OpenSSH introduces a security feature to prevent Side-Channel Attacks

Verschluesselte Kommunikation mit OpenSSH

OpenSSH introduces a new feature to prevent Side-Channel attacks, latest release encrypts secret keys in memory as temporary solution.

Memory side-channel vulnerabilities continue to threaten modern processors, Spectre, and Meltdown, Rowhammer, and RAMBleed are just some samples,

Now OpenSSH encrypts secret keys in memory against Side-Channel attacks.

Many experts demonstrated variants of side-channel attacks against OpenSSH application installed on targeted systems. In the attack scenario, a process owned by an unprivileged attacker exploits memory read vulnerabilities to steal secret SSH private keys from the memory of the target system.

That’s possible because OpenSSH has an agent that keeps a copy of your SSH key in the memory so that you don’t have to type your passphrase every time you want to connect to the same remote server.

To prevent such kind of attacks, modern operating systems by default store sensitive data in the kernel memory that is not accessible by user-level privileged processes.

Anyway, SSH keys are managed on the RAM or CPU memory in plaintext format, allowing attackers to access them with side channel attacks.

The latest version of the OpenSSH addresses this problem by implementing the encryption of the private keys before storing them into the system memory.

“Add protection for private keys at rest in RAM against speculation and memory sidechannel attacks like Spectre, Meltdown, Rowhammer and Rambleed. This change encrypts private keys when they are not in use with a symmetic key that is derived from a relatively large “prekey” consisting of random data (currently 16KB). Attackers must recover the entire prekey with high accuracy before they can attempt to decrypt the shielded private key, but the current generation of attacks have bit error rates that, when applied cumulatively to the entire prekey, make this unlikely.” reads the security advisory published by OpenSSH developer Damien Miller.

“Implementation-wise, keys are encrypted “shielded” when loaded and then automatically and transparently unshielded when used for signatures or when being saved/serialised.”

Miller explained that OpenSSH plans to remove this protection against side-channel attacks in a few years when computer architecture becomes less unsafe.

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Pierluigi Paganini

(SecurityAffairs – OpenSSH, side-channel attacks)

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Pierluigi Paganini

Pierluigi Paganini is member of the ENISA (European Union Agency for Network and Information Security) Threat Landscape Stakeholder Group and Cyber G7 Group, he is also a Security Evangelist, Security Analyst and Freelance Writer. Editor-in-Chief at "Cyber Defense Magazine", Pierluigi is a cyber security expert with over 20 years experience in the field, he is Certified Ethical Hacker at EC Council in London. The passion for writing and a strong belief that security is founded on sharing and awareness led Pierluigi to find the security blog "Security Affairs" recently named a Top National Security Resource for US. Pierluigi is a member of the "The Hacker News" team and he is a writer for some major publications in the field such as Cyber War Zone, ICTTF, Infosec Island, Infosec Institute, The Hacker News Magazine and for many other Security magazines. Author of the Books "The Deep Dark Web" and “Digital Virtual Currency and Bitcoin”.