Mozilla's Firefox browser includes a popup blocker that prevents unsolicited pop‑up windows. A flaw in the Popup Blocker component allows an attacker to bypass this protection, presenting the user with a deceptive popup that appears to originate from a trusted source. This spoofing vulnerability can mislead users into interacting with malicious content or revealing personal information, effectively creating a UI deception that could lead to phishing or fraud.

Affected systems include any installation of Mozilla Firefox older than version 151. Versions prior to the 151 patch are susceptible, regardless of the operating system. The vulnerability is limited to the browser client and does not require privileged access or additional network services.

The EPSS score is currently unavailable and the issue is not listed in CISA's KEV catalog, so exploitation probability is unknown but could exist in the wild given the widespread use of Firefox. The CVSS score is not provided, but the ability to mislead users suggests a high confidentiality and integrity impact and a moderate to high availability impact if used to trick users into insecure actions. The likely attack vector is an active adversary that can host a malicious webpage or engineer phishing links, with the user interacting with that page in their browser.