CVE-2026-100635 in SiYuan
Summary
by MITRE • 09/26/2026
SiYuan before v3.8.4 contains an authentication bypass vulnerability in the publish service where session cookies are issued without Secure or SameSite attributes over plaintext HTTP connections. An on-path attacker can observe a valid publish-visitor-session-id cookie from a Basic Auth exchange and replay it to access authenticated publish endpoints without knowing the account password.
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Analysis
by VulDB Data Team • 09/26/2026
The identified vulnerability in SiYuan versions prior to 3.8.4 represents a critical authentication bypass flaw within its document publishing service, stemming from improper session management practices over unencrypted transport channels. The core technical deficiency lies in the issuance of session cookies without essential security attributes such as Secure and SameSite when transmitted via plaintext HTTP connections. This configuration failure allows an attacker positioned on the network path to intercept valid session identifiers through passive traffic analysis or active man-in-the-middle techniques. Specifically, during a Basic Authentication exchange where credentials are sent in cleartext, the resulting publish-visitor-session-id cookie is exposed alongside other sensitive data, creating a direct pathway for credential theft and subsequent unauthorized access without requiring knowledge of the user's actual password.
From an operational perspective, this vulnerability enables on-path attackers to perform session hijacking attacks with significant ease. By capturing the valid publish-visitor-session-id cookie during transmission, an attacker can replay it against authenticated endpoints within the SiYuan publishing service. This allows the adversary to impersonate legitimate users and access protected content or administrative functions without triggering password-based authentication mechanisms. The impact extends beyond simple data exposure; it compromises the integrity of the published documents and potentially exposes sensitive intellectual property stored within the application, as the attacker gains full authenticated privileges associated with the compromised session token.
This flaw aligns directly with CWE-614, which classifies sessions identified by cookies that lack Secure attributes when transmitted over non-encrypted connections, thereby facilitating interception attacks. Furthermore, it relates to CWE-384 regarding Session Fixation and CWE-200 for Exposure of Sensitive Information through Improper Neutralization of Special Elements in Input. In the context of the MITRE ATT&CK framework, this vulnerability facilitates techniques associated with Credential Access via Network Sniffing (T1056) and Account Manipulation or Unauthorized Access via Session Hijacking (T1528). The absence of SameSite attributes also exposes the application to Cross-Site Request Forgery attacks, although the primary risk here is the interception over HTTP.
To mitigate this vulnerability, organizations must immediately upgrade SiYuan to version 3.8.4 or later where these session management issues have been addressed. In environments where upgrading may not be instantly feasible, it is imperative to enforce HTTPS for all connections serving the publish service, ensuring that cookies are marked with the Secure flag so they are only transmitted over encrypted channels. Additionally, implementing strict SameSite cookie attributes helps prevent cross-site request forgery and limits how cookies are sent in cross-origin contexts. Network-level protections such as TLS inspection or firewall rules should be configured to block plaintext HTTP traffic on ports serving sensitive application services, thereby eliminating the possibility of passive session interception by on-path adversaries.