CVE-2026-87812 in SiYuan
Summary
by MITRE • 09/09/2026
SiYuan before v3.8.2 contains a stored cross-site scripting vulnerability in Bazaar package cards where the iconURL metadata is inserted directly into HTML img src attributes without escaping. Attackers can inject malicious URLs with event handlers that execute JavaScript in the authenticated SiYuan origin when users view Bazaar listings, enabling API requests and application state manipulation.
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Analysis
by VulDB Data Team • 09/09/2026
The vulnerability identified in SiYuan versions prior to 3.8.2 represents a critical stored cross-site scripting flaw located within the Bazaar package management interface. This security defect arises from improper input validation and output encoding mechanisms when handling metadata associated with community-contributed packages, specifically targeting the iconURL field. The application retrieves this URL value directly from its internal data store or user-provided inputs during the rendering of package cards in the marketplace view. Instead of sanitizing or escaping special characters within the URL string before insertion into the Document Object Model, the software embeds the raw value directly into an HTML img tag's src attribute. This architectural oversight allows for the injection of malicious payloads that exploit standard browser parsing behaviors to execute arbitrary JavaScript code within the context of the authenticated SiYuan application origin.
From a technical perspective, this vulnerability is classified under CWE-79, which covers Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting. The specific nature of this flaw aligns with stored XSS patterns where malicious content persists in the backend database or configuration files and is subsequently served to multiple victims upon request. By injecting a crafted URL containing event handlers such onerror or onload, an attacker can bypass standard attribute boundaries because modern browsers may interpret certain malformed src attributes as executable script contexts if not properly escaped. This mechanism enables the execution of JavaScript code every time a user navigates to the Bazaar section and views the compromised package card, effectively turning any visitor into a victim without requiring further interaction beyond page load.
The operational impact of this vulnerability is severe due to its stored nature and high visibility within the application interface. Since SiYuan handles sensitive personal notes, documents, and potentially API keys or authentication tokens, an attacker can leverage the executed JavaScript to perform actions on behalf of the authenticated user. This includes stealing session cookies, hijacking active sessions, exfiltrating local data through fetch requests to external servers, or manipulating the application state by modifying DOM elements directly. The ability to execute code in the trusted origin means that traditional browser same-origin policy protections are bypassed for malicious scripts originating from within the application itself, allowing attackers to interact with internal APIs and read protected content stored locally on the user's device.
This incident maps closely to several techniques defined in the MITRE ATT&CK framework for enterprise security tracking. It corresponds primarily to T1059 Command and Scripting Interpreter under JavaScript execution capabilities, as well as T1204 User Execution if social engineering is used to prompt users to view specific listings. Furthermore, the potential data exfiltration aligns with T1005 Data from Local System, while session hijacking aspects relate to T1539 Steal Web Session Cookie. The persistence of the payload in the Bazaar database ensures that the attack vector remains active until the malicious package is removed or the application updates its rendering logic, creating a long-term risk for all users accessing the marketplace during this window.
Mitigation strategies must focus on both immediate remediation and long-term architectural improvements. Users running versions prior to 3.8.2 should upgrade immediately as version 3.8.2 addresses these input validation flaws by implementing strict output encoding for all dynamic content rendered in HTML contexts. For developers, the fix involves ensuring that any user-supplied data inserted into DOM elements is properly escaped using context-aware encoding techniques rather than simple string concatenation. Implementing a Content Security Policy with nonces or hashes can also provide an additional layer of defense by restricting script execution to only those explicitly authorized by the application server. Regular security audits focusing on template rendering engines and third-party package integrations are essential to prevent similar vulnerabilities in future releases, ensuring that all metadata fields undergo rigorous sanitization before being presented to end-users.