Alexis Kestler
The FREAK attack is a serious vulnerability in SSL/TLS encryption that could allow attackers to intercept encrypted web traffic and steal sensitive user data. When it was disclosed in 2015, over a third of all HTTPS servers were found to be vulnerable, putting millions of users at risk. In this comprehensive guide, I’ll walk you through how to test your systems for the FREAK vulnerability, understand the technical details behind the attack, and implement fixes to protect your websites and users.
A Brief History of Export Encryption and the FREAK Attack Discovery
Before getting into the technical details, it helps to understand the historical context behind the FREAK vulnerability.
Back in the 1990s, the US government imposed restrictions on exporting strong cryptography outside of the country. This meant that in order to sell products and software overseas, companies could only use limited encryption strengths classified as "export-grade". For SSL/TLS, this meant cryptographic keys could only be 40-bit or 512-bit in length.
In the early 2000s, these encryption export regulations were relaxed. However, for compatibility purposes, RSA export cipher suites using 512-bit and 40-bit keys remained supported in many TLS implementations. According to a study by Netcraft, as recently as 2013, about 22% of the top million HTTPS websites still supported these weak export ciphers.
This set the stage for the FREAK attack vulnerability. In 2015, security researchers Karthikeyan Bhargavan and Gaëtan Leurent from INRIA discovered that they could force browsers and websites to use these old 512-bit export ciphers. This would allow them to passively decrypt user traffic and hijack encrypted sessions.
When the vulnerability was publicly disclosed on March 3, 2015, over a third of all HTTPS websites were initially found to be vulnerable, including big names like Yahoo, Amazon and Cloudflare. User traffic could be decrypted, exposing account credentials and other sensitive data.
This highlights why lingering outdated technology, even when thought to be harmless, can come back to bite you years later when newly discovered exploits come to light. Let‘s now dive into the technical details of how the FREAK attack works.
Under the Hood: How Export RSA Ciphers Enable Decryption Attacks
On a technical level, the FREAK attack centers around weaknesses in RSA export cipher suites in SSL/TLS connections. To understand why these ciphers enable decryption, we need to briefly cover how RSA and Diffie-Hellman key exchange work.
In RSA key exchange, the server has a private/public RSA keypair. The server sends the client its public key. The client then generates a random pre-master secret, encrypts it with the server‘s public key, and sends it back. Only the server can decrypt this with its private key and obtain the pre-master secret. This is then used to derive a shared master secret key for encrypting the session.
With Diffie-Hellman key exchange, both client and server generate their own private DH keys. They exchange public keys with each other. The client combines its private key with the server‘s public key to derive a shared pre-master secret. The server does the same in reverse. No public key encryption is used.
RSA export ciphers use 512-bit RSA keys for key exchange. Researchers found these short keys could be cracked by attacking the RSA problem mathematically. This allows the pre-master secret to be obtained by an eavesdropper. With this key, they can now decrypt all communications encrypted with the derived session keys.
For a real-world analogy, imagine RSA encryption is like locking a secret message in a safe exchanged between two parties. The FREAK attack is like forcing the use of a cheap lock that can be easily picked or cracked open by anyone, enabling the message to be stolen.
Testing Your Systems for FREAK Vulnerability
Now that we understand how weak export ciphers can jeopardize security, let‘s discuss how to test your web servers for the FREAK vulnerability:
Use online vulnerability scanners
The easiest way is to use one of the many available online tools:
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SSL Labs Server Test – Tests servers for FREAK and various other vulnerabilities.
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ImmuniWeb SSLScan – Free SSL scanner that detects weak cipher issues.
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GeekFlare TLS Scanner – Checks TLS settings and reports on vulnerability risks.
I recommend testing all Internet-facing servers to identify those still accepting RSA export cipher suites. For load balancers and reverse proxies, test the backend origin servers as well.
Local scanning
For more comprehensive testing, you can scan locally from within your network. Nmap and testssl.sh are two excellent command line tools that support detecting the FREAK vulnerability:
# nmap --script ssl-enum-ciphers -p 443 www.example.com
# testssl.sh -e -E -f freak.txt www.example.com Check handshake logs
If your servers require 508-bit or 40-bit RSA cipher suites, it will show up in the SSL handshake when a client connects.
Monitor your web server logs for attempted use of any cipher string containing "EXPORT" which indicates a vulnerable export cipher suite:
RSA_EXPORT1024_WITH_DES_CBC_SHA
RSA_EXPORT1024_WITH_RC4_56_SHA Any occurrences mean your server does accept export RSA connections, and is likely vulnerable.
Implementing Fixes and Mitigations for FREAK
Once you‘ve identified affected systems, you need to deploy fixes to block export RSA ciphers and mitigate the vulnerability. Here are the recommended remediation steps:
Disable Export Cipher Suites
For Apache web servers, add the following directive to your SSL cipher suite configuration:
SSLCipherSuite !EXPORTOr append !EXPORT to an existing cipher string:
SSLCipherSuite HIGH:!MEDIUM:!aNULL:!MD5:!EXPORT For Nginx, add this line to your SSL server configuration block:
ssl_ciphers ‘!EXPORT‘;After updating, restart your web servers to ensure the new cipher suite rules take effect.
Upgrade Outdated SSL/TLS
Consider upgrading outdated SSL/TLS versions to disable older vulnerable ciphers. Move to TLS 1.2 or TLS 1.3 if possible. Obtain SHA-2 certificates signed with 2048-bit or higher RSA keys.
Tools like the Mozilla SSL Configuration Generator can help securely configure TLS for servers like Nginx.
Use Cryptographic Hardware
For sensitive servers, use cryptographic hardware with certification handling like HSMs or SSL accelerators. These often don‘t support export cipher suites.
Occasional Scanning
Periodically rescan servers to detect new vulnerabilities like FREAK that may emerge. Many tools offer scheduling and automation.
Monitor Industry Resources
Keep up-to-date on new vulnerabilities reported by US-CERT, OWASP, and industry publications. Be ready to quickly test and patch against new threats.
Impact of the FREAK Attack and Lessons Learned
The FREAK attack shed light on the dangers of retaining old insecure technologies that were once thought harmless. Some key stats on its impact:
- Over 36% of HTTPS servers were initially vulnerable when tested by researchers.
- The vulnerability had actually been present for over 15 years before discovery.
- Major sites including Yahoo, Amazon and Cloudflare were found to be exploitable.
- Researchers were able to steal login cookies and user credentials through passive decryption.
This highlighted the need for ongoing vulnerability scanning, patching, and properly decommissioning outdated technologies even if they seem innocuous. Encryption algorithm and protocol weaknesses can be rediscovered and exploited maliciously years later.
In terms of remediation costs, one estimate placed the financial impact as high as $300 million for affected organizations. However, the loss of consumer trust and reputation damage from potential data breaches could be even more severe.
Conclusion
Lingering obsolete encryption protocols and ciphers represent a serious cyber risk even if they appear harmless on the surface. As seen with FREAK, previously insignificant flaws can become deadly vulnerabilities.
I recommend all engineers and security teams closely review their TLS configurations and cipher suite policies in light of this vulnerability. Specifically:
- Audit for export cipher support and disable any weak RSA cipher suites found.
- Upgrade to the latest TLS versions and SHA-2 certificates.
- Routinely scan sites for new TLS vulnerabilities using trusted scanning tools.
- Monitor industry security research and announcements.
Taking a proactive approach will significantly improve resilience and protect your infrastructure from similar encryption flaws and downgrade attacks in the future.
