CVE-2023-6055

Executive Summary

CVE-2023-6055 identifies a critical certificate validation flaw in Bitdefender Total Security's HTTPS scanning functionality. The vulnerability arises when the software fails to properly validate website certificates that lack the "Server Authentication" Extended Key Usage (EKU) extension. This oversight allows an attacker to present a certificate that is technically valid but not authorized for server authentication, enabling a Man-in-the-Middle (MITM) attack. An attacker could intercept, decrypt, and potentially alter HTTPS traffic between the user and legitimate websites, compromising confidentiality and integrity. The vulnerability has a CVSS score of 0.0 (unassigned) but poses a significant risk to user privacy and data security. No public exploit code or patch details are currently available.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It affects a proprietary Windows security product (Bitdefender Total Security). The supply chain risk is limited to the distribution of the Bitdefender software itself, which could be tampered with if the update mechanism is compromised, but the primary attack vector is network-based MITM.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, indicating it has not been officially scored by NVD. However, based on the description, a reasonable assessment would be:

• Attack Vector (AV): Network (N) – The attacker must be on the network path between the user and the target website.
• Attack Complexity (AC): Low (L) – No special conditions required beyond network access.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – The user does not need to perform any action.
• Scope (S): Changed (C) – The attack impacts resources beyond the vulnerable component (the user's HTTPS traffic).
• Confidentiality (C): High (H) – Full decryption of traffic.
• Integrity (I): High (H) – Traffic can be modified.
• Availability (A): None (N) – No impact on availability.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N (Score: 9.1 Critical)

Threat Level: Critical. The vulnerability allows a complete bypass of HTTPS security, enabling credential theft, data exfiltration, and injection of malicious content.

Deep Dive & Mechanics

The vulnerability lies in Bitdefender's HTTPS scanning proxy, which intercepts and inspects encrypted traffic. When a user visits an HTTPS website, the proxy establishes a connection to the server and presents a certificate to the user's browser. The proxy should verify that the server's certificate is valid for the intended purpose (server authentication). The flaw is that the proxy does not check the Extended Key Usage (EKU) extension for the "Server Authentication" OID (1.3.6.1.5.5.7.3.1). This means a certificate that is valid for other purposes (e.g., code signing, email protection) but not for server authentication will be accepted as valid for the website.

Attack Scenario:

1. An attacker on the same network (e.g., public Wi-Fi) performs ARP spoofing or DNS poisoning to redirect the victim's traffic through their machine.
2. The attacker's machine presents a certificate to Bitdefender's proxy that is technically valid (signed by a trusted CA) but lacks the "Server Authentication" EKU.
3. Bitdefender's proxy accepts this certificate as valid and establishes an encrypted tunnel with the attacker.
4. The attacker can now decrypt, read, and modify all HTTPS traffic between the victim and the intended website.

Vulnerable vs. Patched Code

No specific code snippets are available. The vulnerability is in the certificate validation logic of Bitdefender's HTTPS scanning component. The vulnerable code likely resembles:

// Vulnerable pseudo-code
bool ValidateCertificate(X509* cert, string hostname) {
    if (!CheckSignature(cert)) return false;
    if (!CheckHostname(cert, hostname)) return false;
    // Missing: CheckExtendedKeyUsage(cert, "serverAuth");
    return true;
}

A patched version would include:

// Patched pseudo-code
bool ValidateCertificate(X509* cert, string hostname) {
    if (!CheckSignature(cert)) return false;
    if (!CheckHostname(cert, hostname)) return false;
    if (!CheckExtendedKeyUsage(cert, "serverAuth")) return false;
    return true;
}

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Attacker machine on the same network as the victim.
• Ability to perform ARP spoofing or DNS poisoning.
• A valid certificate signed by a trusted CA but without the "Server Authentication" EKU (e.g., a code signing certificate).

Steps:

1. Set up a MITM proxy (e.g., mitmproxy, BetterCAP).
2. Configure the proxy to use the attacker's certificate.
3. Perform ARP spoofing to redirect victim traffic.
4. When the victim accesses an HTTPS site, Bitdefender's proxy will accept the attacker's certificate, allowing decryption.

Example mitmproxy setup:

# Generate a certificate without serverAuth EKU (using openssl)
openssl req -x509 -newkey rsa:2048 -keyout key.pem -out cert.pem -days 365 -nodes -subj "/CN=attacker.com" -addext "extendedKeyUsage=codeSigning"

# Run mitmproxy with the custom certificate
mitmproxy --listen-port 8080 --certs *=cert.pem

Blue Team (Detection & Defense)

Detection:

• Monitor for unexpected certificate validation errors in Bitdefender logs.
• Use network monitoring to detect ARP spoofing or DNS poisoning.
• Check for certificates presented by Bitdefender's proxy that lack the "Server Authentication" EKU.

Defense:

• Apply patches from Bitdefender as soon as they are released.
• Use additional network security controls (e.g., VPN, EDR) to detect MITM attacks.
• Educate users about the risks of using untrusted networks.

Indicators of Compromise (IOCs)

• Network: Unusual ARP replies (ARP spoofing), DNS responses pointing to unexpected IPs.
• Certificate: A certificate presented by Bitdefender's proxy that is valid but lacks the "Server Authentication" EKU (OID 1.3.6.1.5.5.7.3.1).
• Logs: Bitdefender logs showing certificate validation warnings or errors for legitimate websites.
• Process: Unusual processes running on the victim machine that perform packet capture or proxy functions.

Detection Engineering

YARA Rule

rule CVE_2023_6055_Certificate_Validation {
    meta:
        description = "Detects certificates without Server Authentication EKU used in MITM attacks"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "CVE-2023-6055"
    strings:
        $eku_oid = { 06 08 2B 06 01 05 05 07 03 01 } // Server Authentication OID
        $eku_other = { 06 08 2B 06 01 05 05 07 03 03 } // Code Signing OID
    condition:
        // Look for certificates that have EKU but not serverAuth
        for any of ($eku_other*) : (
            not for any of ($eku_oid*) : (true)
        )
}

Sigma Rule

title: Bitdefender HTTPS Scanning Certificate Validation Bypass
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects potential exploitation of CVE-2023-6055 via certificate validation failures
author: Valters IT
logsource:
    product: windows
    service: bitdefender
detection:
    selection:
        EventID: 1001  # Example Bitdefender event ID for certificate errors
        Message|contains: 'Certificate validation failed'
    condition: selection
falsepositives:
    - Legitimate certificate errors (e.g., expired certificates)
level: high

Suricata Rule

alert tcp any any -> any 443 (msg:"CVE-2023-6055 Bitdefender MITM Certificate Bypass"; flow:to_server,established; content:"|16 03|"; depth:2; content:"|0b|"; within:1; content:"|55 1d 23|"; distance:1; within:3; content:"|30|"; distance:1; within:1; content:"|06 08 2b 06 01 05 05 07 03 03|"; distance:1; within:9; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

• Disable HTTPS scanning in Bitdefender Total Security until a patch is available.
• Use a VPN or other encrypted tunnel to protect traffic from MITM attacks.
• Monitor for any suspicious certificate warnings in the browser.

Long-term Fix:

• Apply the official patch from Bitdefender once released.
• Ensure all security software is kept up to date.
• Implement network segmentation and monitoring to detect MITM attempts.

Workaround:

• Temporarily switch to a different antivirus solution that properly validates certificates.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert