CVE-2016-6515

Think of your SSH login service as a door with a sign-in form. This bug is like a flaw in the sign-in form that can be overwhelmed by sending an extremely long password—enough to knock the door system offline. The main problem is availability: your SSH login service can stop working until it recovers or is restarted.

This matters if your small business runs an SSH server using OpenSSH (sshd) version before 7.3, especially where password-based login is allowed. It does not require the attacker to have an account or to trick a user into clicking anything. Because it is reachable over the network with no authentication needed, it becomes a risk when your SSH service is exposed to network traffic and password authentication is enabled.

This is AMBER because attackers can trigger a crash remotely by repeatedly attempting SSH logins with extremely long passwords, and public exploitation exists. Even without confirmed “official” listing in CISA KEV, the combination of easy reachability (network, no login required) and known public exploit code makes it important to address quickly to protect availability.

1. Confirm exposure (your side):

   • Identify the SSH server software and version: run sshd -V (or check your OS package inventory for the OpenSSH server version).
   • Verify whether password authentication is enabled by reviewing sshd_config (look for PasswordAuthentication yes).
   • Confirm the service is reachable from the network where attackers can reach it (for example, SSH exposed to the internet).

2. Determine which systems need upgrading:

   • OpenSSH hosts: if OpenSSH is before 7.3 and password authentication is enabled, they are in scope.
   • Siemens SCALANCE X-200RNA devices: if you are running affected firmware versions, treat V3.2.7 as the minimum fixed level per vendor guidance.

3. Apply the fix:

   • OpenSSH: upgrade to 7.3 or later so that password authentication input handling enforces length limits.
   • Siemens SCALANCE X-200RNA: upgrade to V3.2.7 or later.

4. If you cannot patch immediately (temporary containment):

   • Disable password authentication if you can switch to stronger alternatives (for example, key-based SSH authentication), because the bug is specifically triggered during password authentication.
   • Restrict network access to SSH (allow only trusted source IPs/VPN) so the unauthenticated remote attacker cannot reach the service.

5. After changes:

   • Re-check sshd -V and confirm PasswordAuthentication setting.
   • Monitor for SSH daemon crashes/restarts and authentication attempts with abnormal login payload sizes (if your logs show repeated unusual login failures close to crash events).

Severity is HIGH with an availability impact (denial of service). The weakness is CWE-20 (improper input validation): sshd’s password authentication path (auth_password in auth-passwd.c) does not limit password length for password authentication in OpenSSH versions before 7.3. Attack vector is network-based and does not require prior authentication or user interaction, meaning an attacker can send an excessively long password to trigger a crash. Exploitation status: public exploit code is available (4 known). KEV status: not listed in CISA KEV. A patch exists via vendor and upstream changes; for Siemens SCALANCE X-200RNA, remediation guidance calls out upgrading to at least V3.2.7.