CVE-2020-12413: Firefox Information Disclosure Vulnerability

CVE-2020-12413 Overview

CVE-2020-12413 describes the Raccoon attack, a timing attack vulnerability that targets Diffie-Hellman Ephemeral (DHE) ciphersuites inherent in the TLS specification. This side-channel vulnerability allows attackers to potentially recover the premaster secret by exploiting timing differences in how TLS servers process DHE key exchanges. To mitigate this vulnerability, Mozilla Firefox disabled support for DHE ciphersuites entirely.

Critical Impact

Attackers exploiting this timing vulnerability could decrypt sensitive TLS-protected communications by recovering cryptographic secrets through precise timing measurements of DHE key exchange operations.

Affected Products

• Mozilla Firefox (all versions prior to mitigation)
• Mozilla Firefox ESR (all versions prior to mitigation)

Discovery Timeline

• 2023-02-16 - CVE-2020-12413 published to NVD
• 2025-03-19 - Last updated in NVD database

Technical Details for CVE-2020-12413

Vulnerability Analysis

The Raccoon attack exploits a fundamental weakness in the TLS specification's handling of Diffie-Hellman Ephemeral (DHE) key exchanges. This timing attack vulnerability (CWE-203: Observable Discrepancy) allows an attacker to observe subtle timing differences in how servers process the premaster secret during DHE operations.

The vulnerability stems from the fact that TLS 1.2 and earlier versions do not mandate constant-time processing of Diffie-Hellman computations. When the premaster secret begins with zero bytes, servers may process it differently, leading to measurable timing variations. An attacker positioned to observe these timing differences across multiple connections could potentially reconstruct the premaster secret and decrypt captured TLS sessions.

The attack requires precise timing measurements and the ability to observe many TLS connections, making it challenging to execute in practice. However, the theoretical risk to confidentiality warranted Mozilla's decision to disable DHE ciphersuites altogether in Firefox.

Root Cause

The root cause lies in the TLS specification's allowance for non-constant-time handling of DHE premaster secrets. When the computed shared secret has leading zero bytes, the length of the secret may vary, causing timing discrepancies during cryptographic operations. This information leakage through timing side-channels enables the Raccoon attack.

Attack Vector

This is a network-based timing attack that requires the attacker to:

1. Position themselves to intercept encrypted TLS traffic
2. Trigger multiple TLS handshakes with the target server using DHE ciphersuites
3. Perform precise timing measurements of server responses during DHE key exchanges
4. Accumulate sufficient timing samples to statistically determine the premaster secret
5. Use recovered secrets to decrypt captured TLS sessions

The attack requires high complexity to execute successfully, as it depends on precise network timing measurements and significant computational resources for cryptanalysis. More information about the attack methodology is available at the Raccoon Attack Overview page.

Detection Methods for CVE-2020-12413

Indicators of Compromise

• Unusual patterns of repeated TLS handshake attempts from the same source
• High volume of connections using DHE ciphersuites specifically
• Network traffic analysis showing abnormal timing measurement activity

Detection Strategies

• Monitor for repeated TLS connection attempts that may indicate timing measurement activity
• Audit TLS configurations to identify use of vulnerable DHE ciphersuites
• Deploy network intrusion detection systems configured to alert on suspicious handshake patterns
• Review TLS session logs for anomalous connection behavior

Monitoring Recommendations

• Implement continuous monitoring of TLS cipher suite negotiation across enterprise systems
• Track Firefox and Firefox ESR version deployment to ensure updated versions without DHE support
• Monitor security advisories from Mozilla for related cryptographic vulnerabilities

How to Mitigate CVE-2020-12413

Immediate Actions Required

• Update Mozilla Firefox and Firefox ESR to the latest versions where DHE ciphersuites are disabled
• Audit server-side TLS configurations and disable DHE ciphersuites where possible
• Consider migrating to ECDHE (Elliptic Curve Diffie-Hellman Ephemeral) ciphersuites as a more secure alternative
• Review TLS 1.3 adoption, which addresses this class of vulnerabilities through improved protocol design

Patch Information

Mozilla addressed this vulnerability by disabling DHE ciphersuite support in Firefox entirely. Users should update to the latest Firefox or Firefox ESR release to benefit from this mitigation. Additional details are available through the Mozilla Bugzilla tracking entry.

Workarounds

• Disable DHE ciphersuites on all TLS servers through configuration changes
• Configure browsers and clients to prefer ECDHE ciphersuites over DHE
• Implement TLS 1.3 which is not vulnerable to the Raccoon attack
• Use network-level controls to limit exposure of TLS-protected services to untrusted networks

# Example: Disable DHE ciphersuites in Apache HTTPD
# Add to ssl.conf or virtual host configuration
SSLCipherSuite ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:!DHE
SSLProtocol all -SSLv2 -SSLv3 -TLSv1 -TLSv1.1