A signal handler in sshd(8) may call a logging function that is not async-signal-safe. The signal handler is invoked when a client does not authenticate within the LoginGraceTime seconds (120 by default). This signal handler executes in the context of the sshd(8)s privileged code, which is not sandboxed and runs with full root privileges.
This issue is another instance of the problem in CVE-2024-6387 addressed by FreeBSD-SA-24:04.openssh. The faulty code in this case is from the integration of blacklistd in OpenSSH in FreeBSD.
As a result of calling functions that are not async-signal-safe in the privileged sshd(8) context, a race condition exists that a determined attacker may be able to exploit to allow an unauthenticated remote code execution as root.
The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Freebsd | Freebsd | * | 13.0 (excluding) |
| Freebsd | Freebsd | 13.1 (including) | 13.3 (excluding) |
| Freebsd | Freebsd | 13.3-p1 (including) | 13.3-p1 (including) |
| Freebsd | Freebsd | 13.3-p2 (including) | 13.3-p2 (including) |
| Freebsd | Freebsd | 13.3-p3 (including) | 13.3-p3 (including) |
| Freebsd | Freebsd | 13.3-p4 (including) | 13.3-p4 (including) |
| Freebsd | Freebsd | 14.0-beta5 (including) | 14.0-beta5 (including) |
| Freebsd | Freebsd | 14.0-p1 (including) | 14.0-p1 (including) |
| Freebsd | Freebsd | 14.0-p2 (including) | 14.0-p2 (including) |
| Freebsd | Freebsd | 14.0-p3 (including) | 14.0-p3 (including) |
| Freebsd | Freebsd | 14.0-p4 (including) | 14.0-p4 (including) |
| Freebsd | Freebsd | 14.0-p5 (including) | 14.0-p5 (including) |
| Freebsd | Freebsd | 14.0-p6 (including) | 14.0-p6 (including) |
| Freebsd | Freebsd | 14.0-p7 (including) | 14.0-p7 (including) |
| Freebsd | Freebsd | 14.0-p8 (including) | 14.0-p8 (including) |
| Freebsd | Freebsd | 14.0-rc3 (including) | 14.0-rc3 (including) |
| Freebsd | Freebsd | 14.0-rc4-p1 (including) | 14.0-rc4-p1 (including) |
| Freebsd | Freebsd | 14.1-p1 (including) | 14.1-p1 (including) |
| Freebsd | Freebsd | 14.1-p2 (including) | 14.1-p2 (including) |
This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider. A race condition occurs within concurrent environments, and is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:
A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. Programmers may assume that certain code sequences execute too quickly to be affected by an interfering code sequence; when they are not, this violates atomicity. For example, the single “x++” statement may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read (the original value of x), followed by a computation (x+1), followed by a write (save the result to x). The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.