SiYuan CVE-2026-74905

SafeMode must be explicitly enabled on the SiYuan instance - this is the hard prerequisite, as SafeMode is what invokes SSRFSafeDialer and the vulnerable isPrivateIP check. Without SafeMode, the code path containing the vulnerability is not active (though SSRF protections would also be absent). The attacker must be authenticated per the CVE description; however, the provided CVSS 4.0 vector specifies PR:N (no privileges required), creating an unresolved conflict - authentication requirements should be verified against the vendor advisory before assuming unauthenticated exploitation is possible. The attacker must control a URL submitted to one of three endpoints: the network forward proxy, WebSocket proxy, or SSE proxy. Exploitation further requires that the network path from the SiYuan host supports routing of IPv6 transition addresses (NAT64, 6to4, or Teredo) to their embedded private IPv4 destinations, or that the DNS resolver returns such transition addresses for attacker-controlled hostnames. Cloud-hosted instances with access to IMDSv1 endpoints represent the highest-impact targets; air-gapped or IPv4-only environments with no IPv6 transition routing are not exploitable via this technique.

The provided CVSS 4.0 vector (PR:N) directly conflicts with the CVE description, which states 'an authenticated attacker' - this discrepancy must be confirmed against the vendor advisory at GHSA-qq8m-8p8v-x4xg before making access-control decisions. If authentication is required (PR:L, as stated in the description), the realistic attacker pool is limited to users with existing SiYuan credentials, substantially reducing exposure for single-user self-hosted deployments. For team or shared SiYuan instances - particularly those cloud-hosted - the full-read SSRF capability is high-severity: reaching IMDSv1 endpoints (169.254.169.254) yields IAM credentials enabling lateral movement far beyond the SiYuan host itself. The SafeMode prerequisite is an additional limiting factor; environments that have not enabled SafeMode are not affected by this bypass (though they may have unrestricted SSRF exposure separately). No EPSS score is available in the provided intelligence, no KEV listing exists, and no public POC has been identified - urgency is moderate rather than immediate, though the detailed technical description substantially lowers the bar for independent exploit development. The CVSS 4.0 base score of 6.9 may understate cloud-hosted impact where credential theft has significant downstream consequences.

An authenticated SiYuan user on an instance with SafeMode enabled submits a crafted URL - for example, one whose hostname resolves to 64:ff9b::a9fe:a9fe (the NAT64 encoding of 169.254.169.254) - to the network forward proxy endpoint. The isPrivateIP function evaluates the resolved IPv6 address, does not recognize it as a transition address encoding a private IPv4 destination, and permits the outbound connection. The proxy returns the full HTTP response body, yielding AWS instance metadata including IMDSv1 IAM credentials, which the attacker can immediately use for lateral movement within the cloud environment. No public POC exists at time of analysis, but the CVE description provides the precise address ranges and affected code path needed to construct one.