OpenSSH Flaw Lets Attackers Log Into Password-Disabled Linux Accounts

A newly disclosed OpenSSH flaw dubbed “*sshole” can allow attackers to log in to password-disabled accounts on certain musl-based Linux systems simply by submitting a password longer than 256 bytes. “Apparently certain varieties of musl Linux (Alpine, OpenWrt, Buildroot, Yocto, etc.) have had this OpenSSH vulnerability for about a decade,” writes longtime Slashdot reader AlteredEgg. “Upgrade now!” BleepingComputer reports: Security researcher Ozgun Kultekin, who discovered and released details on the vulnerability today, says the flaw has existed since 2016 and stems from how OpenSSH handles password verification errors on systems using the musl C library. Linux accounts can be configured to prevent password logins by replacing their password hash with an asterisk (*) in the /etc/shadow file. Because this is not a valid password hash, no password should allow access to those accounts. However, a patch introduced in OpenSSH in 2016 to fix a username enumeration vulnerability tracked as CVE-2016-6210 changed how OpenSSH handled accounts without usable password hashes.

To prevent attackers from identifying valid usernames through different response times, OpenSSH was modified to perform password hashing even when an account had no usable password hash, including accounts configured with * to block password logins. The problem is that the musl C library’s crypt() function also returns * when password hashing fails, including when a submitted password exceeds its 256-byte limit. This means an attacker can submit an oversized password and cause OpenSSH to compare the returned * against the account’s stored *, resulting in a successful login.

To be clear, the flaw does not allow attackers to bypass passwords on accounts with valid password hashes. […] Admins who can’t immediately install the update can mitigate the issue by turning off SSH password authentication with PasswordAuthentication no in their SSH server configuration and restarting SSH. Kultekin says admins can also check for signs of exploitation by examining SSH authentication logs for password-based logins for accounts that should not allow them.

Read more of this story at Slashdot.