detailed structural data regardless of the document's actual access restrictions. This design oversight effectively creates a side-channel through which an adversary can map out the internal structure and interconnectivity of private content without ever needing to authenticate against the protected resource itself.
From a technical perspective, this vulnerability exploits the decoupling between public-facing API endpoints and the underlying permission checks that should govern data retrieval operations. When an attacker sends a POST request containing the ID of a published document, the server responds with refIDs and refCount values corresponding to blocks within hidden or protected documents that reference the published one. This information disclosure is particularly dangerous because it reveals not just the existence of private content but also its structural relationship to public data. By analyzing these backlink block IDs and their associated counts, an attacker can infer the scope of sensitive information stored in non-public sections of the workspace. For instance, a high reference count might indicate that a particular piece of confidential documentation is heavily linked from various other areas, suggesting it contains critical intellectual property or personal data worth targeting further. This level of granularity provides attackers with valuable reconnaissance intelligence that significantly lowers the barrier for subsequent exploitation attempts.
The operational impact of this vulnerability extends beyond simple information leakage to include potential facilitation of more severe attacks such as privilege escalation or targeted social engineering. Knowledge of which private documents reference public ones allows an attacker to prioritize their efforts on specific targets, increasing the likelihood of successful compromise. Furthermore, understanding the block-level structure can aid in crafting precise injection payloads if other vulnerabilities exist within the rendering engine or database layer. The ability to enumerate hidden content also violates fundamental security principles regarding data isolation and least privilege access. Users who rely on SiYuan for managing sensitive corporate or personal information face a significant risk of exposure, as their expectation that password-protected documents remain inaccessible is fundamentally undermined by this API flaw. In enterprise environments where compliance with standards like GDPR or HIPAA may be required, such unauthorized disclosure could lead to regulatory penalties and loss of trust among stakeholders who depend on the platform's security guarantees.
To mitigate this vulnerability, immediate action must focus on patching the SiYuan installation to version 3.8.5 or later, where these access control checks have been corrected. Administrators should ensure that all instances are updated promptly to close the gap in API-level permission validation. In addition to applying patches, organizations should implement network-level controls such as Web Application Firewalls (WAFs) to monitor and restrict unusual POST requests targeting block information endpoints. It is also advisable to review logging mechanisms to detect any potential exploitation attempts that may have occurred prior to remediation. Long-term improvements involve adopting a defense-in-depth strategy where API endpoints are rigorously audited for proper authorization checks before returning sensitive metadata. Regular security assessments and penetration testing should be conducted to identify similar logic flaws in other parts of the application stack, ensuring that confidentiality boundaries remain intact even when public-facing interfaces are accessible.
This incident aligns with CWE-200, which classifies information exposure as a category where an attacker gains access to sensitive data through improper configuration or implementation errors. It also relates to ATT&CK technique T1537, involving the transfer of data over a different channel than intended for primary communication, although in this case it is more accurately described as unauthorized discovery of hidden resources via API abuse. The failure to enforce consistent permission checks across all entry points highlights the importance of centralized access control policies within application architecture. Developers must ensure that every API endpoint validates user permissions against the specific resource being accessed, rather than relying on implicit assumptions about document status. By addressing these root causes through code reviews and automated security testing, organizations can prevent similar information disclosure vulnerabilities from compromising their data integrity and confidentiality in future deployments of SiYuan or similar knowledge management platforms.