SiYuan CVE-2026-100637

Requires an authenticated administrator account on the SiYuan instance (CVSS PR:H) and a reachable checkoutRepo endpoint. The attacker must supply a sessionID parameter containing directory-traversal sequences (../). Exploitation is limited to overwriting JSON files that reside in directories the SiYuan kernel already has write access to and that already exist - it cannot create files in arbitrary locations or write non-JSON content, and it grants no read/confidentiality access. The dominant limiting factor is the administrator privilege requirement, which restricts this to insider/rogue-admin abuse or post-credential-compromise activity rather than opportunistic remote attack.

This is a moderate, not top-tier, priority. The CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:H/UI:N) is network-reachable and low-complexity, but the PR:H metric is the key limiter: exploitation requires an already-authenticated ADMINISTRATOR of the SiYuan instance, sharply narrowing the realistic threat to a rogue admin or an attacker who has already compromised admin credentials/session. Impact is integrity-focused (VI:H/SI:H) with only minor availability (VA:L) and zero confidentiality (VC:N) - the flaw writes/overwrites JSON files, it does not read data. There is no EPSS score, no CISA KEV listing, and no public exploit code provided, so there is no public exploit identified at time of analysis and no evidence of active or automatable exploitation. The vendor's 8.3 (High) score is on the generous side given the admin prerequisite and write-only-JSON constraint; the practical risk is closer to medium. No signals conflict, but the absence of EPSS/POC data means exploitation likelihood is inferred from the vector alone.

An authenticated SiYuan administrator (or an attacker who has hijacked an admin session) sends a request to the checkoutRepo endpoint with a sessionID parameter containing directory-traversal sequences (e.g. ../../). The kernel then writes attacker-controlled JSON content outside the intended workspace, overwriting an existing JSON file in another kernel-writable directory - potentially tampering with configuration or state files. No public proof-of-concept is available at time of analysis.