Mozilla Firefox Built-in PDF Viewer Same Origin Policy (SOP) Bypass and Arbitrary File Disclosure (CVE-2015-4495)
Executive & SecOps Summary
SECOPS ASSESSMENTMozilla Firefox before 39.0.3, Firefox ESR 38.x before 38.1.1, and Firefox OS before 2.2
Remote attackers can bypass the Same Origin Policy (SOP) to read arbitrary local files (e.g., SSH keys, configuration files) and exfiltrate them without user interaction
Immediately update Mozilla Firefox and Firefox ESR to the latest secure versions (39.0.3 / 38.1.1 or higher)
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
For Debian/Ubuntu-based Linux distributions, update the Firefox package to the latest secure version using the package manager:
# Update package lists and upgrade Firefox to the latest version
sudo apt-get update
sudo apt-get --only-upgrade install firefox -yFor RedHat/CentOS-based systems, execute the following command:
sudo yum update firefox -y2. Temporary Workaround & Hardening Configuration
If an immediate upgrade is not feasible, mitigate the risk by disabling the built-in PDF viewer in Firefox via the configuration preferences:
# Disable the built-in PDF viewer by appending the preference to user.js
echo "user_pref('pdfjs.disabled', true);" >> ~/.mozilla/firefox/*.default/user.js3. Log Audit & IoC Verification Commands
Use the following script to verify the currently installed Firefox version and determine if the system is vulnerable:
# Script to check the installed Firefox version and vulnerability status
FIREFOX_VER=$(firefox --version 2>/dev/null | awk '{print $3}')
if [ -z "$FIREFOX_VER" ]; then
echo '[INFO] Firefox is not installed or not in the system PATH.'
else
echo "[INFO] Installed Firefox version: $FIREFOX_VER"
# Compare version to check if it is below 39.0.3
dpkg --compare-versions "$FIREFOX_VER" "lt" "39.0.3" && echo '[WARNING] Vulnerable version detected! Immediate update required.' || echo '[OK] Firefox is patched and secure.'
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Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2015-4495) exists in the built-in JavaScript-based PDF reader (pdf.js) of Mozilla Firefox. Due to an implementation flaw in how the PDF viewer executes within a local context, the Same Origin Policy (SOP) is not properly enforced when interacting with specific native setters and crafted JavaScript code (CWE-346). This allows scripts running in a web content context to cross security boundaries and access the highly privileged local file system.
2. Exploitation Mechanics & Attack Scenario
An attacker hosts a malicious website or injects malicious JavaScript into an advertisement network. When a victim visits the page, the script triggers the vulnerability by interacting with a native setter in the context of the built-in PDF viewer. This interaction bypasses the browser's security checks, allowing the script to access local files via the 'file://' protocol. The exploit runs silently in the background without displaying any download prompts or warnings, reading sensitive files (such as '/etc/passwd', '.ssh/id_rsa', or Windows configuration files) and exfiltrating them to a remote attacker-controlled server.
3. Infrastructure & Business Impact
This vulnerability was actively exploited in the wild as a zero-day in August 2015. If a developer, system administrator, or privileged user's workstation is compromised, critical assets such as SSH private keys, cloud credentials, and database configuration files can be stolen. This poses a severe business risk, potentially leading to full infrastructure compromise and subsequent lateral movement within the corporate network.
Official Vendor Advisories & References
The PDF reader in Mozilla Firefox before 39.0.3, Firefox ESR 38.x before 38.1.1, and Firefox OS before 2.2 allows remote attackers to bypass the Same Origin Policy, and read arbitrary files or gain privileges, via vectors involving crafted JavaScript code and a native setter, as exploited in the wild in August 2015.
Frequently Asked SecOps Questions (CVE-2015-4495)
FAQQ1. How critical is CVE-2015-4495 and is it actively exploited in the wild?
CVE-2015-4495 is rated CVSS 8.8 (HIGH) with a 30-day EPSS exploitation probability of 68.6%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2015-4495?
Primary impacted vendors include mozilla across firefox, firefox_esr, firefox_os. Scope summary: Mozilla Firefox versions prior to 39.0.3, Firefox ESR 38.x prior to 38.1.1, and Firefox OS prior to 2.2
Q3. What temporary mitigations or workarounds exist for CVE-2015-4495 before patching?
[Mitigation] Immediately update Mozilla Firefox and Firefox ESR to the latest secure versions (39.0.3 / 38.1.1 or higher) Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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