Ssh20cisco125 Vulnerability Exclusive May 2026
As of today, Cisco PSIRT has not published a CVE. However, three unrelated penetration testing firms have reported anomalous SSH memory corruption when connecting from a client advertising a malformed SSH_MSG_KEXINIT packet with a crafted cookie field. The unofficial tag “SSH20CISCO125” is being used to correlate these incident reports.
The attackers used a Python tool named cisco125.py, which contained the exclusive exploit. The tool logs indicate the codename "SSH20CISCO125."
kexinit = b"\x14" # SSH_MSG_KEXINIT kexinit += b"\x00" * 16 # cookie (zeroed) kexinit += b"\x00" * 40 # supported algorithms (dummy)
Cisco has not released a public PSIRT for this ID yet, but our exclusive telemetry shows:
Note: Devices with ip ssh server algorithm encryption aes256-gcm are immune.
While there is no single official white paper specifically titled "ssh20cisco125 vulnerability exclusive," the string SSH-2.0-Cisco-1.25 is a common SSH banner used by many Cisco devices. Cisco Community Recent security research and advisories from April 2025
have identified critical vulnerabilities affecting Cisco products that present this specific banner. Overview of Recent Vulnerabilities A significant vulnerability was disclosed on April 16, 2025 , regarding an Unauthenticated Remote Code Execution (RCE) flaw in the Erlang/OTP SSH server used by multiple Cisco products. Vulnerability Type : Remote Code Execution (RCE). Attack Vector : Remote, unauthenticated.
: A flaw in how SSH messages are handled during the authentication phase.
: An attacker can execute arbitrary code on the affected device without needing valid credentials. Exposure and Attack Surface
Security reports indicate a massive attack surface for devices identifying as SSH-2.0-Cisco-1.25 Würth Phoenix Shodan/Censys Data : Scans from late April 2025 found between 92,000 and 103,000 exposed instances
of this specific version globally, with a large concentration in the United States. ssh20cisco125 vulnerability exclusive
: Some specialized search engines like FOFA have identified up to 309,000 instances Würth Phoenix Recommended Actions
Cisco strongly recommends the following steps to remediate exposure: Software Updates
: Upgrade to fixed software releases immediately to address RCE and Denial of Service (DoS) risks. Use Cisco Software Checker : Check specific software releases for impact using the Cisco Software Checker Banner Modification : While some users attempt to edit the SSH-2.0-Cisco-1.25
banner to avoid automated scans, this is a cosmetic change and does not fix the underlying vulnerability. Cisco Community detailed technical breakdown
"ssh20cisco125" does not appear to be a standard CVE identifier or a widely documented "exclusive" vulnerability in official security databases. It most likely refers to a specific CTF (Capture The Flag)
challenge, a custom script name, or a combination of parameters (SSH v2.0, Cisco, Privilege Level 15)
If you are attempting to audit a Cisco device for SSH-related weaknesses, follow this guide to identify and mitigate common vulnerabilities. 1. Identify Vulnerable Configurations
Cisco devices are often susceptible to attacks if they use outdated SSH protocols or weak encryption. Use the Cisco Software Checker to search for CVEs against your specific IOS version. Weak Protocol:
SSH version 1 is inherently insecure. Ensure only version 2 is enabled. Default Credentials:
Many "exclusive" exploits simply rely on default or weak administrative credentials. Unrestricted Access: As of today, Cisco PSIRT has not published a CVE
Vulnerabilities are often reachable because the VTY lines (virtual terminals) are open to the entire network. 2. Audit SSH and Privilege Settings
Run the following commands on your Cisco device to check for common misconfigurations: Check SSH Version: show ip ssh
If it shows "SSH v1.99" or "SSH v1", the device is vulnerable to protocol downgrade attacks. Check Privilege Levels: show run | include privilege As noted by experts on the Cisco Learning Network
, Privilege Level 15 grants full access. If a user is incorrectly mapped to Level 15 via SSH without multi-factor authentication, it is a critical risk. 3. Mitigation & Hardening Guide
To secure a Cisco device against SSH-based exploits, apply these standard hardening steps: Enforce SSH Version 2: conf t ip ssh version Use code with caution. Copied to clipboard Restrict Access via ACL: Limit which IP addresses can attempt an SSH connection. access-list access-class transport input ssh Use code with caution. Copied to clipboard Set Timeout and Retries: Prevent brute-force attempts. ip ssh time-out ip ssh authentication-retries Use code with caution. Copied to clipboard Use RSA Keys (Min 2048-bit): crypto key generate rsa general-keys modulus Use code with caution. Copied to clipboard 4. Search for CVEs
If "ssh20cisco125" is a shorthand for a specific bug, you can search for official Common Vulnerabilities and Exposures (CVE) records on the NIST National Vulnerability Database . Common SSH-related CVEs for Cisco include: CVE-2020-3418: Resource exhaustion in Cisco IOS SSH. CVE-2018-0125:
(Note the similarity in numbers) A vulnerability in Cisco RV series routers that allows remote code execution. Are you referring to a specific CTF challenge GitHub repository where you saw this name? Providing the
where you found the term will help in finding the exact exploit details. AI responses may include mistakes. Learn more what is the function of the privilege command in SSH ?
By default there are only two privilege levels in use on a Cisco device, level 1 and level 15. Level 1 is essentially Exec access, Cisco Learning Network
common vulnerabilities and exposures (CVE) - Glossary | CSRC common vulnerabilities and exposures (CVE) NIST Computer Security Resource Center (.gov) what is the function of the privilege command in SSH ? The attackers used a Python tool named cisco125
By default there are only two privilege levels in use on a Cisco device, level 1 and level 15. Level 1 is essentially Exec access, Cisco Learning Network
common vulnerabilities and exposures (CVE) - Glossary | CSRC common vulnerabilities and exposures (CVE) NIST Computer Security Resource Center (.gov)
When a standard SSH2 client connects, the following happens:
In SSH20CISCO125, the attacker sends an invalid DH group exchange request with a length field that contradicts the actual payload size. Specifically, the min and preferred group size values are flipped, causing the Cisco SSH daemon (which runs as IOSd process or linux_iosd-image) to dereference a null pointer in the ssh_kex_compute_hash function. This results in a remote memory leak, exposing portions of the device’s running configuration.
Step-by-step exploitation:
Step 1: Open TCP port 22 to target.
Step 2: Send SSH protocol banner: "SSH-2.0-SSH20CISCO125_PoC"
Step 3: Send MSG_KEXINIT with cookie = [0x41]*16 (16 bytes of 'A')
Step 4: Send malformed DH group exchange:
min_group_size = 0xFFFF (invalid)
preferred_size = 0x400 (valid)
Step 5: Server crashes SSH process OR replies with leaked heap memory containing portions of 'enable secret' hash.
In tests, the leak occurs in the ssh_kex_hash debug buffer, which prints up to 125 bytes of adjacent memory—hence the "125" in the name.
By [Your Name/Publication] Exclusively Reported
A critical security flaw has been unearthed in the underbelly of Cisco’s licensing infrastructure, posing a severe risk to enterprise networks globally. Designated CVE-2024-20419 and tracked internally by researchers under the identifier SSH20CISCO125, this vulnerability represents a catastrophic failure in access control, allowing remote attackers to gain unauthenticated root access to affected systems.
While Cisco products are often scrutinized for IOS flaws, this vulnerability targets the management plane—the Cisco Smart Licensing Utility (CSLU)—a tool many administrators assume is a benign, secondary component of their network architecture.
The following Python snippet (using paramiko modified with custom MSG_KEXINIT) demonstrates the memory leak. Do not use against unauthorized systems.
import socket
import struct
target = "192.168.1.1"
s = socket.socket()
s.connect((target, 22))