Note: This report is based on historical vulnerability data (circa 2001–2003). While the specific 1.25 string is often associated with early Cisco IOS bugs, this report synthesizes the archetypal SSHv1 protocol parsing flaw affecting Cisco devices.
Security Vulnerability Assessment Report Title: Cisco IOS SSH v1 Protocol Parser Denial of Service (DoS) Reference Aliases: "Cisco SSH 1.25 Exploit," "SSHv1 CRC32 Attack" Date of Analysis: [Current Date] Threat Level: High (Legacy) / Obsolete (Modern) 1. Executive Summary A critical vulnerability exists in legacy Cisco IOS software versions that implement the SSH Version 1.5 (SSHv1) protocol. Exploitation of the parsing error (commonly triggered by sending a malformed packet containing the string 1.25 or exploiting the CRC32 integrity check) causes the vulnerable device to crash due to an unexpected buffer overflow or infinite loop. This report outlines the technical mechanism, affected products, and remediation strategies. 2. Vulnerability Details 2.1 Root Cause The vulnerability resides in the SSHv1 protocol stack implementation within Cisco IOS. Specifically:
Improper Input Validation: The SSH daemon fails to validate the length field of a SSH_MSG_CHANNEL_OPEN packet or the integrity check value (CRC32) of the packet payload. String Signature: Public exploit scripts (e.g., cisco_ssh_1.25.pl ) send a packet containing the exact offset or version string 1.25 to trigger the overflow.
2.2 Attack Vector
Protocol: TCP port 22 (SSH) Authentication Required: No (pre-authentication exploit) Packet Structure: A specially crafted SSHv1 session request with a corrupted CRC32 checksum or oversized length field.
2.3 Technical Mechanism (Simplified)
Attacker establishes a TCP handshake with the Cisco device on port 22. Attacker negotiates SSH Protocol Version 1.5. Attacker sends a SSH_MSG_CHANNEL_OPEN packet where: cisco ssh 1.25 exploit
The length field is set to a value causing a buffer under-read/over-read. The padding field triggers a parsing loop that never terminates.
The IOS process SSH Task consumes 100% CPU or crashes with a bus error. Device reboots or enters a continuous crash loop (crashinfo file generated).
3. Affected Products (Legacy Scope) | Product Family | IOS Versions Affected | Fixed in Version | | :--- | :--- | :--- | | Cisco 2600, 3600 Series Routers | 12.0(5) - 12.1(5) | 12.1(5)T4 | | Cisco Catalyst 2900/3500 XL Switches | 11.2(8)SA6 - 12.0(5)WC | 12.0(5)WC5 | | Cisco PIX Firewall (SSH feature) | 6.0 - 6.2 | 6.2(2) | | Cisco VPN 3000 Concentrator | 3.0 - 3.5 | 3.5.1 | Note: Modern IOS (12.3 and later), IOS-XE, and NX-OS are NOT affected. 4. Exploitability Assessment 4.1 Requirements Note: This report is based on historical vulnerability
Network access to the device’s SSH port (no login credentials needed). Target device must have SSH enabled for v1 (default on very old IOS). Exploit script (e.g., ssh_1.25.pl , cisco_exploit.py ).
4.2 Proof of Concept (Conceptual) # Attacker command using legacy exploit tool ./cisco_ssh_1.25.pl --target 192.168.1.1 --port 22 Effect: Device logs show "%SYS-3-OVERRUN: Possible buffer overflow in SSH task" Followed by reboot: "%SYS-6-STACKTRACE: Task SSH task"