Understanding SSH Banner Composition

SSH banners are essentially greeting messages that serve as the initial communication statement when an SSH client connects to an SSH server. These banners provide basic information about the server's SSH protocol version and may also reveal the server type or software version. In SSH protocol exchanges, version identifiers such as SSH-2.0-OpenSSH_7.9 can be instrumental in understanding the server configurations.

For instance, SSH-2.0 indicates the SSH version, and OpenSSH_7.9 provides information about the specific server software and its version. Sometimes, custom messages follow this standard format, which can include additional text specified by the server administrator. An understanding of these compositions allows an attacker to infer potential features and vulnerabilities.

Tools for SSH Banner Grabbing

Several tools can efficiently retrieve SSH banners for analysis. Netcat (nc) is a simple utility that can connect to a target's port 22 (the default SSH port) to fetch the banner. The basic usage would involve:

nc -v TARGET_IP 22

Identifying SFTP Capabilities in Banners

Once captured, SSH banners should be scrutinized for indicative markers suggesting SFTP services. Typically, a banner might directly or indirectly hint at protocols or versions that support SFTP.

If a banner or subsequent handshake includes references to subsystem sftp, it strongly suggests that SFTP capabilities are enabled.

SSH Banner Example with SFTP:

SSH-2.0-OpenSSH_8.0p1 Ubuntu-20.04
subsystem sftp /usr/lib/openssh/sftp-server

Additionally, pay attention to vendor-specific implementations or unique identifiers that correspond to servers notoriously offering SFTP. The presence of OpenSSH or another SSH implementation like Dropbear can be an indicator, as these often support SFTP natively. Consulting documentation of known SSH server software can help in correlating these hints with actual SFTP support.

Bypassing SSH Banner Masking Techniques

SSH administrators sometimes obscure or modify banners to protect against reconnaissance. Masking techniques might include omitting version numbers or adding misleading details. To counteract this, alternative fingerprinting methods such as analyzing timing of connection responses or discrepancies in packet sizes can provide context.

Additionally, behavioral analysis, comparing connection sequences and server responses for anomalies, helps infer details beyond the visible banner. These techniques can mitigate the effectiveness of masking and provide a clearer picture of the server's capabilities.