configure terminal crypto key zeroize rsa ! WARNING: This removes all existing RSA keys – do this out-of-hours crypto key generate rsa modulus 2048

The SSH-2-Cisco-125 vulnerability is a critical security threat that affects certain versions of Cisco IOS software running on various Cisco routers and switches. A successful exploit of this vulnerability could allow an attacker to gain unauthorized access to a vulnerable device, potentially leading to a complete compromise of the system. To mitigate this vulnerability, it is essential to apply the patch released by Cisco and implement additional mitigation strategies, such as disabling SSHv2 and implementing access controls.

If an attacker has recorded encrypted SSH sessions weeks ago (e.g., via a SPAN port or a compromised switch), and later factors the weak 125-byte modulus, they can decrypt those sessions and obtain:

| Vulnerability / Advisory | Year | Product(s) Affected | Primary Risk (CIA Impact) | Key Detail | | :--- | :--- | :--- | :--- | :--- | | | 2002 | Cisco IOS, Catalyst switches | Denial of Service | A crafted SSH packet could cause a device to crash and reload. | | cisco-sa-20050406-ssh | 2005 | Cisco IOS with TACACS+ | Denial of Service | Vulnerabilities in the SSH server could lead to resource exhaustion and DoS. | | CVE-2018-0482 | 2018 | Cisco IOS XE Software (SSH client) | Privilege Escalation | A local attacker could gain shell access with root privileges. | | CVE-2025-20163 | 2025 | Cisco Nexus Dashboard Fabric Controller (NDFC) | Information Leak, Credential Capture | A 8.7 CVSS flaw for machine-in-the-middle attacks due to insufficient SSH host key validation. | | CVE-2025-20309 | 2025 | Cisco Unified CM, IM&P | Remote Code Execution | Vulnerability due to hardcoded, unchangeable default root credentials , leading to complete system compromise. | | Erlang/OTP SSH Server RCE (in Cisco) | 2025 | Cisco products using Erlang/OTP | Remote Code Execution | A CVSS 10.0 flaw allowing unauthenticated RCE by sending malicious messages during the authentication phase. |

Confirm the new key:

Note: this post analyzes the widely referenced SSH server identification string "SSH-2.0-Cisco-1.25" and associated vulnerabilities that have appeared in advisories and exploit literature. It explains what the identification string means, the types of issues commonly associated with specific SSH server implementations (including some Cisco SSH products), real-world impact, detection, and step-by-step mitigation and hardening guidance. Assumptions: reader has basic networking and SSH familiarity.

Legacy software versions should be inventoried immediately. You can check the current running version and active SSH configuration by executing the following commands in the Cisco privileged EXEC mode: Router# show version Router# show ip ssh Use code with caution.

To verify if a device is exposing this banner, a penetration tester or administrator can perform a simple banner grab using standard tools like Netcat or Telnet on port 22.

Modify default SSH configurations to limit the capabilities of potential automated attack scripts. Enforce maximum timeouts and lower authentication retry thresholds to terminate malicious connection attempts early:

On the Cisco device:

: Refers to SSH Version 2.0 . While vastly superior to the cryptographically broken SSH v1, standard SSH v2 can still host outdated key exchange algorithms (Diffie-Hellman Group 1), weak ciphers (3DES, RC4), or vulnerable software daemons.

no ip http server and no ip http secure-server

First Published: April 22, 2025 | Last Updated: June 11, 2026

Ssh20cisco125 Vulnerability Jun 2026

configure terminal crypto key zeroize rsa ! WARNING: This removes all existing RSA keys – do this out-of-hours crypto key generate rsa modulus 2048

The SSH-2-Cisco-125 vulnerability is a critical security threat that affects certain versions of Cisco IOS software running on various Cisco routers and switches. A successful exploit of this vulnerability could allow an attacker to gain unauthorized access to a vulnerable device, potentially leading to a complete compromise of the system. To mitigate this vulnerability, it is essential to apply the patch released by Cisco and implement additional mitigation strategies, such as disabling SSHv2 and implementing access controls.

If an attacker has recorded encrypted SSH sessions weeks ago (e.g., via a SPAN port or a compromised switch), and later factors the weak 125-byte modulus, they can decrypt those sessions and obtain:

| Vulnerability / Advisory | Year | Product(s) Affected | Primary Risk (CIA Impact) | Key Detail | | :--- | :--- | :--- | :--- | :--- | | | 2002 | Cisco IOS, Catalyst switches | Denial of Service | A crafted SSH packet could cause a device to crash and reload. | | cisco-sa-20050406-ssh | 2005 | Cisco IOS with TACACS+ | Denial of Service | Vulnerabilities in the SSH server could lead to resource exhaustion and DoS. | | CVE-2018-0482 | 2018 | Cisco IOS XE Software (SSH client) | Privilege Escalation | A local attacker could gain shell access with root privileges. | | CVE-2025-20163 | 2025 | Cisco Nexus Dashboard Fabric Controller (NDFC) | Information Leak, Credential Capture | A 8.7 CVSS flaw for machine-in-the-middle attacks due to insufficient SSH host key validation. | | CVE-2025-20309 | 2025 | Cisco Unified CM, IM&P | Remote Code Execution | Vulnerability due to hardcoded, unchangeable default root credentials , leading to complete system compromise. | | Erlang/OTP SSH Server RCE (in Cisco) | 2025 | Cisco products using Erlang/OTP | Remote Code Execution | A CVSS 10.0 flaw allowing unauthenticated RCE by sending malicious messages during the authentication phase. | ssh20cisco125 vulnerability

Confirm the new key:

Note: this post analyzes the widely referenced SSH server identification string "SSH-2.0-Cisco-1.25" and associated vulnerabilities that have appeared in advisories and exploit literature. It explains what the identification string means, the types of issues commonly associated with specific SSH server implementations (including some Cisco SSH products), real-world impact, detection, and step-by-step mitigation and hardening guidance. Assumptions: reader has basic networking and SSH familiarity.

Legacy software versions should be inventoried immediately. You can check the current running version and active SSH configuration by executing the following commands in the Cisco privileged EXEC mode: Router# show version Router# show ip ssh Use code with caution. configure terminal crypto key zeroize rsa

To verify if a device is exposing this banner, a penetration tester or administrator can perform a simple banner grab using standard tools like Netcat or Telnet on port 22.

Modify default SSH configurations to limit the capabilities of potential automated attack scripts. Enforce maximum timeouts and lower authentication retry thresholds to terminate malicious connection attempts early:

On the Cisco device:

: Refers to SSH Version 2.0 . While vastly superior to the cryptographically broken SSH v1, standard SSH v2 can still host outdated key exchange algorithms (Diffie-Hellman Group 1), weak ciphers (3DES, RC4), or vulnerable software daemons.

no ip http server and no ip http secure-server

First Published: April 22, 2025 | Last Updated: June 11, 2026 To mitigate this vulnerability, it is essential to