SiYuan EUVD-2026-64270

Exploitation requires same-origin access to the SiYuan bazaar install HTTP endpoints, meaning the attacker must be able to issue HTTP requests that satisfy the browser same-origin policy - achievable via a malicious script in an open SiYuan browser tab, a compromised browser extension, or direct API access to the localhost server port. The attacker must know the packageName of at least one already-installed, trusted plugin to target for overwrite. The CVSS 4.0 vector confirms low privileges (PR:L), partial user interaction (UI:P), and specific attack prerequisites (AT:P) are required. Exploitation is constrained to local-access scenarios (AV:L) with high complexity (AC:H), substantially narrowing the threat surface compared to fully remote unauthenticated flaws. Single-user deployments with no shared browser sessions and network-restricted SiYuan instances face materially reduced exposure.

The vendor-assigned CVSS 4.0 vector (AV:L/AC:H/AT:P/PR:L/UI:P) scoring 5.9 accurately captures the constrained exploitation surface: local access, high attack complexity, specific preconditions, low privilege, and partial user interaction all limit the realistic threat population. The vulnerability does not appear in the CISA KEV catalog, no POC code is referenced in the available intelligence, and EPSS data was not provided - leaving exploitation probability unquantified but inferably low given the access requirements. Despite the moderate score, the integrity and persistence impact is disproportionate: an attacker who meets prerequisites achieves durable plugin substitution that survives restarts, which is a meaningful foothold in environments where SiYuan plugins interact with sensitive notes or external data. The CVSS 4.0 subsequent-system integrity metric (SI:H) reflects this. Risk is highest in multi-user or team SiYuan deployments where a malicious insider or a compromised same-origin browser session can reach the Bazaar API; single-user, air-gapped, or network-restricted deployments face materially lower exposure. No data conflicts exist between available signals, though the absence of EPSS and KEV data leaves real-world exploitation frequency unconfirmed.

An attacker with same-origin access to a SiYuan instance - for example, a malicious script running in the same browser session as an open SiYuan tab, or a local network actor who can reach the SiYuan HTTP API directly - crafts a bazaar install request specifying a trusted, already-installed plugin's packageName alongside a repoURL pointing to attacker-controlled malicious code. SiYuan fetches and installs the malicious code into the trusted plugin's directory without detecting the mismatch, after which the malicious plugin executes automatically on every subsequent application restart under the trust level of the overwritten plugin.