FreeScout CVE-2026-53592

Exploitation requires the attacker to hold at minimum a low-privilege authenticated FreeScout account (confirmed by PR:L in the CVSS vector). The victim must actively visit a crafted URL that causes main.js to parse attacker-controlled query parameters - a deliberate user interaction step (confirmed by UI:R). The vulnerable surface is any FreeScout page that loads /public/js/main.js, which covers standard help desk views; no special non-default server configuration is required for the client-side code to execute. Exploitation is bounded by: (1) the need for an existing authenticated account, limiting anonymous internet-facing abuse; (2) required victim interaction, preventing fully automated exploitation; and (3) scope limited to the victim's browser session (S:U), meaning the server itself and other users' sessions are not directly affected. Organizations restricting FreeScout to internal or VPN-only access further reduce the exploitable population.

The CVSS 3.1 score of 4.6 (Medium) with vector AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:L/A:L correctly captures the real-world constraints: high attack complexity reflects the need to deliver a crafted URL to a victim, required user interaction limits opportunistic mass exploitation, and the low-privilege authentication prerequisite narrows the attacker pool to existing FreeScout users. Scope is unchanged (S:U), confining impact to the victim's browser session rather than the server. No EPSS score data was provided in the input, preventing probability-based triage. The vulnerability is not listed in CISA KEV, and no public exploit code has been identified. A notable compounding risk factor is patch confusion: organizations that applied the 1.8.139 incomplete fix may incorrectly believe they are fully remediated. Prototype pollution alone rarely achieves critical impact without chaining to a downstream vulnerability, so realistic risk is moderate-to-low for well-monitored environments, but the bypass-of-existing-fix angle warrants prompt patching over deferral.

An authenticated low-privilege FreeScout user (e.g., a support agent) crafts a URL such as https://helpdesk.example.com/conversations?b[__proto__][polluted]=PWNED and delivers it to a colleague or administrator via internal chat or email. When the victim clicks the link and the page loads main.js, getQueryParam parses the nested query key and writes the attacker-controlled value into Object.prototype, injecting a polluted property into every subsequently created JavaScript object in that browser session, which could be leveraged to alter application behavior or escalate to further client-side attacks. No public POC is currently known, but the nested __proto__ bypass technique is thoroughly documented in open security research.