SSH situation is insane

Summary

The video explains a serious SSH password-authentication vulnerability caused by interacting behaviors in musl libc and certain OpenSSH configurations. Despite involving very long password input, the presenter emphasizes that it is not a buffer overflow. It is a logic flaw caused by assumptions in separate functions.

How the flaw works

• In /etc/shadow, a * can mark an account as locked or as having no usable password, preventing normal password login.
• Historically, musl’s crypt() returned NULL on errors. It was later changed to return * instead.
• OpenSSH also performs a dummy password-hash calculation for accounts that do not exist or have no password, partly to reduce timing-based username enumeration.
• With certain long inputs, musl’s SHA-512 crypt routine returns an error. That error can become *, which may then be compared with the * stored for a locked account and mistakenly treated as a match.

Under the right conditions, an attacker may be able to authenticate by entering a password longer than 256 characters.

Who may be affected

The presenter says the vulnerable setup requires several conditions, including:

• OpenSSH built with musl
• Password authentication enabled
• No PAM authentication modules in use
• A locked account marked with *, alongside an account using a real SHA-512 password hash

Alpine Linux is highlighted as a notable example because its default Docker image configuration reportedly meets these conditions. The video says other configurations—such as GNU libc systems, PAM-based setups, and systems using Dropbear or OpenBSD—are not affected by this particular issue. The presenter stresses that exposure depends on configuration, not simply on whether a system uses SSH.

Response and broader analysis

The video reports that emergency fixes for OpenSSH and musl libc have been issued and urges administrators of affected systems to patch.

The presenter describes the bug as an example of security contracts drifting across software components: each function’s behavior may seem reasonable in isolation, but together their error-handling assumptions create an authentication bypass. The presenter also notes that this is not the kind of memory-safety issue Rust is specifically designed to prevent.

The video includes a brief sponsor segment about Flare’s threat-intelligence platform, which monitors dark-web and illicit-channel activity and tracks exploitation of known vulnerabilities.

Main speakers and sources

• Main speaker: The Low Level channel presenter, explaining the vulnerability and affected configurations.
• Research source: Ozgun Call Tekken (name as heard in the subtitles), whose research and repository the presenter uses for the walkthrough.