Spring4shell with Hmac Signatures

Spring4shell exploits a deserialization vulnerability in Spring Framework versions prior to 5.3.18 and 5.2.20. When combined with Hmac Signatures, the attack surface expands significantly because Hmac implementations often rely on Java object serialization for signature verification and payload integrity checks.

The core vulnerability exists in the way Spring handles class parameter parsing in certain request contexts. An attacker can craft a malicious payload that, when processed through Hmac verification logic, triggers remote code execution. This becomes particularly dangerous when Hmac signatures are used to verify webhook payloads or API requests containing serialized objects.

Consider this vulnerable Hmac implementation pattern:

public class VulnerableHmacVerifier {
    public boolean verifySignature(String payload, String signature, String secret) {
        // Spring deserialization vulnerability can be triggered here
        byte[] decodedPayload = Base64.getDecoder().decode(payload);
        
        // If payload contains malicious serialized object, Spring4shell executes
        Object deserialized = deserialize(decodedPayload);
        
        // Hmac calculation on potentially malicious object
        String calculated = calculateHmac(deserialized, secret);
        return calculated.equals(signature);
    }
}

The critical issue occurs when the deserialization process itself becomes the attack vector. Spring4shell can be triggered during the deserialize() call, allowing an attacker to execute arbitrary code before the Hmac verification even completes. This bypasses traditional security controls since the vulnerability manifests during the object reconstruction phase.

Another manifestation occurs in Hmac-based API authentication systems where the signature verification process accepts serialized Java objects as the message to be signed. An attacker can embed Spring4shell payloads within these objects, causing execution during the verification process:

// Vulnerable pattern in Hmac authentication
public class ApiAuthFilter {
    public boolean authenticate(HttpServletRequest request) {
        String payload = request.getParameter("payload");
        String signature = request.getParameter("signature");
        
        // Deserialization happens before Hmac verification
        Object message = deserializeFromBase64(payload);
        
        // Spring4shell executes during deserialization
        return verifyHmacSignature(message, signature, getSecret());
    }
}

The combination is particularly insidious because Hmac signatures are designed to ensure data integrity and authenticity. When the verification process itself is vulnerable to deserialization attacks, the entire security model collapses. The attacker doesn't need to break the Hmac algorithm—they exploit the implementation's handling of the signed data.

Detecting Spring4shell in Hmac implementations requires specialized scanning that understands both the Hmac verification workflow and Spring's deserialization behavior. Traditional vulnerability scanners often miss these combinations because they don't understand the specific interaction between Hmac processing and Spring's object handling.

middleBrick's scanning approach for this specific vulnerability includes several Hmac Signatures-specific checks:

Deserialization Point Analysis - The scanner identifies all points in your Hmac implementation where deserialization occurs before signature verification. This includes Base64 decoding of payloads, object reconstruction from byte arrays, and any custom deserialization logic used in your Hmac verification pipeline.

Spring Version Detection - The scanner checks for vulnerable Spring Framework versions by analyzing classpath information, configuration files, and runtime behavior. It specifically looks for versions between 5.0.0 and 5.3.17 (inclusive) and 5.2.0 to 5.2.19 (inclusive).

Payload Structure Analysis - The scanner tests for malicious payload structures that could trigger Spring4shell. This includes testing with crafted serialized objects that contain specific class references known to trigger the vulnerability when processed through Hmac verification.

Hmac Verification Flow Testing - The scanner simulates attack scenarios where malicious payloads are submitted with valid Hmac signatures. It verifies whether the implementation properly validates the signature before processing the payload content.

Here's how you can use middleBrick to scan your Hmac implementation:

# Install middleBrick CLI
npm install -g middlebrick

# Scan your Hmac endpoint
middlebrick scan https://yourapi.com/v1/auth/verify \
  --category=hmac \
  --test=spring4shell \
  --output=json > scan-report.json

The scanner provides detailed findings including:

The scanner also provides specific remediation guidance for each finding, including code snippets and configuration changes needed to secure your Hmac implementation against Spring4shell attacks.

Remediating Spring4shell in Hmac implementations requires a multi-layered approach that addresses both the immediate vulnerability and the underlying design issues. The following Hmac Signatures-specific fixes focus on secure implementation patterns.

Upgrade Spring Framework - The first and most critical step is upgrading to Spring Framework 5.3.18+ or 5.2.20+. These versions include the patch for the deserialization vulnerability. For Hmac implementations, this means:

// Maven dependency upgrade
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-core</artifactId>
    <version>5.3.18</version>
</dependency>

// Gradle dependency upgrade
implementation 'org.springframework:spring-core:5.3.18'

Implement Secure Deserialization - Never deserialize objects before verifying Hmac signatures. Use a two-step verification process:

public class SecureHmacVerifier {
    public boolean verifyAndProcess(String base64Payload, String signature, String secret) {
        // Step 1: Verify signature on raw data
        byte[] payloadBytes = Base64.getDecoder().decode(base64Payload);
        String calculated = calculateHmac(payloadBytes, secret);
        
        if (!constantTimeCompare(calculated, signature)) {
            return false; // Invalid signature
        }
        
        // Step 2: Only deserialize if signature is valid
        try {
            Object deserialized = deserializeSafely(payloadBytes);
            processDeserializedObject(deserialized);
            return true;
        } catch (InvalidObjectException e) {
            return false; // Deserialization failed
        }
    }
}

Use Safe Serialization Libraries - Replace Java serialization with safer alternatives like JSON or Protocol Buffers:

// Using JSON instead of Java serialization
public class JsonHmacVerifier {
    public boolean verifyJsonPayload(String jsonPayload, String signature, String secret) {
        // Calculate Hmac on JSON string directly
        String calculated = calculateHmac(jsonPayload, secret);
        
        if (!constantTimeCompare(calculated, signature)) {
            return false;
        }
        
        // Parse JSON safely
        MyObject obj = objectMapper.readValue(jsonPayload, MyObject.class);
        return validateObject(obj);
    }
}

Implement Input Validation - Validate all Hmac payloads before processing:

public class ValidatedHmacVerifier {
    public boolean verifyWithValidation(String payload, String signature, String secret) {
        // Validate payload structure
        if (!isValidPayloadFormat(payload)) {
            return false;
        }
        
        // Verify signature
        String calculated = calculateHmac(payload, secret);
        if (!constantTimeCompare(calculated, signature)) {
            return false;
        }
        
        // Safe processing
        return processPayload(payload);
    }
    
    private boolean isValidPayloadFormat(String payload) {
        // Check for valid JSON, XML, or expected format
        // Reject suspicious patterns, excessive size, etc.
        return payload.length() < MAX_PAYLOAD_SIZE && 
               payload.matches(VALID_PAYLOAD_PATTERN);
    }
}

Enable Security Manager - Use Java Security Manager to restrict deserialization capabilities:

public class SecureHmacService {
    private static final SecurityManager securityManager = new SecurityManager();
    
    static {
        System.setSecurityManager(securityManager);
    }
    
    public boolean verifyWithSecurityManager(String payload, String signature, String secret) {
        try {
            // Security manager will restrict dangerous operations
            return verifyHmacSignature(payload, signature, secret);
        } catch (SecurityException e) {
            log.warn("Security violation detected: {}", e.getMessage());
            return false;
        }
    }
}

Continuous Monitoring with middleBrick - After remediation, use middleBrick's continuous monitoring to ensure your Hmac implementation remains secure:

# Set up continuous monitoring
middlebrick monitor https://yourapi.com/v1/auth/verify \
  --schedule=daily \
  --alert=slack \
  --threshold=medium

This monitoring will alert you to any new vulnerabilities, configuration changes, or suspicious activity patterns that could indicate Spring4shell or similar attacks targeting your Hmac implementation.