CVE-2025-62003

Executive Summary

CVE-2025-62003 is a medium-severity vulnerability in BullWall Server Intrusion Protection that allows a remote, authenticated attacker to bypass Multi-Factor Authentication (MFA) for RDP connections during a configuration-dependent delay window. The vulnerability arises because the system introduces a noticeable delay before enforcing MFA checks, creating a temporal window where an attacker with valid credentials can establish an RDP session without completing the second factor. This bypass is particularly dangerous in environments where MFA is the primary defense against credential theft or lateral movement. No public exploit code or proof-of-concept is available, and the vendor has not released a patch. The EPSS score is 0.0%, indicating low current exploitation activity, but the risk increases if credentials are compromised.

CVSS Breakdown & Threat Level

• CVSS Score: 7.5 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
• Attack Vector: Network (AV:N) – The vulnerability is exploitable remotely over the network.
• Attack Complexity: Low (AC:L) – No special conditions or race conditions beyond the inherent delay.
• Privileges Required: Low (PR:L) – The attacker must have valid RDP credentials (e.g., stolen or guessed).
• User Interaction: None (UI:N) – No user action required.
• Scope: Unchanged (S:U) – The vulnerable component and the impacted component are the same.
• Confidentiality Impact: High (C:H) – Successful exploitation allows full access to the RDP session and its data.
• Integrity Impact: None (I:N) – The attacker cannot modify system data.
• Availability Impact: None (A:N) – The attack does not affect system availability.
• Threat Level: High – The vulnerability undermines MFA, a critical security control, and is easy to exploit once credentials are obtained.

Deep Dive & Mechanics

The BullWall Server Intrusion Protection system is designed to enforce MFA for RDP connections, typically requiring a one-time password (OTP) or push notification after initial credential validation. The vulnerability stems from a configuration-dependent delay introduced before the MFA challenge is presented. This delay can be caused by factors such as:

• Network latency in contacting the MFA provider.
• Internal processing delays (e.g., logging, session initialization).
• Misconfiguration of timeout or retry settings.

During this delay, the RDP session is in a partially authenticated state. If the attacker sends the RDP connection request and immediately begins session activity (e.g., executing commands or transferring files), the system may not enforce the MFA check before allowing the session to proceed. The attacker must have valid credentials (username/password) to initiate the RDP connection, but the MFA bypass occurs because the check is deferred or not completed before the session is established.

The vulnerability is configuration-dependent, meaning not all installations are equally affected. Systems with longer delays (e.g., due to high latency to the MFA server or custom timeout settings) are more vulnerable. The affected versions (4.6.0.0, 4.6.0.6, 4.6.0.7, 4.6.1.4) likely share a common code path where the MFA enforcement is not atomic with the RDP session creation.

Vulnerable vs. Patched Code

No patch is available, and the vendor has not released a fix. The vulnerable code likely resides in the RDP authentication handler, where the MFA challenge is initiated asynchronously or with a delay. A patched version would need to:

• Ensure MFA is enforced synchronously before the RDP session is fully established.
• Remove or minimize any configurable delay that could be exploited.
• Implement a timeout that denies the session if MFA is not completed within a short, fixed window.

Without access to the source code, the exact vulnerable code path cannot be identified. The following pseudocode illustrates the likely vulnerable pattern:

# Vulnerable pseudocode
def handle_rdp_connection(credentials):
    if validate_credentials(credentials):
        # Delay before MFA check
        time.sleep(config.get('mfa_delay', 5))  # Configurable delay
        session = create_rdp_session()
        # MFA check happens after session creation
        if not perform_mfa_check(session):
            # Session already active, MFA failure may not terminate it
            log_warning("MFA failed but session active")
        return session

A patched version would look like:

# Patched pseudocode
def handle_rdp_connection(credentials):
    if validate_credentials(credentials):
        # MFA check before session creation
        if not perform_mfa_check():
            return None
        session = create_rdp_session()
        return session

Exploitation & Lab Setup

Red Team

Prerequisites:

• Valid RDP credentials for a target BullWall-protected system.
• Network access to the RDP port (typically 3389).
• Knowledge of the MFA delay (can be determined by probing or reconnaissance).

Exploitation Steps:

1. Reconnaissance: Use Nmap or a custom script to measure the delay between RDP connection initiation and MFA prompt. Send an RDP connection request and time the response.
2. Credential Acquisition: Obtain valid credentials via phishing, password spraying, or credential dumping.
3. Exploit: Use a modified RDP client (e.g., FreeRDP with custom timing) to send the connection request and immediately begin session activity (e.g., execute a command via xfreerdp with a script). The goal is to complete the session setup before the MFA check is enforced.

Example using FreeRDP:

# Measure delay (adjust as needed)
time xfreerdp /v:target_ip /u:user /p:password /cert-ignore

# Exploit: Send connection and immediately run a command
xfreerdp /v:target_ip /u:user /p:password /cert-ignore /app:"cmd.exe /c whoami > C:\temp\pwned.txt" &
sleep 1  # Adjust based on delay
# The command may execute before MFA is enforced

Note: The exact timing depends on the configuration. The attacker may need to script multiple attempts with varying delays.

Blue Team

Detection:

• Monitor for RDP connections that are established but do not have a corresponding MFA success event.
• Look for multiple failed MFA attempts followed by a successful RDP session (indicating bypass).
• Alert on RDP sessions that start immediately after credential validation without an MFA challenge.

Hunting:

• Search for RDP event logs (Event ID 4624 for successful logon) without a corresponding MFA event (e.g., from Duo or RSA logs).
• Analyze network flows for RDP traffic that shows session establishment before MFA server communication.

Mitigation:

• Reduce or eliminate the MFA delay by tuning BullWall configuration (e.g., set mfa_delay to 0 if possible).
• Implement network segmentation to limit RDP access to trusted hosts only.
• Use conditional access policies to require MFA for all RDP connections, enforced at the network level (e.g., VPN with MFA).

Indicators of Compromise (IOCs)

• Network:
  • RDP connections (TCP/3389) from unusual IP addresses or geographic locations.
  • RDP traffic patterns showing session establishment before MFA server communication (e.g., no traffic to MFA provider IPs before RDP session start).
• Host:
  • Event ID 4624 (successful logon) for RDP sessions without a corresponding MFA success event.
  • Event ID 4648 (logon using explicit credentials) for RDP sessions.
  • Presence of tools like xfreerdp or rdesktop on endpoints.
• Logs:
  • BullWall logs showing RDP connections with missing or delayed MFA checks.
  • MFA provider logs (e.g., Duo, RSA) showing no challenge for a successful RDP session.

Detection Engineering

YARA Rule

rule CVE_2025_62003_BullWall_MFA_Bypass {
    meta:
        description = "Detects BullWall RDP MFA bypass indicators"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-62003"
    strings:
        $s1 = "BullWall" nocase
        $s2 = "MFA" nocase
        $s3 = "RDP" nocase
        $s4 = "delay" nocase
    condition:
        all of ($s*) and filesize < 1MB
}

Sigma Rule

title: BullWall RDP MFA Bypass Detection
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects RDP sessions without MFA challenge in BullWall environment
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-62003
logsource:
    product: windows
    service: security
detection:
    selection:
        EventID: 4624
        LogonType: 10  # RemoteInteractive
    filter:
        - EventID: 4624
          LogonType: 10
          # Add MFA success event ID (e.g., from Duo: 4105)
    condition: selection and not filter
falsepositives:
    - Legitimate MFA failures not logged
level: high

Suricata Rule

alert tcp $EXTERNAL_NET any -> $HOME_NET 3389 (msg:"CVE-2025-62003 BullWall RDP MFA Bypass Attempt"; flow:to_server,established; content:"|03 00 00|"; depth:3; content:"|02 f0 80|"; within:5; reference:cve,2025-62003; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable MFA delay: If possible, configure BullWall to enforce MFA immediately (set mfa_delay to 0).
2. Network segmentation: Restrict RDP access to only authorized IPs via firewall rules.
3. Enable logging: Ensure detailed logging of RDP and MFA events for forensic analysis.

Long-term Fix:

• Monitor the vendor for a patch. If none is released, consider replacing BullWall with an alternative MFA solution that enforces MFA synchronously.
• Implement additional controls such as:
  • VPN with MFA for all remote RDP access.
  • Just-in-time (JIT) access for RDP sessions.
  • Endpoint detection and response (EDR) to monitor for anomalous RDP behavior.

Workaround:

• Use a network-level MFA proxy (e.g., Duo Network Gateway) that enforces MFA before allowing RDP traffic to reach the BullWall server.

Valters IT
DevSecOps & Cyber Threat Intelligence