CVE-2026-91021 in Notes
Summary
by MITRE • 09/14/2026
Trilium Notes, version v0.103.0 and earlier, contains a stored cross-site scripting (XSS) vulnerability in the share renderer for webView notes due to improper HTML escaping of user-controlled #webViewSrc values. This vulnerability allows attackers with note-authoring privileges to inject arbitrary JavaScript that executes for any user who opens the shared note, including administrators.
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Analysis
by VulDB Data Team • 09/14/2026
The security flaw identified in Trilium Notes versions v0.103.0 and earlier represents a critical stored cross-site scripting vulnerability located within the share renderer component responsible for handling webView notes. This specific implementation defect stems from an improper HTML escaping mechanism applied to user-controlled input fields, specifically the #webViewSrc parameter. In web application architecture, parameters designated as source URLs or script sources are often treated with higher trust levels because they are intended to load external resources such as iframes or scripts. However, when these inputs are not rigorously sanitized against special characters and malicious payloads before being rendered in a browser context, they become vectors for code injection. The vulnerability allows an attacker who possesses note-authoring privileges within the Trilium Notes instance to embed arbitrary JavaScript code directly into the shared content of a note.
The operational impact of this vulnerability is severe due to its stored nature and broad attack surface. Unlike reflected XSS where the payload must be delivered via a crafted link, stored XSS persists in the application's database or file system. Consequently, any user who accesses the compromised shared note will have their browser execute the injected script automatically without further interaction from the attacker. This includes administrators of the Trilium Notes instance, which significantly escalates the potential damage. An administrator account typically holds elevated privileges that may allow access to sensitive data, configuration settings, or other users' notes. By exploiting this flaw, an attacker can perform actions such as session hijacking by stealing authentication cookies, performing unauthorized administrative actions on behalf of the victim, defacing shared content, or redirecting users to malicious phishing sites designed to harvest credentials.
From a classification perspective, this vulnerability aligns with CWE-79 Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting. The specific mechanism involves insufficient validation and encoding of user-supplied input before it is included in the HTML output sent to the client browser. Furthermore, within the context of the MITRE ATT&CK framework, this attack vector maps to T1059 Command and Control via Web Protocols or potentially T1204 User Execution if the payload relies on social engineering to trigger additional actions after execution. The persistence aspect places it firmly in the category of stored attacks where the malicious code remains active until the vulnerable note is deleted or the vulnerability is patched, creating a long-term risk for all users interacting with shared content.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. For administrators using affected versions, the most effective immediate action is to upgrade Trilium Notes to version v0.103.1 or later where this issue has been resolved through improved input sanitization protocols. In environments where upgrading is not immediately feasible, implementing a Web Application Firewall with rules capable of detecting and blocking common XSS payloads in note content fields can provide temporary protection. Additionally, enforcing strict Content Security Policy headers that restrict the execution of inline scripts and limit allowed sources for iframes can mitigate the impact even if an injection occurs. Developers should also adopt secure coding practices such as context-aware output encoding rather than relying solely on generic escaping functions to ensure that all user-controlled data is properly neutralized before rendering in HTML, JavaScript, or CSS contexts.