ProtonMail is not vulnerable to the recently announced Linux TCP Vulnerability

Earlier this week, a rather serious Linux TCP Vulnerability was disclosed (CVE-2016-5696) by security researchers in the US. As a result, ProtonMail’s security team did an analysis of this bug to see if it compromises the integrity of ProtonMail’s encrypted email service. Our analysis shows that this bug did not pose a threat to users of our encrypted email service. Nevertheless, we have taken additional action to further harden ProtonMail’s servers.

Linux TCP Vulnerability

The vulnerability which was discovered has been present in the Linux kernel since 2012 and did impact the kernel version that ProtonMail is running. In order to exploit this vulnerability, an attacker only needs to have the IP addresses of the client and the server, which are not to difficult for a sophisticated attacker to obtain. Due to a rate limit enforced by Linux on TCP challenge ACK packets, it is possible to hijack the TCP connection between server and client. This can be used for example, to hijack HTTP (web) connections to insert malicious code and data into the communications stream.

More critically, this vulnerability can be exploited without needing to have man-in-the-middle (MITM) capabilities. Thus, the attack can also be performed “off-path” without the ability to eavesdrop on the network between server and client, significantly lowering the difficulty of the attack. Additional details can be found in the original research paper.

Protecting our Encrypted Email Service

While this vulnerability sounds severe, its impacted on ProtonMail’s secure email service is quite limited because of the encryption that we utilize. In particular, we enforce HSTS on all of our web servers so all connections must go through HTTPS instead of HTTP. This means that during the period in which ProtonMail was vulnerable, the worst that could be done with this attack is to break the connection. The connection could not be hijacked or have malicious code inserted. You can learn more about HSTS from the following blog post made by ProtonMail Security Contributor Mazin Ahmed:

For your protection, ProtonMail uses Swiss SSL certificates with the highest strength ciphers. Our SSL certificates are issued by Swiss SSL certificate provider QuoVadis Trustlink Schweiz AG and in addition to HSTS, we also use Extended Validation (EV), 4096-bit RSA, SHA-256 hash, and Certificate Transparency (CT) with our SSL certificate. For security reasons, we utilize a very select group of SSL ciphers and use those with Perfect Forward Secrecy as often as possible.

SSL is only one security layer for securing our email service. Along with SSL, we also implement end-to-end encryption with PGP, and we are also the maintainers of OpenPGPjs, the world’s most widely used open source PGP library. This combination of factors means that we are highly confident that no ProtonMail accounts were compromised as a result of this Linux TCP vulnerability.

Security Improvements

Because CVE-2016-5696 has only just been reported, official security patches for the vulnerability have not yet been officially released upstream. However, we take a very proactive approach to security so our Security Team manually modified the running Linux kernel on all of our servers in order to make them immune to this security flaw. In order to make ProtonMail the world’s most secure email service, our team monitors security developments 24/7 so we can move quickly to mitigate any issues that may come up. For the additional security news and updates, you can also follow us on Twitter.

Best Regards,

The ProtonMail Security Team