Attacker Impact
An attacker positioned as a Man-in-the-Middle (MitM) can bypass integrity checks within the SSH transport protocol [S1]. By omitting specific packets during the extension negotiation phase, the attacker can force the connection to use downgraded security features without detection [S1]. This compromise allows for the disabling of security extensions that would otherwise protect the session [S1].
Root Cause
The vulnerability, CVE-2023-48795, is caused by the improper handling of sequence numbers during the SSH handshake [S1]. This allows an attacker to inject or drop packets before encryption is established in a manner that keeps sequence numbers synchronized once encryption begins [S1]. Consequently, the integrity of the handshake negotiation is compromised, leading to a security downgrade (CWE-354) [S1].
Concrete Fixes
- Implement Strict Key Exchange: Update SSH clients and servers to versions that support the "strict KEX" extension, which prevents sequence number manipulation during the handshake [S1].
- Configuration Hardening: Ensure that both peers are configured to require strict key exchange to maintain protocol integrity [S1].
- Cipher Selection: In environments where patching is delayed, disabling vulnerable ciphers like ChaCha20-Poly1305 and CBC-mode ciphers with Encrypt-then-MAC (EtM) can reduce the attack surface [S1].
How FixVibe could detect it
FixVibe could potentially identify this vulnerability through:
- Banner Analysis: Identifying SSH service versions that are known to lack the strict key exchange implementation [S1].
- Configuration Auditing: Scanning system configurations and repository manifests for the absence of strict key exchange requirements or the presence of vulnerable cipher suites [S1].