CWE-415: Double Free
Learn about CWE-415 (Double Free), its security impact, exploitation methods, and prevention guidelines.
What is Double Free?
• Overview:
- A Double Free vulnerability occurs when a program attempts to free the same memory address more than once.
- This can lead to undefined behavior, program crashes, or even arbitrary code execution.
• Exploitation Methods:
- Attackers can exploit this vulnerability by manipulating memory management to execute malicious code.
- Common attack patterns include heap corruption, where attackers overwrite memory management data structures.
• Security Impact:
- Direct consequences of successful exploitation include application crashes and arbitrary code execution.
- Potential cascading effects involve compromising system integrity and security by allowing further exploitation of memory.
- Business impact could include data breaches, loss of customer trust, and financial loss due to system downtime or data recovery efforts.
• Prevention Guidelines:
- Specific code-level fixes include ensuring each dynamically allocated memory block is freed only once.
- Security best practices involve rigorous code reviews, using smart pointers in C++, and implementing proper memory management checks.
- Recommended tools and frameworks include static analysis tools like Valgrind and AddressSanitizer to detect memory management issues.
Technical Details
Likelihood of Exploit:
Affected Languages: C, C++
Affected Technologies: Not specified
Vulnerable Code Example
// Example of a Double Free vulnerability in C
#include <stdio.h>
#include <stdlib.h>
void process_data() {
int *data = (int *)malloc(sizeof(int) * 10);
if (data == NULL) {
fprintf(stderr, "Memory allocation failed\n");
return;
}
// Simulate processing data
free(data); // First free
// Some other operations...
free(data); // Double free vulnerability
}
int main() {
process_data();
return 0;
}
In this vulnerable code example, free(data); is called twice on the same pointer. This is known as a double free vulnerability. Double freeing memory can lead to undefined behavior, including program crashes and potential security risks like arbitrary code execution.
How to fix Double Free?
To fix the Double Free vulnerability, ensure that the free() function is called only once for a given memory allocation. After freeing a memory block, set the pointer to NULL. This prevents accidental double freeing because freeing a NULL pointer has no effect. It's a good practice to always set pointers to NULL after freeing them, as this can also help prevent use-after-free vulnerabilities.
Fixed Code Example
// Fixed version of the code demonstrating proper handling of allocated memory
#include <stdio.h>
#include <stdlib.h>
void process_data() {
int *data = (int *)malloc(sizeof(int) * 10);
if (data == NULL) {
fprintf(stderr, "Memory allocation failed\n");
return;
}
// Simulate processing data
free(data); // First and only free
data = NULL; // Set pointer to NULL to prevent double free
// Some other operations...
// No need to free again, but if attempted, it will be safe due to NULL check
if (data != NULL) {
free(data); // This check prevents double free
}
}
int main() {
process_data();
return 0;
}
In the fixed code example, the line data = NULL; ensures that the pointer is set to NULL immediately after it is freed. This prevents the second call to free(data); from attempting to free an already-freed memory block, thereby avoiding the double free vulnerability. Additionally, the if check before the second free(data); call ensures that no action is taken if data is NULL. Following these practices ensures safe memory management and prevents security vulnerabilities related to double freeing of memory in C programs.