Double Free with Hmac Signatures

Double free vulnerabilities in HMAC signature implementations occur when memory allocated for cryptographic operations is freed more than once, leading to heap corruption, memory leaks, or potential code execution. In HMAC signature contexts, this typically manifests through specific code patterns and library behaviors.

The most common scenario involves HMAC context objects or buffers being freed in multiple code paths. Consider a typical HMAC verification function where the HMAC context might be freed in both success and failure cases:

HMAC_CTX *hmac = HMAC_CTX_new();
if (!hmac_verify(hmac, message, key)) {
    HMAC_CTX_free(hmac); // First free
    return -1;
}
HMAC_CTX_free(hmac); // Second free - potential double free
return 0;

Another manifestation occurs when HMAC operations are wrapped in exception handling or error recovery code. If an error occurs after the first free but before cleanup completes, a subsequent free call can trigger the vulnerability:

HMAC_CTX *hmac = NULL;
try {
    hmac = HMAC_CTX_new();
    if (!HMAC_Init_ex(hmac, key, key_len, EVP_sha256(), NULL)) {
        throw std::runtime_error("HMAC init failed");
    }
    // Error occurs here, control jumps to catch block
    if (!HMAC_Update(hmac, data, data_len)) {
        throw std::runtime_error("HMAC update failed");
    }
    HMAC_CTX_free(hmac); // Normal cleanup
} catch (...) {
    HMAC_CTX_free(hmac); // Double free if hmac was already freed
}

Memory pool management in cryptographic libraries can also introduce double free risks. Some HMAC implementations use custom allocators or memory pools. If the pool management is flawed, the same memory block might be returned to the pool multiple times:

// Pool allocator bug example
void* pool_malloc(size_t size) {
    void *ptr = malloc(size);
    if (ptr) {
        // Mark as allocated
        allocated_blocks[ptr] = size;
    }
    return ptr;
}

void pool_free(void *ptr) {
    if (allocated_blocks.find(ptr) != allocated_blocks.end()) {
        // Free memory
        free(ptr);
        // Remove from tracking
        allocated_blocks.erase(ptr);
        // BUG: Control flow issue could cause this to execute twice
        free(ptr); // Second free - double free vulnerability
    }
}

Function pointers and callbacks in HMAC implementations can also create double free scenarios. If a callback function frees the HMAC context and the main function also attempts to free it, the vulnerability occurs:

typedef void (*hmac_cleanup_callback)(HMAC_CTX *);

void hmac_verify_with_callback(const char *message, const char *key, hmac_cleanup_callback cb) {
    HMAC_CTX *hmac = HMAC_CTX_new();
    if (!hmac_verify(hmac, message, key)) {
        cb(hmac); // Callback frees the context
        return -1;
    }
    cb(hmac); // Callback frees again - double free
    return 0;
}

void cleanup_callback(HMAC_CTX *hmac) {
    HMAC_CTX_free(hmac); // Single responsibility for freeing
}

Detecting double free vulnerabilities in HMAC signature implementations requires a combination of static analysis, dynamic testing, and runtime monitoring. The detection approach must account for the specific memory management patterns and error handling in cryptographic code.

Static analysis tools can identify suspicious patterns in HMAC-related code. Look for these specific indicators:

# Using clang static analyzer with HMAC-specific checks
clang --analyze -Xanalyzer -analyzer-checker=core,unix.Malloc \n    -Xanalyzer -analyzer-checker=crypto.HMAC.DoubleFree \n    hmac_sign.c

# Custom grep patterns for double free in HMAC code
# Pattern 1: Multiple frees on same variable
grep -r "HMAC_CTX_free.*HMAC_CTX_free" . --include="*.c"

# Pattern 2: Free in both success and failure paths
grep -r "if.*HMAC_CTX_free" . --include="*.c" | grep -A5 -B5 "else"

# Pattern 3: Free in error handling
grep -r "catch.*HMAC_CTX_free" . --include="*.c"

Dynamic analysis using fuzzing tools can trigger double free conditions through unexpected input sequences. The AFL++ fuzzer with custom HMAC mutators can effectively find these vulnerabilities:

# AFL++ configuration for HMAC fuzzing
export AFL_HMAC_MUTATORS=1
export AFL_INST_LIBS=1

# Run fuzzer with HMAC-specific instrumentation
afl-fuzz -m none -o findings_dir \n    -M fuzzer01 -t 1000+ \n    -x hmac_dict.txt \n    -- ./hmac_target @@

# Custom mutator for HMAC edge cases
# Generates inputs that trigger early exits and multiple cleanup paths

Runtime memory monitoring tools like AddressSanitizer (ASan) can detect double free conditions during normal operation:

# Compile with AddressSanitizer
clang -fsanitize=address -g hmac_sign.c -o hmac_sign

# Run with ASan
ASAN_OPTIONS=detect_leaks=1 ./hmac_sign

# ASan will report:
# ERROR: AddressSanitizer: attempting double-free
# Double-free can be dangerous!

middleBrick's black-box scanning approach can identify HMAC signature vulnerabilities through runtime API testing. The scanner examines HTTP headers and request/response patterns for HMAC implementations:

{
  "scan_results": {
    "hmac_security": {
      "double_free_risk": "medium",
      "issues_found": [
        {
          "severity": "high",
          "type": "potential_double_free",
          "description": "HMAC context freed in multiple error paths",
          "remediation": "Consolidate cleanup logic in single exit point"
        }
      ],
      "recommendations": [
        "Use RAII pattern for HMAC context management",
        "Implement centralized error handling",
        "Add memory pool integrity checks"
      ]
    }
  }
}

Coverage-guided fuzzing with sanitizers provides the most comprehensive detection. Combine AFL++ with ASan and custom HMAC mutators:

# Comprehensive HMAC double free detection
export AFL_USE_ASAN=1
afl-fuzz -m none -o findings_dir \n    -M fuzzer01 -t 1000+ \n    -x hmac_dict.txt \n    -- ./hmac_target @@

# Monitor ASan output for double free reports
# Look for "ERROR: AddressSanitizer: attempting double-free"

Remediating double free vulnerabilities in HMAC signature implementations requires systematic changes to memory management patterns. The most effective approach uses RAII (Resource Acquisition Is Initialization) principles and centralized cleanup logic.

The RAII pattern ensures resources are automatically cleaned up when objects go out of scope. In C++, this translates to using smart pointers or custom RAII wrappers:

class HMACContext {
private:
    HMAC_CTX *ctx;
public:
    HMACContext() {
        ctx = HMAC_CTX_new();
        if (!ctx) {
            throw std::bad_alloc();
        }
    }
    
    ~HMACContext() {
        if (ctx) {
            HMAC_CTX_free(ctx);
        }
    }
    
    // Delete copy operations to prevent double free
    HMACContext(const HMACContext&) = delete;
    HMACContext& operator=(const HMACContext&) = delete;
    
    // Allow move operations
    HMACContext(HMACContext&& other) noexcept {
        ctx = other.ctx;
        other.ctx = nullptr;
    }
    
    HMACContext& operator=(HMACContext&& other) noexcept {
        if (this != &other) {
            if (ctx) HMAC_CTX_free(ctx);
            ctx = other.ctx;
            other.ctx = nullptr;
        }
        return *this;
    }
    
    HMAC_CTX* get() { return ctx; }
    const HMAC_CTX* get() const { return ctx; }
};

// Usage - automatic cleanup
int hmac_verify_safe(const char *message, const char *key) {
    try {
        HMACContext hmac;
        if (!HMAC_Init_ex(hmac.get(), key, strlen(key), EVP_sha256(), NULL)) {
            return -1; // RAII will free on scope exit
        }
        if (!HMAC_Update(hmac.get(), message, strlen(message))) {
            return -1; // RAII will free on scope exit
        }
        return 0;
    } catch (...) {
        return -1; // RAII handles cleanup
    }
}

For C codebases, implement centralized cleanup functions with goto-based error handling. This pattern ensures single cleanup points:

int hmac_verify_centralized(const char *message, const char *key) {
    int result = -1;
    HMAC_CTX *hmac = NULL;
    unsigned char *digest = NULL;
    unsigned int digest_len = 0;
    
    hmac = HMAC_CTX_new();
    if (!hmac) goto cleanup;
    
    if (!HMAC_Init_ex(hmac, key, strlen(key), EVP_sha256(), NULL)) {
        goto cleanup;
    }
    
    if (!HMAC_Update(hmac, message, strlen(message))) {
        goto cleanup;
    }
    
    digest_len = EVP_MAX_MD_SIZE;
    digest = malloc(digest_len);
    if (!digest) goto cleanup;
    
    if (!HMAC_Final(hmac, digest, &digest_len)) {
        goto cleanup;
    }
    
    result = 0; // Success
    
cleanup:
    if (digest) free(digest);
    if (hmac) HMAC_CTX_free(hmac); // Single cleanup point
    return result;
}

Implement defensive programming with null checks and state tracking to prevent multiple frees:

typedef struct {
    HMAC_CTX *ctx;
    int is_freed;
} SafeHMAC;

SafeHMAC* safe_hmac_new() {
    SafeHMAC *hmac = malloc(sizeof(SafeHMAC));
    if (!hmac) return NULL;
    hmac->ctx = HMAC_CTX_new();
    hmac->is_freed = 0;
    return hmac;
}

void safe_hmac_free(SafeHMAC *hmac) {
    if (hmac && !hmac->is_freed) {
        if (hmac->ctx) {
            HMAC_CTX_free(hmac->ctx);
        }
        hmac->is_freed = 1;
        free(hmac);
    }
}

// Usage with safety checks
SafeHMAC *hmac = safe_hmac_new();
if (!hmac) return -1;

// Multiple calls are safe
safe_hmac_free(hmac); // First call frees
safe_hmac_free(hmac); // Second call is no-op

Static analysis integration helps catch double free patterns during development:

// Static analysis annotations
#include <sal.h>

// Annotate that this function frees the HMAC context
__declspec(alloc_text(PAGE, hmac_verify_annotated))
int hmac_verify_annotated(
    _In_z_ const char *message,
    _In_z_ const char *key,
    _In_ _Post_ptr_invalid_ HMAC_CTX **hmac_out) {
    
    HMAC_CTX *hmac = HMAC_CTX_new();
    if (!hmac) return -1;
    
    // ... HMAC operations ...
    
    *hmac_out = hmac; // Transfer ownership
    return 0;
}

// Caller must free the returned context
void caller_function() {
    HMAC_CTX *hmac = NULL;
    if (hmac_verify_annotated("msg", "key", &hmac) == 0) {
        // hmac is now owned by caller
        HMAC_CTX_free(hmac); // Single responsibility
    }
}