Race Condition within a Thread

Description

Race Condition within a Thread is a vulnerability that occurs when two or more threads of execution use a resource simultaneously without proper synchronization, leading to undefined behavior and inconsistent state. While typically race conditions involve multiple threads, this weakness focuses on situations where the resource access pattern within a single thread's logical flow creates conditions where another thread can interfere, or where asynchronous operations within the same thread conflict with each other. The fundamental issue is that data may be read or modified while it is in an invalid or inconsistent state due to concurrent access.

Risk

Thread race conditions can cause data corruption, security bypasses, and use-after-free vulnerabilities. When one thread destroys a resource while another is using it, crashes or exploitable memory corruption occurs. In web browsers, race conditions between threads handling the same resource have led to critical security vulnerabilities. Data integrity is compromised when multiple threads modify shared state without synchronization - financial calculations, authentication states, and security decisions can all be corrupted. The intermittent nature of race conditions makes them difficult to detect in testing but attackers can sometimes reliably trigger them.

Solution

Implement proper locking mechanisms around code sections that modify or read shared data. Use mutexes, read-write locks, or other synchronization primitives appropriate for the access pattern. Consider using lock-free data structures where appropriate. Follow consistent locking hierarchies to prevent deadlocks. Use thread-safe data structures from standard libraries. Implement resource ownership patterns that clearly define which thread owns a resource at any time. Consider using memory-safe languages with built-in race condition protection. Employ static analysis and dynamic tools like ThreadSanitizer to detect race conditions.

Common Consequences

Impact	Details
Integrity	Scope: Integrity Data can be altered while in an inconsistent state, compromising logical flow and causing unexpected behavior.
Availability	Scope: Availability Use-after-free conditions from race conditions cause crashes and system instability.

Example Code

Vulnerable Code

// Vulnerable: Shared resource without synchronization
typedef struct {
    int *data;
    int size;
    int ref_count;
} SharedBuffer;

SharedBuffer *global_buffer = NULL;

// Thread 1
void vulnerable_use_buffer() {
    if (global_buffer && global_buffer->data) {
        // Vulnerable: Another thread may free buffer here
        int value = global_buffer->data[0];
        process(value);
    }
}

// Thread 2
void vulnerable_free_buffer() {
    if (global_buffer) {
        free(global_buffer->data);
        global_buffer->data = NULL;  // Race: Thread 1 may crash
        free(global_buffer);
        global_buffer = NULL;
    }
}

// Vulnerable: Check-then-act without synchronization
public class VulnerableCache {
    private Map<String, Object> cache = new HashMap<>();

    public Object get(String key) {
        // Vulnerable: Another thread may remove key
        if (cache.containsKey(key)) {
            return cache.get(key);  // May return null!
        }
        return null;
    }

    public void remove(String key) {
        cache.remove(key);  // Races with get()
    }
}

# Vulnerable: Shared counter without lock
import threading

counter = 0

def vulnerable_increment():
    global counter
    # Vulnerable: Read-modify-write is not atomic
    temp = counter
    temp += 1
    counter = temp  # Another thread may have changed counter

Fixed Code

// Fixed: Proper synchronization with mutex
#include <pthread.h>

typedef struct {
    int *data;
    int size;
    int ref_count;
    pthread_mutex_t lock;
} SharedBuffer;

SharedBuffer *global_buffer = NULL;
pthread_mutex_t buffer_lock = PTHREAD_MUTEX_INITIALIZER;

void secure_use_buffer() {
    pthread_mutex_lock(&buffer_lock);

    if (global_buffer && global_buffer->data) {
        int value = global_buffer->data[0];
        pthread_mutex_unlock(&buffer_lock);
        process(value);
    } else {
        pthread_mutex_unlock(&buffer_lock);
    }
}

void secure_free_buffer() {
    pthread_mutex_lock(&buffer_lock);

    if (global_buffer) {
        free(global_buffer->data);
        free(global_buffer);
        global_buffer = NULL;
    }

    pthread_mutex_unlock(&buffer_lock);
}

// Fixed: Thread-safe operations
import java.util.concurrent.ConcurrentHashMap;

public class SecureCache {
    private ConcurrentHashMap<String, Object> cache = new ConcurrentHashMap<>();

    public Object get(String key) {
        // Fixed: Atomic operation
        return cache.get(key);
    }

    public Object getOrCompute(String key, Supplier<Object> supplier) {
        // Fixed: Atomic compute-if-absent
        return cache.computeIfAbsent(key, k -> supplier.get());
    }

    public void remove(String key) {
        cache.remove(key);
    }
}

# Fixed: Thread-safe counter with lock
import threading

counter = 0
counter_lock = threading.Lock()

def secure_increment():
    global counter
    with counter_lock:  # Fixed: Atomic operation
        counter += 1

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CVE Examples

• CVE-2022-2621 — Browser race condition led to use-after-free.

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References

1. MITRE Corporation. "CWE-366: Race Condition within a Thread." https://cwe.mitre.org/data/definitions/366.html
2. Google. "ThreadSanitizer." https://github.com/google/sanitizers