SiYuan EUVD-2026-57809

Exploitation requires only network-level HTTP access to a SiYuan instance running any version prior to v3.7.4 - no account, no session token, and no special configuration is required, as the vulnerability is reachable by anonymous readers. The specific prerequisite is that SiYuan's HTTP interface is reachable by the attacker, which applies to instances bound to a non-loopback interface without a network-layer authentication proxy. Exploitation is strictly limited to filesystem path existence probing: file contents are not disclosed, and no write or availability impact occurs. Instances bound exclusively to localhost (127.0.0.1) are not remotely exploitable. Instances behind an authenticating reverse proxy that rejects unauthenticated requests before they reach SiYuan are also not directly exploitable.

The CVSS 4.0 score of 6.9 Medium appropriately reflects a vulnerability that is trivially reachable but limited in direct damage. The critical risk signals are the zero-authentication, zero-complexity, network-accessible attack surface (AV:N/AC:L/AT:N/PR:N/UI:N): any attacker with HTTP access to a SiYuan instance can immediately exploit this without any account or special tooling. The impact ceiling is low - filesystem existence disclosure only, with no integrity or availability consequence - but reconnaissance capability is a genuine force multiplier for follow-on attacks targeting the same host. No EPSS score is provided in the input data, so probabilistic exploitation likelihood cannot be quantified; the absence of a KEV listing and no identified public POC reduce immediate urgency. Deployments that bind SiYuan only to localhost or place it behind a network-layer authentication proxy are not exposed to remote exploitation, substantially narrowing the real-world at-risk population. For internet-exposed or LAN-accessible instances, the zero-barrier nature elevates practical priority above what the Medium score alone would suggest.

An unauthenticated attacker identifies a SiYuan instance exposed on the network and sends a series of crafted HTTP requests to the getUniqueFilename endpoint, each supplying a different absolute filesystem path such as /etc/shadow, /root/.ssh/authorized_keys, /home/user/.aws/credentials, or Windows equivalents. The endpoint returns responses that reveal whether each queried path exists, allowing the attacker to methodically map the host's filesystem structure, discover installed software, locate sensitive configuration files, and build a targeting profile for a subsequent, more damaging attack against the same system. No public proof-of-concept code has been identified, but the attack requires only a basic HTTP client.