CVE-2023-6209

What it actually does

Incorrect parsing of relative URLs beginning with three slashes permits a "/../" sequence in the path to override the intended host, enabling path traversal that can redirect requests or bypass host-based security controls in affected browsers.

Why it matters to the business

Organizations relying on older browser versions face elevated risk of data exposure or unauthorized resource access when users visit malicious or compromised sites, potentially leading to compliance issues or loss of sensitive information.

How an attack unfolds

An attacker crafts a specially formatted URL containing three leading slashes followed by a traversal sequence that resolves to an attacker-controlled or unintended host; when a user opens the link in an affected browser the malformed URL is parsed incorrectly, causing the browser to fetch content from the overridden location and potentially leaking data or bypassing origin checks.

How to fix it

Upgrade Firefox to version 120 or later, Firefox ESR to 115.5.0 or later, and Thunderbird to 115.5 or later; enforce browser updates through enterprise management tools and disable outdated versions where possible.

How to confirm the fix worked

Verify browser versions via about:support or enterprise inventory tools; confirm that URLs containing three leading slashes followed by traversal sequences no longer resolve to an overridden host after the update.

Typical attack path

1. Attacker identifies a target web application or user base using vulnerable browser versions.
2. Attacker crafts a malicious link or web page embedding a URL with three leading slashes and a "/../" traversal payload targeting a different host.
3. Victim is tricked into clicking the link or visiting the page via phishing, compromised site, or social engineering.
4. Vulnerable browser parses the URL incorrectly and resolves it to the attacker-specified host.
5. Attacker receives the request or exfiltrates data that should have been restricted to the original host.

How to detect it

Examine browser history, proxy logs, and web server access logs for URLs containing three consecutive slashes followed by path traversal sequences; correlate with user reports of unexpected redirects or data leakage.

What your security team should do

• Prioritize immediate deployment of the fixed browser versions across all endpoints.
• Monitor internal software inventory for any remaining instances of Firefox < 120, ESR < 115.5.0, or Thunderbird < 115.5.
• Educate users on avoiding suspicious links until updates are complete.
• Review web application code that relies on browser-enforced host isolation for additional hardening.