Ssh-2.0-cisco-1.25 Vulnerability May 2026
If you are required to submit this as a formal paper for academic or professional use, I strongly recommend that you:
Would you like me to help you instead:
The string SSH-2.0-Cisco-1.25 SSH server banner typically seen when connecting to Cisco IOS or IOS-XE devices. This banner itself is a version string, not a specific vulnerability, but its presence indicates the device is running a version of the Cisco SSH implementation that may be susceptible to several known protocol-level and implementation-specific vulnerabilities. Devolutions Forum Key Vulnerabilities Associated with Cisco SSH
If your device reports this version string, it may be affected by the following vulnerabilities depending on the specific software release (IOS/IOS-XE): RSA-Based Authentication Bypass (CVE-2015-6280)
: A flaw in the SSHv2 public key authentication implementation could allow a remote attacker to bypass user authentication by using a crafted private key. This requires the attacker to know a valid username and the corresponding public key. SSH Denial of Service (CVE-2020-3200)
: A vulnerability in the SSH state machine of Cisco IOS and IOS-XE Software could allow an authenticated, remote attacker to cause the device to reload by sending a specific traffic pattern, leading to a Denial of Service (DoS). Terrapin Attack (CVE-2023-48795)
: A prefix truncation weakness in the SSH protocol that could allow a man-in-the-middle attacker to downgrade the connection's security by deleting messages from the beginning of the secure channel. Erlang SSH Remote Code Execution (RCE)
: Recent reports in April 2025 highlight a critical RCE vulnerability in the Erlang-based SSH server used in some Cisco product lines. This is a "Perfect 10" severity flaw that allows unauthenticated code execution. Cisco Community How to Verify and Mitigate SSH Terrapin Prefix Truncation Weakness - Cisco Community
In the world of network security, few things cause a spike in adrenaline quite like an unfamiliar banner appearing in your vulnerability scanner. For many system administrators and security analysts, the string "ssh-2.0-cisco-1.25" is one such trigger. Scrolling through a Nessus, OpenVAS, or Qualys report, this identifier often appears under "SSH Server Version Information," flagged with a medium or high-severity warning.
But is this a critical zero-day exploit? A backdoor? A misconfiguration?
The short answer is more nuanced. The "ssh-2.0-cisco-1.25 vulnerability" is not a singular, unpatched software flaw. Rather, it is a version fingerprint associated with specific Cisco operating systems (primarily older versions of Cisco IOS and Cisco NX-OS) that historically contained several known, documented vulnerabilities.
This article will dissect exactly what SSH-2.0-Cisco-1.25 means, explore the real vulnerabilities tied to this SSH implementation, distinguish between myth and fact, and provide a definitive guide to remediation.
An attacker sending a single crafted SSHv2 packet can crash the device. No logs may be left before crash.
Rosa was the network engineer for a small regional hospital. One quiet Sunday she noticed unusual login attempts on a Cisco router that connected the hospital’s outpatient clinics. The logs showed a banner string: “SSH-2.0-Cisco-1.25.” She recognized the banner from a vendor advisory she’d skimmed weeks earlier but had never fully investigated.
That morning she made a quick plan. First, she isolated the affected device by moving management access to an alternate path and restricting SSH access in the firewall to only her workstation’s IP. She then pulled the exact firmware and configuration versions from the router and compared them against the vendor’s advisory. The advisory described a flaw in certain Cisco SSH implementations where malformed negotiation packets could cause a buffer overflow, allowing unauthenticated attackers to crash the SSH service or execute code.
Rosa followed these concrete steps:
Two things made the difference: quick containment and a tested patch plan. Because Rosa prioritized limiting access first, even if an exploit existed, attackers had far fewer opportunities. Because she tested upgrades in a lab, the hospital avoided a surprise outage. ssh-2.0-cisco-1.25 vulnerability
Moral: Treat unexpected SSH banners as a signal to investigate, not ignore. With containment, identification, mitigations, timely patching, and improved processes, small teams can keep critical infrastructure safe.
Understanding the "SSH-2.0-Cisco-1.25" Banner and Modern Security Risks
If you have recently run a vulnerability scan like Nessus or OpenVAS against your Cisco infrastructure, you may have seen a reference to SSH-2.0-Cisco-1.25. While this string is actually a version banner rather than a single specific "vulnerability," it often serves as a primary indicator for several critical security flaws affecting Cisco’s SSH implementation. What is SSH-2.0-Cisco-1.25?
This is a software banner identifying the SSH server running on your Cisco device. SSH-2.0: Indicates the device is running SSH Version 2.
Cisco-1.25: Refers to a specific legacy version of the Cisco SSH stack found in various Cisco IOS, IOS XE, and older PIX/ASA software releases.
Because this version is dated, it is frequently flagged by scanners because it supports weak cryptographic algorithms or is susceptible to protocol-level attacks discovered in recent years. Top Vulnerabilities Linked to This Version
When security professionals discuss the "Cisco-1.25 vulnerability," they are typically referring to one of the following critical issues: 1. The Terrapin Attack (CVE-2023-48795)
Many Cisco devices running the 1.25 stack are vulnerable to the Terrapin attack, a prefix truncation weakness.
The Risk: A Man-in-the-Middle (MitM) attacker can downgrade the connection's security by deleting specific protocol messages during the handshake without the client or server noticing. Cisco Bug ID: CSCwi61646. 2. Unauthenticated Remote Code Execution (CVE-2025-32433)
Recent advisories have highlighted a maximum-severity flaw (CVSS 10.0) in certain Cisco SSH implementations (specifically those utilizing Erlang/OTP libraries).
The Risk: Attackers can execute arbitrary code on the target system without needing to authenticate first.
Affected Banner: This has been observed in environments reporting the SSH-2.0-Cisco-1.25 banner. 3. Weak Cryptographic Algorithms
Older Cisco SSH stacks often default to algorithms now considered "broken" or "weak":
KEX Algorithms: Support for diffie-hellman-group1-sha1 or diffie-hellman-group-exchange-sha1.
Ciphers: Continued use of CBC-mode ciphers (e.g., aes128-cbc), which are susceptible to side-channel attacks. How to Secure Your Cisco Device
If your scanner has flagged this banner, follow these steps to mitigate the risk: Step 1: Update Your IOS/IOS XE Software If you are required to submit this as
The most effective fix is to upgrade to a modern, patched version of Cisco software. Check the Cisco Security Advisory for your specific hardware to find the recommended "Gold Star" release. Step 2: Harden the SSH Configuration
If you cannot upgrade immediately, manually disable weak algorithms in the CLI:
# Disable weak Diffie-Hellman groups ip ssh dh min size 2048 # Specify secure ciphers (prefer CTR or GCM modes) ip ssh server algorithm encryption aes256-ctr aes192-ctr aes128-ctr # Specify secure Message Authentication Codes (MACs) ip ssh server algorithm mac hmac-sha2-256 hmac-sha2-512 Use code with caution. Copied to clipboard Step 3: Obfuscate the Banner (Optional)
While "security by obscurity" isn't a primary defense, you can prevent casual scanning from identifying your exact version. On some platforms, you can customize or suppress parts of the SSH banner via the banner command, though the protocol-level version string (Cisco-1.25) is often hard-coded into the stack. Summary Table Vulnerability Mitigation Terrapin (CVE-2023-48795) Security Downgrade Disable ChaCha20-Poly1305 and CBC ciphers. RCE (CVE-2025-32433) Full System Takeover Immediate software update/patching. Weak KEX/Ciphers Data Decryption Update ip ssh settings to use SHA-2 and CTR.
Are you seeing this alert on a specific model, like a Catalyst switch or an ASA firewall? Providing the hardware type can help narrow down the exact patch you need.
The string SSH-2.0-Cisco-1.25 is not a specific vulnerability itself, but rather the version banner
that a Cisco device sends when a connection is initiated over port 22. Cisco Community
While the banner is a standard part of the SSH handshake, it is frequently flagged by security scanners (like Nessus or Qualys) as "potentially vulnerable" because it reveals that the device is running an older or specific version of the Cisco SSH server. Cisco Community Understanding the Banner : Indicates the device is using SSH Protocol Version 2.0. Cisco-1.25
: This is the internal version of the Cisco SSH software implementation. Cisco Community Why Scanners Flag This
Security tools often alert on this banner because it helps attackers perform fingerprinting
—identifying the exact operating system and software version to find matching exploits. Several critical vulnerabilities have affected Cisco devices running versions associated with this banner over the years: NetCom Learning SSH Terrapin Prefix Truncation Weakness - Cisco Community
The string SSH-2.0-Cisco-1.25 is not a specific vulnerability itself, but rather the software version banner
that a Cisco device displays when you connect to its SSH server.
Security scanners (like Nessus or Qualys) often flag this banner because it reveals the device's operating system and version, which can help an attacker identify known vulnerabilities. Below is a breakdown of what this banner means and the actual vulnerabilities often associated with it. What is SSH-2.0-Cisco-1.25?
When an SSH client connects to a Cisco router or switch, the two devices exchange "version strings" to ensure they can talk to each other.
: Indicates the device is using SSH protocol version 2.0 (more secure than 1.x). Cisco-1.25 Would you like me to help you instead:
: This is the specific internal version of the Cisco SSH server software running on the device. Why do scanners flag it? (The "Vulnerability")
Security audits often list this as a "medium" or "low" risk because of Information Disclosure
. By advertising the exact version of the SSH server, the device tells a potential attacker exactly which bugs might be exploitable on that specific system.
However, "Cisco-1.25" is found across many different IOS versions. Depending on which IOS version you are running, your device might be vulnerable to several real, documented threats: SSH Terrapin Prefix Truncation Weakness - Cisco Community
The identifier "SSH-2.0-Cisco-1.25" is a software version string returned by the SSH banner on many Cisco IOS-based devices. While not a specific vulnerability name itself, this version string is frequently associated with several critical security flaws that affect the SSH implementation in Cisco IOS and IOS XE software. Notable Vulnerabilities Associated with Cisco SSH
Security researchers and automated scanners often flag devices displaying this banner because they may be susceptible to the following high-impact issues:
Authentication Bypass (CVE-2015-0923): A significant vulnerability in the SSH version 2 protocol implementation allows unauthenticated, remote attackers to bypass user authentication. To exploit this, an attacker must know a valid username configured for RSA-based authentication.
Denial of Service (CVE-2020-3200): A flaw in the SSH server code allows an authenticated remote attacker to cause a device reload. This occurs due to an internal state machine error that can be triggered by specific traffic patterns, leading to a DoS condition.
Remote Code Execution (CVE-2025-32433): Recent reports have identified a critical vulnerability (CVSS 10.0) in certain Cisco products using the Erlang/OTP SSH implementation. It allows unauthenticated remote code execution by sending connection protocol messages before authentication occurs.
Resource Exhaustion: Older Cisco IOS releases using SSH with TACACS+ authentication are vulnerable to resource exhaustion, which can lead to spontaneous reloads. Scope and Exposure
Scanning tools like Shodan and Censys have identified over 100,000 exposed instances globally of the "SSH-2.0-Cisco-1.25" banner. This broad exposure makes these devices prime targets for automated exploit scripts. Remediation and Best Practices
Cisco has released software updates to address these vulnerabilities across its product lines. Administrators are advised to:
Upgrade Firmware: Consult the Cisco Security Advisories page to identify the fixed release for your specific hardware.
Restrict Management Access: Use Access Control Lists (ACLs) to limit SSH access to known, trusted management IP addresses.
Disable Vulnerable Features: If immediate patching is not possible, consider temporarily disabling RSA-based public key authentication if it is the primary vector for a known bypass. CVE-2020-3200 Detail - NVD
Older Cisco SSH implementations typically support legacy ciphers such as Arcfour (RC4), Blowfish, and 3DES (specifically the CBC mode).