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SSHhole: OpenSSH root login with any 257+ char password on musl

2026-11-06 Fri ⏱ 12 min theprimeagen

Prime walks through "SSHhole", a vulnerability found by Ozgun (sshhole.com): on Alpine, OpenWrt or any musl-based box running sshd with password auth and without PAM, anyone can log in as root by sending a password longer than 256 characters. The bug is not one mistake but three independently reasonable decisions, spread over 40 years, that line up.

Who is affected

  • sshd on musl (Alpine - including the default Alpine Docker image - OpenWrt, etc.), password authentication enabled, PAM disabled.
  • With PAM, OpenSSH delegates the check (and gets 2FA etc. for free); without PAM it uses its own hand-rolled password comparison - that is the vulnerable path.

The ingredients

  • Asterisk as "no login" (1980): Bill Joy set the daemon account's password field to *; no real hash can equal *, so nobody can log in. Password check = crypt(password, salt) compared with the hash in /etc/shadow.
  • crypt returns * on failure (~2012): crypt used to return NULL on errors (unknown algorithm etc.), crashing callers; it switched to returning *.
  • musl's crypt rejects long passwords: hashing is CPU-heavy, so a 10,000-char password could pin a core per request (a DoS similar to ReDoS). musl caps input at 256 chars and returns * ("bad password") above that.
  • Fake-user timing defence (CVE-2016-6210): non-existent users returned instantly while real ones took a slow hash, enabling user enumeration. OpenSSH now hashes the supplied password with some other user's algorithm to simulate real timing.
  • Root password set to * (2022): a 2019 change put ! in root's password field without PAM, which broke sshd; in 2022 it became *.

The exploit

  1. Log in as root with a password longer than 256 chars.
  2. Root has no usable hash, so sshd borrows another user's algorithm and calls crypt.
  3. musl crypt sees >256 chars and returns *.
  4. sshd string-compares the result with root's stored "hash" - also * - match, root shell.

Takeaways

  • Patch/update any Alpine/OpenWrt host or container exposing sshd with password auth; prefer key-only auth and PAM.
  • The discoverer reportedly found it by throwing ~17,000 weird logins at OpenSSH with an agent swarm.
  • Prime's point: none of the pieces is obviously wrong in isolation; sentinel values (*) reused across layers turned "failure" into "match".