Older YubiKeys compromised by unpatchable 2FA bug — side-channel attack is critical, but expensive and difficult to execute

A critical security vulnerability has been discovered affecting many YubiKey two-factor authentication devices, breaking their security with no patch in sight. Yubico's security advisory confirmed that Yubikey 5 and Security Key Series prior to firmware 5.7 are forever vulnerable to a high-level cloning attack. However, the average user should not worry too much about the vulnerability.

Yubikey 5 series, YubiHSM 2, and other two-factor authentication products by Yubico and other vendors utilizing the Infineon SLB96xx series TPM chip are vulnerable to the newly found attack. Security researchers at NinjaLab tested Yubikey 5 products — due to them being the most common FIDO authenticator tools — and found that an issue in Infineon's library allows bad actors to clone the keys. All Infineon chips, going back 14 years, which run any version of its cryptographic library, are vulnerable to the same attack.

Physical two-factor authentication FIDO devices like Yubikeys are hugely valuable conveniences for users who want to save time compared to using an authenticator app when logging into secure computers, websites, or apps. Potential users include anywhere from government workers with sensitive secrets to those with nothing to hide but think turning on their computer with a key is cool. 

This cloning attack is a severe weakness for any 2FA tool, though the materials needed to perform it make the weakness a non-issue for most consumers. The attack first requires bad actors to obtain the key, at which point the key is already completely compromised. Then, after opening the key, the Yubikey device must be connected to a $45,000 setup (though researchers believe an $11,000 setup would function just fine) to read electromagnetic side-channel measurements. This process takes an hour to capture EM emissions and then a day to clone the key. Now that the Infineon chip has been successfully breached, the key can be cloned, and the original can be reassembled and sneakily returned to its owner.

The complexity of the steps required to perform the attack makes its real-world risk close to zero for most owners of a Yubikey. However, those with highly sensitive information, such as government employees, journalists, or healthcare workers, may need to consider phasing out affected hardware for newer hardware without the vulnerability. When we asked Yubico for comment, a company spokesperson gave the following:

"This issue was discovered in Infineon's cryptographic library, which is used in older versions of Yubico devices. Yubico's latest YubiKey 5 Series and Security Key Series hardware security keys that are currently available for purchase on Yubico.com include 5.7 firmware. The 5.7 firmware contains Yubico's own cryptographic library and these new devices are not impacted by Infineon's vulnerability.

FIDO is the strongest, phishing-resistant protocol. Yubico (and the researchers in their report) highly recommend continued use of FIDO authenticators over weaker authentication methods like OTP or SMS.

To help avoid local and physical threats, users should continue to take precautions to maintain physical control of their YubiKeys. In the event a YubiKey is lost or stolen, users should always promptly deregister keys with associated applications and services. This also supports the recommended best practice of having a primary and a backup key."

Yubico has been selling products with its 5.7.0 firmware and newer since May of this year. For security reasons, the firmware cannot be retroactively updated to older products, so those interested in replacing affected products should look to Yubico products with firmware 5.7.0 or newer or to other 2FA key manufacturers.