CVE-2015-4495 is a security feature bypass vulnerability in Mozilla Firefox. CISA added it to the Known Exploited Vulnerabilities catalogue on 25 May 2022, which means attackers have used it in real attacks. Moxilla Firefox allows remote attackers to bypass the Same Origin Policy to read arbitrary files or gain privileges.
What this vulnerability means in practice
Same Origin Policy is one of the core browser trust boundaries. When a browser flaw lets remote content bypass that boundary, the attack can cross from a web page into local data access or privileged browser behaviour, which is why even a “feature bypass” can be high impact in a widely used client.
For Firefox, the key security question is not just whether the browser crashes or behaves oddly, but whether an attacker can use the flaw to break containment between origins. That can turn a routine browsing session into a path for reading files, stealing data, or reaching actions the browser should have blocked.
Why this class of weakness is dangerous
A same-origin bypass weakens the assumption that web content is isolated by site boundary. Once that assumption fails, an attacker may be able to pivot from hostile content to local resources, session material, or browser state that was intended to stay protected.
This is dangerous because the browser is a high-trust application sitting between users and many other systems. A successful bypass can expose data that was never meant to leave the local machine and may also become a stepping stone into adjacent accounts, internal applications, or files that are reachable through the browser context.
For current record details, use the CVE Program entry and the NIST National Vulnerability Database rather than relying on informal summaries.
How organisations check exposure and reduce blast radius
Exposure review starts with inventory: identify where Firefox is installed, which versions are present, and which endpoints are exposed to untrusted browsing or shared-user use. Internet exposure matters because the exploit path begins with remote content, but local privilege and file-access conditions can determine how far the impact reaches.
When patching is delayed, compensating controls should focus on shrinking the browser’s reach, limiting risky browsing contexts, and reducing the value of any data that the browser can touch. Administrators should also review vendor advisories and the CISA KEV entry to confirm which deployments are urgent for remediation.
The practical response is strongest when vulnerability data is tied to asset management and prioritisation. NHIMG’s Vulnerability Management Guide is useful here because it frames KEV-driven triage, verification, and exception handling as an operational process rather than a one-time patch task.
Detection, compromise review, and recovery considerations
If exploitation is suspected, defenders should look beyond the browser itself and review for signs that a session was used to access unexpected files, internal resources, or data outside normal browsing patterns. Post-patch review matters because a fixed browser does not undo any data exposure that already occurred.
Recovered environments should be checked for evidence of follow-on activity from stolen data, exposed credentials, or copied files. A browser bypass can be the first step in a broader intrusion chain, so the response should include validation that no additional persistence or lateral movement followed the initial access.
For broader attack-chain context, NHIMG’s State of NHI & AI Agent Breach Report shows how stolen secrets and compromised access often become the real business impact after an initial vulnerability is exploited.
Risk and Threat Considerations
A browser-origin bypass can expose local files, session material, or other browser-accessible data, and the impact is often larger than the browser bug itself. Because the weakness is reachable remotely, an attacker only needs a successful lure or delivery path to convert a browsing session into a data access problem.
Failure mechanism: The attack works by breaking the browser’s origin separation, so hostile content can act outside its intended security boundary and access resources that should be isolated.
Impact: That can lead to disclosure of sensitive local information, credential or session abuse, and in some environments escalation into broader system or account compromise.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
NIST SP 800-53 Rev 5, CIS Controls v8 and OWASP ASVS set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| NIST SP 800-53 Rev 5 | SI-2 — Flaw Remediation | CVE exposure requires timely vulnerability remediation and tracking. |
| AC-4 — Information Flow Enforcement | Same Origin Policy is a browser information-flow boundary relevant to this bypass. | |
| AU-6 — Audit Review, Analysis, and Reporting | Post-exploitation review depends on reviewing browser and endpoint activity evidence. | |
| Recommendation — Prioritise and verify remediation for the Firefox flaw under SI-2. Enforce information-flow restrictions to limit cross-origin access under AC-4. Review logs and endpoint evidence under AU-6 for signs of browser compromise. | ||
| CIS Controls v8 | CIS-7 — Continuous Vulnerability Management | KEV-listed browser vulnerabilities need asset inventory, prioritisation and remediation. |
| Recommendation — Use CIS-7 to identify exposed Firefox installations and drive urgent remediation. | ||
| OWASP ASVS | V12 — Secure Communication | Browser-origin separation and trust boundaries align with secure communication expectations. |
| Recommendation — Verify origin and boundary handling under V12 where the browser enforces trust separation. | ||
Practitioner Guidance
Why practitioners should care: Treat this kind of browser flaw as a containment failure, not just an application defect. The real decision is whether the vulnerable browser is allowed to handle sensitive workflows before remediation is complete.
What to watch for: Prioritise systems where Firefox is used for privileged portals, internal tools, or file-heavy workflows, because those environments increase the value of any same-origin bypass. Use the vendor advisory and the KEV record to anchor patch urgency.
Practitioner takeaway: The fastest risk reduction comes from combining inventory, urgent remediation, and a review of what the browser could already reach if exploitation occurred.