Memory Corruption Due to Use-After-Free Pointer Dereference

Critical Risk Memory Safety
ccppuse-after-freememory-corruptionrceheapdangling-pointer

What it is

Use-after-free vulnerabilities occur when code dereferences a pointer after the memory it points to has been freed. This can enable memory corruption, crashes, data corruption, or remote code execution when freed memory is reallocated and controlled by an attacker. The dangling pointer may point to reclaimed memory containing attacker-controlled data.

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

// VULNERABLE: Use-after-free
void process_data() {
    char* buffer = malloc(100);
    if (buffer == NULL) return;
    
    strcpy(buffer, "Hello");
    free(buffer);
    
    // USE-AFTER-FREE!
    printf("%s\n", buffer);
    strcpy(buffer, "Bad");
}
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

// SECURE: Proper pointer management
void process_data() {
    char* buffer = malloc(100);
    if (buffer == NULL) return;
    
    strcpy(buffer, "Hello");
    
    // Use before freeing
    printf("%s\n", buffer);
    
    free(buffer);
    buffer = NULL;  // Prevent reuse
    
    // Safe: won't execute
    if (buffer != NULL) {
        strcpy(buffer, "Won't execute");
    }
}

💡 Why This Fix Works

The vulnerable code dereferences buffer after calling free(), causing use-after-free. The secure version uses the buffer before freeing it and sets the pointer to NULL afterwards to prevent accidental reuse.

Why it happens

Calling free() on a pointer but continuing to dereference it afterwards without reallocating.

Root causes

Freeing Memory But Continuing to Use Pointer

Calling free() on a pointer but continuing to dereference it afterwards without reallocating.

Missing NULL Assignment After Free

Not setting pointers to NULL after freeing them, leaving dangling pointers.

Complex Ownership Semantics

Unclear memory ownership patterns where multiple code paths may free the same memory.

Fixes

1

Set Pointers to NULL After Free

Always set pointers to NULL immediately after calling free() to prevent accidental reuse.

2

Use AddressSanitizer

Enable AddressSanitizer (-fsanitize=address) to detect use-after-free at runtime during testing.

3

Establish Clear Ownership

Define clear ownership patterns and lifetimes for dynamically allocated memory.

Detect This Vulnerability in Your Code

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