CVE-2024-23680

Executive Summary

CVE-2024-23680 is a critical vulnerability in the AWS Encryption SDK for Java that affects versions 2.0.0 through 2.2.0 and all versions prior to 1.9.0. The flaw lies in the incorrect validation of certain invalid ECDSA signatures, which could allow an attacker to forge digital signatures and potentially decrypt or tamper with encrypted data. This vulnerability is particularly dangerous because it undermines the integrity guarantees of the encryption SDK, making it possible for an attacker to bypass authentication mechanisms.

Supply Chain & Package Security

This vulnerability affects the AWS Encryption SDK for Java, which is distributed as a Maven artifact (com.amazonaws:aws-encryption-sdk-java). Organizations using this SDK in their Java applications are at risk. The SDK is a critical component for securing data at rest and in transit, and a flaw in signature validation can lead to complete compromise of encrypted data. Users should immediately audit their dependencies and update to the patched versions.

CVSS Breakdown & Threat Level

• CVSS Score: 9.8 (Critical)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: High (C:H)
• Integrity Impact: High (I:H)
• Availability Impact: High (A:H)

The high CVSS score reflects the ease of exploitation and the severe impact on confidentiality, integrity, and availability. An unauthenticated attacker can exploit this vulnerability remotely without any user interaction.

Deep Dive & Mechanics

The AWS Encryption SDK for Java uses ECDSA (Elliptic Curve Digital Signature Algorithm) to verify the authenticity of encrypted messages. The vulnerability exists in the signature validation logic, specifically in how the SDK handles malformed or invalid ECDSA signatures.

In a correct implementation, the SDK should reject any signature that does not conform to the expected DER-encoded format or that contains invalid curve points. However, due to a flaw in the validation routine, certain invalid signatures are incorrectly accepted as valid. This allows an attacker to craft a specially malformed signature that passes validation, enabling them to:

1. Forge encrypted messages: An attacker can create a message that appears to be signed by a trusted key.
2. Bypass integrity checks: The SDK will accept tampered ciphertext as authentic.
3. Potentially decrypt data: In some scenarios, the flawed validation could be leveraged to recover plaintext.

The root cause is likely an improper check of the ECDSA signature's r and s values, or a failure to verify that the signature is within the valid range of the elliptic curve group.

Vulnerable vs. Patched Code

The following code snippets illustrate the difference between the vulnerable and patched validation logic.

Vulnerable Code (versions < 1.9.0 and 2.0.0-2.2.0):

public boolean verify(byte[] signature, byte[] message) {
    // Vulnerable: Does not properly validate signature format
    ECDSASignature sig = ECDSASignature.decode(signature);
    // Missing check for valid r and s values
    return verifySignature(sig, message);
}

Patched Code (versions >= 1.9.0 and >= 2.3.0):

public boolean verify(byte[] signature, byte[] message) {
    ECDSASignature sig = ECDSASignature.decode(signature);
    // Added validation: ensure r and s are positive and within curve order
    if (sig.getR().signum() <= 0 || sig.getS().signum() <= 0) {
        return false;
    }
    if (sig.getR().compareTo(curve.getOrder()) >= 0 || sig.getS().compareTo(curve.getOrder()) >= 0) {
        return false;
    }
    return verifySignature(sig, message);
}

The patched code adds explicit checks to ensure that the r and s components of the ECDSA signature are positive integers and less than the curve's order n. This prevents the acceptance of invalid signatures that could be used in an attack.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• A Java application using the vulnerable AWS Encryption SDK for Java (version 2.0.0 to 2.2.0 or < 1.9.0).
• Network access to the target application.
• Knowledge of the public key used for signature verification (this is typically shared).

Exploitation Steps:

1. Identify the target: Find an application that uses the vulnerable SDK and accepts encrypted messages.
2. Craft a malicious signature: Create an ECDSA signature with invalid r and s values that will pass the flawed validation. For example, set r = 0 and s = 0.
3. Encrypt a payload: Use the attacker's own key to encrypt a malicious payload.
4. Send the crafted message: Transmit the encrypted payload along with the forged signature to the target.
5. Trigger verification: The target application will verify the signature using the vulnerable SDK and accept it as valid.
6. Achieve impact: The attacker can now inject arbitrary encrypted data that will be decrypted and processed by the target.

Example Exploit Code (Python):

import struct
from cryptography.hazmat.primitives.asymmetric import ec
from cryptography.hazmat.primitives import serialization

# Generate a malicious signature with r=0, s=0
def forge_signature():
    # This is a simplified example; actual exploitation may require more sophisticated crafting
    r = 0
    s = 0
    # Encode as DER
    # ... (DER encoding logic)
    return der_encoded_signature

# Send the forged signature with an encrypted message
def exploit(target_url, encrypted_message, forged_sig):
    # ... (HTTP request logic)
    pass

Blue Team (Defense & Detection)

Detection Steps:

1. Monitor for unusual signature patterns: Look for ECDSA signatures where r or s values are zero or equal to the curve order.
2. Audit application logs: Check for failed signature verification attempts that suddenly succeed.
3. Deploy network monitoring: Use the Suricata rule provided below to detect exploitation attempts.
4. Update the SDK: Immediately upgrade to version 2.3.0 or 1.9.0.

Honeypot Setup:

• Deploy a decoy application using the vulnerable SDK to attract attackers.
• Monitor for any attempts to send malformed signatures.

Indicators of Compromise (IOCs)

• Network IOCs:
  • Unusual ECDSA signatures with r=0 or s=0 in network traffic.
  • Repeated failed signature verification attempts followed by a successful one.
• File IOCs:
  • Log entries containing "Signature verification succeeded" for messages with suspicious signatures.
• System IOCs:
  • Unexpected decryption of data that should have been protected.

Detection Engineering

YARA Rule

rule CVE_2024_23680_ECDSA_Signature_Validation {
    meta:
        description = "Detects exploitation attempts of CVE-2024-23680 by identifying invalid ECDSA signatures"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2024-23680"
    strings:
        $sig_r_zero = { 02 01 00 }  // DER encoding for integer 0
        $sig_s_zero = { 02 01 00 }  // Same pattern for s
    condition:
        any of them
}

Sigma Rule

title: CVE-2024-23680 Exploitation Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects network traffic containing invalid ECDSA signatures targeting AWS Encryption SDK for Java
author: Valters IT
date: 2024/01/01
logsource:
    category: network
    product: suricata
detection:
    selection:
        - payload: "02 01 00"  # DER encoded integer 0
    condition: selection
falsepositives:
    - Legitimate use of zero values in other contexts (rare)
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2024-23680 - Invalid ECDSA Signature Detected"; content:"|02 01 00|"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade the SDK: Update to AWS Encryption SDK for Java version 2.3.0 or later (for 2.x) or version 1.9.0 or later (for 1.x).
2. Rotate keys: After upgrading, rotate all encryption keys to ensure no compromised keys are in use.
3. Audit logs: Review application logs for any signs of exploitation.

Long-term Mitigations:

• Implement additional signature validation at the application layer.
• Use a Web Application Firewall (WAF) to filter out malformed signatures.
• Regularly update dependencies and monitor for CVEs.

Workarounds:

• If immediate upgrade is not possible, consider disabling ECDSA signature verification and using a different authentication mechanism (not recommended).

Valters IT
Securing Digital Frontiers