SiYuan versions before v3.8.1 contain a server-side request forgery vulnerability in the http_request and web_fetch agent tools that perform DNS resolution only at guard time without validating the connect-time resolution. Attackers can use DNS rebinding to answer the guard resolution with a public IP and the connect resolution with a private or metadata IP, bypassing the SSRF defense to access cloud instance metadata and internal services.

SiYuan versions prior to v3.8.1 are affected by a server-side request forgery (SSRF) vulnerability in the http_request and web_fetch agent tools. The flaw arises because DNS resolution is performed only at guard time, without re-validating the resolved IP at connection time. This creates a time-of-check to time-of-use (TOCTOU) race condition that attackers can exploit via DNS rebinding. By initially resolving a domain to a public IP during the guard check and then switching the DNS response to a private or cloud metadata IP at connection time, attackers can bypass SSRF defenses. Successful exploitation allows access to cloud instance metadata services (e.g., AWS IMDSv1) and internal network services. The vulnerability has been patched in SiYuan v3.8.1. Users are strongly advised to upgrade immediately.