VyOS and Debian-Based Systems Dropbear SSH Key Reuse Vulnerability Allowing Man-in-the-Middle Attacks

A vulnerability exists in VyOS versions 1.3 through 1.5, as well as in any Debian-based system using Dropbear with live-build, where the same private host keys are used across different installations. This key reuse can lead to active man-in-the-middle attacks on SSH connections if Dropbear is enabled as the SSH daemon. While this is not the default for system SSH on VyOS, it is the default for the console service. In VyOS, the private keys are generated during the image build process and not regenerated on first boot, causing all systems deployed from the same image to use identical keys. An attacker could exploit this by intercepting and modifying traffic between the server and client, impersonating the server during the key exchange phase.

Impact

Exploitation of this vulnerability allows for active man-in-the-middle attacks on SSH connections to a Dropbear server, with the attacker able to intercept, modify, and potentially impersonate the server during the key exchange process.

Reproduction

After installing a vulnerable VyOS system image, enable the Dropbear SSH server. The private keys can be extracted and will show that the same keys are used across different installations from the same image. This key reuse can be verified by comparing the extracted keys with those generated on a fresh installation.

Remediation

Users can update to VyOS 1.4.2 or the latest VyOS 1.5 release, both of which include the necessary fix. Alternatively, users can manually remove the pre-generated Dropbear keys and replace them with newly generated keys before using Dropbear as the SSH daemon.