Binance founder Changpeng Zhao (CZ) has warned server operators about a critical OpenSSH authentication vulnerability affecting certain Alpine Linux systems, urging them to install security fixes after developers discovered that the flaw had existed for nearly 10 years.
Summary
- Binance founder CZ has urged server operators to patch a decade-old OpenSSH authentication bypass vulnerability immediately.
- The flaw affects certain musl-based Linux servers using password authentication without PAM protection enabled during logins.
- Attackers could access vulnerable accounts using oversized passwords without knowing their actual credentials or private keys.
- OpenSSH released version 10.6p2, while Alpine Linux published security fixes for affected operating system releases.
- Some Alpine cloud configurations could expose administrative privileges, although no confirmed exploitation had been publicly reported.
In a November 6 post on X, Zhao drew attention to the security issue after OpenSSH and Alpine Linux released fixes on November 5. The vulnerability can allow attackers to access certain accounts without knowing their passwords, including accounts normally configured to accept only SSH keys.
“Alpine Linux OpenSSH security bug that’s been there for 10 years. Apply the fixes if you haven’t,” CZ wrote.
The warning concerns a software vulnerability affecting some server configurations. Neither Zhao’s post nor the security notices established that Binance systems had been compromised through the flaw.
OpenSSH flaw allows unauthorized access to vulnerable Linux servers
According to an official security advisory, attackers can bypass authentication on affected Alpine Linux servers by supplying a password between 257 and 1,024 bytes long.
The vulnerability affects systems running OpenSSH without Pluggable Authentication Modules (PAM), provided password authentication remains enabled and certain account configurations exist.
An attacker needs to know the username of an account whose stored password field contains an asterisk, indicating that no password has been set. The machine must have another local account with a compatible SHA-256 or SHA-512 password hash.
Under those conditions, the server may accept the oversized password even though it does not match legitimate credentials. The advisory states that exploitation requires only one connection, without repeated password guesses.
However, not every Alpine Linux server is vulnerable. Systems using PAM for password checks, those with password authentication disabled and containers without a running SSH server are not affected.
The flaw can extend beyond Alpine to other Linux distributions using musl libc, including certain OpenWrt, Buildroot and Yocto configurations.
OpenWrt normally uses Dropbear for SSH connections, which the researcher said is not affected by this particular issue.
Security researcher explains decade-old authentication bug
Security researcher Özgün Kültekin of Wakefield identified the vulnerability, which he named “*sshole,” and traced its origins to changes made between 2012 and 2016.
In his technical write-up, Kültekin explained that musl’s password-hashing function returns an asterisk when passwords exceed its 256-byte processing limit.
OpenSSH can borrow another account’s password salt when checking accounts without valid password hashes. The combination creates a situation where the returned asterisk matches the stored password field, allowing an invalid login to succeed.
The researcher linked the behavior to an OpenSSH change introduced in July 2016 to address a separate security problem involving username discovery.
OpenSSH acknowledged the vulnerability in its release notes but attributed the underlying cause to code outside the SSH server.
“We consider the root cause of this bug to be outside sshd,” the development team stated in its security notice.
Developers described the new release as a workaround that prevents the authentication error from granting access.
Alpine Linux cloud images face possible root access
Certain Alpine Linux cloud images present another concern because an account exposed through the vulnerability may have administrative permissions.
The Alpine advisory identifies tiny-cloud images from versions 3.18.11 through 3.24.3 as potentially affected. Their default alpine account has passwordless access to the doas utility, which allows users to execute commands with administrative privileges.
Although those images are not vulnerable on their first boot, adding an account with a compatible password hash can create the conditions for exploitation.
An attacker who successfully accesses the alpine account could then use its existing permissions to gain root access.
The advisory distinguishes these configurations from systems where direct root logins remain restricted. OpenSSH’s default settings prevent password-based root access, although another compromised administrative account could still provide elevated privileges.
The disclosure arrives as CZ has continued issuing security warnings to cryptocurrency users. On November 5, crypto.news reported his warning about a fraudulent Trust Wallet website designed to deceive users into connecting their wallets.
OpenSSH releases emergency security fix
OpenSSH released version 10.6p2 on November 5 to address the authentication bypass. Alpine Linux published patched packages covering current releases and older versions dating back to 3.16.
Its advisory instructs administrators to update affected packages and restart the SSH service, since installing an update alone does not replace a running server process.
For operators unable to update immediately, the developers recommend disabling password authentication by setting PasswordAuthentication no and restarting sshd after confirming SSH key access works.
Alpine identified versions 3.21.10, 3.22.8, 3.23.8 and 3.24.4 as releases containing the fixed packages.
Administrators were advised to review SSH authentication logs for unexpected password-based access to accounts that should not accept passwords.
The security advisory said operators unable to rule out unauthorized access should treat the affected machine as compromised.
