Description

SiYuan is a self-hosted personal knowledge management system. In versions up to and including 3.8.3, the kernel's authentication guards (CheckAuth in kernel/model/session.go and IsSessionOriginAllowed in kernel/util/net.go) fail open when the HTTP Origin header is absent, on the incorrect assumption that any browser-initiated cross-site request carries an Origin. Because browsers omit Origin on cross-site top-level GET navigations and no-cors GET subresource loads — and the session cookie is SameSite=Lax — a single cross-site GET issued from any attacker-controlled web page is granted RoleAdministrator, both on default installations with no access-authorization code and on password-protected instances with a live session. Combined with content-type sniffing on the /api/network/proxy endpoint, which allows attacker-controlled HTML to be served under SiYuan's own origin, this permits an unauthenticated remote attacker to execute arbitrary script in the SiYuan origin (http://127.0.0.1:6806), invoke administrator APIs, and exfiltrate the persistent kernel API token. This issue is fixed in version 3.8.4.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "8.1",
  "pubDate": "2026-09-26T14:16:46.900Z",
  "pubdate": "2026-09-26T14:16:46.900Z",
  "executiveSummary": "SiYuan versions up to and including 3.8.4 are susceptible to an authentication bypass vulnerability stemming from flawed logic in the session validation mechanism. The vulnerability exists within the application's kernel, specifically where HTTP Origin header validation is performed. By failing to authenticate requests when the Origin header is absent—an occurrence common in cross-site GET navigations—the system erroneously grants administrative privileges to unauthenticated, attacker-controlled requests.\nThis vulnerability is critical because it allows a remote attacker to bypass authentication on both default and password-protected instances. When combined with content-type sniffing capabilities present in the /api/network/proxy endpoint, an attacker can achieve Cross-Site Scripting (XSS) within the SiYuan origin. The potential impact is significant, as it enables the execution of arbitrary scripts, unauthorized invocation of administrator-level APIs, and the exfiltration of the persistent kernel API token, effectively granting an attacker full control over the user's personal knowledge management data.",
  "technicalDetails": "The vulnerability originates from an insecure implementation of authentication guards in the SiYuan kernel, specifically within kernel/model/session.go (CheckAuth) and kernel/util/net.go (IsSessionOriginAllowed). The application logic incorrectly assumes that all cross-site, browser-initiated requests must include an HTTP Origin header. Consequently, when a request is made without this header—such as during top-level GET navigations or no-cors GET subresource loads—the security check fails open, bypassing the mandatory session validation.\nBecause SiYuan uses SameSite=Lax for session cookies, these cross-site GET requests are permitted to transmit authentication cookies, allowing the application to treat the incoming request as legitimate. An attacker can exploit this by forcing a victim's browser to navigate to a malicious URL, which then triggers a request to the SiYuan instance (http://127.0.0.1:6806). The kernel validates the session as if it were an authorized administrator, despite the cross-site nature of the request.\nThe attack is exacerbated by content-type sniffing vulnerabilities in the /api/network/proxy endpoint. By leveraging this, an attacker can serve arbitrary HTML content that is rendered within the context of the SiYuan origin. This creates a powerful XSS vector. The full attack flow involves: 1) The attacker initiates a cross-site GET request to a target SiYuan instance to trigger the authentication bypass; 2) The attacker leverages the /api/network/proxy endpoint to serve malicious content that is interpreted as part of the SiYuan origin; 3) The malicious script executes in the user's browser, gaining access to the SiYuan session context; 4) The script invokes administrative API calls or exfiltrates the persistent kernel API token.\nThis vulnerability affects SiYuan versions up to and including 3.8.3. It requires no specific authentication or pre-existing privileges from the attacker; the requirement is simply that the target user has a live session or that the instance is in a default installation state where the authorization check is bypassed entirely. The network exposure is limited to instances accessible by the victim's browser, typically local network or exposed cloud instances, but the impact is severe, resulting in complete compromise of the knowledge base."
}