SiYuan EUVD-2026-74840

The attacker must hold low-privilege access to the SiYuan workspace sufficient to place or upload text-based asset files (PR:L per CVSS 4.0); external unauthenticated exploitation is not possible. The victim must actively trigger the search asset preview feature and view the crafted asset in the preview panel (UI:A per CVSS 4.0) - passive browsing without previewing the asset does not trigger the payload. The vulnerability is confined to the search asset preview rendering path; normal document editing and rendering are not reported as affected. Exploitation is further bounded to users sharing the same SiYuan instance and workspace.

The CVSS 4.0 score of 8.4 (AV:N/AC:L/AT:N/PR:L/UI:A/VC:H/VI:H/VA:N) is a reasonable characterization of this stored XSS: network-reachable, low attacker complexity, but constrained by both a low-privilege workspace access requirement (PR:L) and mandatory victim interaction to trigger the preview (UI:A). No public exploit code has been identified and SiYuan is not listed in the CISA KEV catalog at time of analysis, which lowers immediate exploitation urgency compared to the raw score. EPSS data was not included in the source intelligence, so automated exploitation probability cannot be quantified. The meaningful limiting factor is PR:L - an external, unauthenticated attacker cannot exploit this without first gaining workspace membership; however, even a mid-sized municipal IT team in leszno running a shared SiYuan instance for collaborative note-taking should treat this as urgent because the payload delivers full authenticated API access once triggered, enabling silent exfiltration of all indexed workspace content. The high C/I impact metrics are well-grounded given that SiYuan's API exposes notebook content, configuration, and asset management endpoints accessible from the victim's session.

A low-privileged workspace member crafts a plain-text file embedding an HTML or JavaScript payload and uploads it to a shared SiYuan workspace. When a victim searches the workspace and the asset preview panel renders the file, SiYuan passes the raw content to innerHTML, executing the attacker's script within the SiYuan browser origin. The script can then silently call SiYuan's authenticated REST API to enumerate notebooks, exfiltrate note content, or modify workspace data - all without further attacker interaction.