CVE-2025-35021

Executive Summary

CVE-2025-35021 is an authentication bypass vulnerability in Abilis CPX devices. When the device is in an unconfigured state, an attacker can exploit a flaw in the SSH authentication mechanism. After three failed login attempts, the fourth attempt grants access to a restricted shell, from which the attacker can relay network connections. This allows an unauthenticated, remote attacker to gain a foothold on the device and potentially pivot to internal networks. The vulnerability has a CVSS score of 6.5 (Medium) and currently has no known patch. The EPSS score is 0.0%, indicating low exploitation likelihood in the wild, but the attack vector is trivial.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The vulnerability is exploitable remotely over the network.
• Attack Complexity (AC): Low (L) – No special conditions are required; the attacker simply needs to attempt SSH login.
• Privileges Required (PR): None (N) – No authentication is needed to start the attack.
• User Interaction (UI): None (N) – No user action is required.
• Scope (S): Unchanged (U) – The exploit does not cross a trust boundary.
• Confidentiality (C): Low (L) – Limited information disclosure (restricted shell access).
• Integrity (I): Low (L) – The attacker can relay connections, potentially modifying traffic.
• Availability (A): None (N) – No impact on availability.

Threat Level: Medium. The attack is simple and requires no authentication, but the impact is limited to a restricted shell and connection relay. However, this can be a stepping stone for deeper network compromise.

Deep Dive & Mechanics

The vulnerability resides in the SSH daemon of unconfigured Abilis CPX devices. The device's authentication logic incorrectly handles the "unconfigured" state. When the device has no configuration (e.g., factory default), the SSH service enters a special mode. In this mode, the first three authentication attempts are rejected, but the fourth attempt is automatically accepted, granting access to a restricted shell. This shell is not a full system shell; it is a limited environment that allows the user to execute a predefined set of commands, including the ability to relay TCP connections (e.g., using a built-in relay command). This allows an attacker to use the device as a proxy to reach other hosts on the internal network.

The root cause is a logic error in the authentication state machine. The counter for failed attempts is not reset after a successful login, and the "unconfigured" flag bypasses normal credential validation after three failures.

Vulnerable vs. Patched Code

No patch is currently available. The vulnerable code is in the SSH daemon's authentication handler. A conceptual fix would involve:

• Resetting the failed attempt counter after any successful login.
• Removing the automatic bypass after three failures in unconfigured mode.
• Requiring explicit configuration of a default password before SSH access is granted.

Vulnerable logic (pseudocode):

if (device_unconfigured) {
    if (failed_attempts < 3) {
        reject();
        failed_attempts++;
    } else {
        grant_restricted_shell();
    }
}

Patched logic (pseudocode):

if (device_unconfigured) {
    // Do not allow any SSH access until configured
    reject();
}

Exploitation & Lab Setup

Red Team

Lab Setup:

1. Obtain an Abilis CPX device (or use a virtualized instance if available).
2. Ensure the device is in factory default state (no configuration applied).
3. The device should be reachable on the network (e.g., IP 192.168.1.100).

Exploitation Steps:

1. From an attacker machine, initiate SSH connections to the device.
2. For the first three attempts, use any credentials (e.g., root:root). Each attempt will fail.
3. On the fourth attempt, the SSH connection will succeed, granting a restricted shell.
4. Once inside the restricted shell, use the relay command to forward connections to internal hosts.

Example SSH session:

# Attempt 1
ssh root@192.168.1.100
# (password prompt, enter wrong password)
# Attempt 2
ssh root@192.168.1.100
# (password prompt, enter wrong password)
# Attempt 3
ssh root@192.168.1.100
# (password prompt, enter wrong password)
# Attempt 4 - success
ssh root@192.168.1.100
# (password prompt, enter any password - it will be accepted)
# You are now in a restricted shell
restricted> relay 10.0.0.5 3389
# This relays connections to 10.0.0.5:3389 (RDP)

Blue Team

Detection:

• Monitor for multiple failed SSH authentication attempts from the same source IP to an Abilis CPX device, followed by a successful login.
• Look for unusual SSH sessions where the user is in a restricted shell (e.g., commands like relay).
• Network traffic analysis: Detect unexpected TCP connections originating from the Abilis device to internal hosts.

Hunting:

• Search logs for patterns: 3 failed SSH logins followed by a successful login within a short time window.
• Check for the presence of the relay command in SSH session logs.

Prevention:

• Immediately configure the Abilis CPX device with a strong password and disable default settings.
• If possible, restrict SSH access to trusted IPs only.
• Monitor for unconfigured devices on the network.

Indicators of Compromise (IOCs)

• Network:
  • Multiple failed SSH authentication attempts (3) from the same source IP to an Abilis CPX device.
  • Successful SSH login from the same source IP immediately after the failures.
  • TCP connections from the Abilis device to internal hosts on non-standard ports (e.g., 3389, 22, 445).
• Host (on Abilis device):
  • SSH logs showing Failed password entries followed by Accepted password for the same user.
  • Presence of relay command in shell history.
• Logs:
  • /var/log/auth.log or equivalent: pattern of 3 failures then 1 success.

Detection Engineering

YARA Rule

rule CVE_2025_35021_Abilis_SSH_Bypass {
    meta:
        description = "Detects SSH authentication bypass pattern for CVE-2025-35021"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-35021"
    strings:
        $fail = "Failed password"
        $success = "Accepted password"
        $relay = "relay"
    condition:
        #fail == 3 and #success == 1 and $relay
}

Sigma Rule

title: Abilis CPX SSH Authentication Bypass
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects 3 failed SSH logins followed by a successful login to an Abilis CPX device
author: Valters IT
logsource:
    product: linux
    service: sshd
detection:
    selection:
        EventID: 22  # Failed login
        count: 3
        timeframe: 5m
    selection2:
        EventID: 23  # Successful login
        count: 1
        timeframe: 5m
    condition: selection and selection2
falsepositives:
    - Legitimate user forgetting password
level: high

Suricata Rule

alert tcp $EXTERNAL_NET any -> $HOME_NET 22 (msg:"CVE-2025-35021 Abilis CPX SSH Bypass Attempt"; flow:to_server; content:"SSH"; nocase; threshold: type both, track by_src, count 3, seconds 300; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Configure the device: Apply a basic configuration (e.g., set an admin password) to take the device out of the unconfigured state. This immediately closes the vulnerability.
2. Restrict SSH access: Use firewall rules to limit SSH access to trusted management IPs only.
3. Monitor logs: Enable detailed SSH logging and monitor for the pattern of 3 failures + 1 success.

Long-term:

• Contact Abilis for a firmware patch. Since no patch is currently available, consider replacing the device if it cannot be configured.
• Implement network segmentation to isolate Abilis CPX devices from critical internal networks.
• Use an intrusion detection system (IDS) to alert on the specific SSH pattern.

Workaround:

• Disable SSH entirely if not needed, and use a serial console for management.

Valters IT