Is CVE-2015-4520 in the CISA KEV catalog?

No. CVE-2015-4520 is not currently on the CISA Known Exploited Vulnerabilities list.

What is the CVSS severity of CVE-2015-4520?

CVE-2015-4520 has a CVSS v2 base score of 6.4 (MEDIUM).

What is the EPSS score for CVE-2015-4520?

CVE-2015-4520 has an EPSS score of 3.09% (86th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2015-4520?

Upgrade firefox on ubuntu 14.04 to 41.0+build3-0ubuntu0.14.04.1 or later. Fixes are available for 2 affected products.

CVE & CISA-KEV Catalog

CVE-2015-4520

MEDIUMEPSS 86th pctl

6.4

CVSS v2

NVD

Description

Mozilla Firefox before 41.0 and Firefox ESR 38.x before 38.3 allow remote attackers to bypass CORS preflight protection mechanisms by leveraging (1) duplicate cache-key generation or (2) retrieval of a value from an incorrect HTTP Access-Control-* response header.

CWE-254

How to fix

Remediation Available

firefoxUbuntu

Fixed in:41.0+build3-0ubuntu0.14.04.1USN-2743-1

thunderbirdUbuntu

Fixed in:1:38.3.0+build1-0ubuntu0.14.04.1USN-2754-1

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v2 Vector

No CVSS vector data available.

Exploit Intelligence

3.09%probability of exploitation in 30 days

86thpercentile

Elevated risk: more likely to be exploited than 86% of all known CVEs.

References

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-05-06.