Devices running some configurations of Alpine Linux, and possibly other distributions, are affected by a decade-old vulnerability that allows attackers to log into devices with no password.

The vulnerability exists in the interaction between OpenSSH, the code library implementing SSH connections, and musl, a lightweight alternative to the libc library that, unlike most other Linux distributions, is used in Alpine. Further, OpenSSH using musl (pronounced muscle) must not use PAM (pluggable authentication modules), a framework that handles user authentication and account-access checks. Alpine is a lightweight, widely used Linux distribution. Other platforms may be affected if they use the same configurations.

There are two other conditions necessary for successful exploitation. First, a system must have at least one user account that’s accessible using a password that is hashed using the SHA256 or SHA512 functions. Second, the target system account must be configured to be passwordless, by way of specifying a "*" in the password field in /etc/shadow, the Linux file where password hashes are stored. Passwordless accounts are commonly used for accounts that use certificate-based authentication. This setting is common for certain types of accounts.

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