Example - 1

The following code shows a simple example of a double free vulnerability.

free(ptr);
char* ptr = (char*)malloc (SIZE);...if (abrt) {}...free(ptr);

Double free vulnerabilities have two common (and sometimes overlapping) causes:

Error conditions and other exceptional circumstancesConfusion over which part of the program is responsible for freeing the memory

Although some double free vulnerabilities are not much more complicated than this example, most are spread out across hundreds of lines of code or even different files. Programmers seem particularly susceptible to freeing global variables more than once.

Example - 2

This code binds a server socket to port 21, allowing the server to listen for traffic on that port.

// /*unlink the socket if already bound to avoid an error when bind() is called*/// 
int server_sockfd;int server_len;struct sockaddr_in server_address;unlink("server_socket");server_sockfd = socket(AF_INET, SOCK_STREAM, 0);server_address.sin_family = AF_INET;server_address.sin_port = 21;server_address.sin_addr.s_addr = htonl(INADDR_ANY);server_len = sizeof(struct sockaddr_in);bind(server_sockfd, (struct sockaddr *) &s1, server_len);void bind_socket(void) {}

This code may result in two servers binding a socket to same port, thus receiving each other's traffic. This could be used by an attacker to steal packets meant for another process, such as a secure FTP server.