Use-After-Free

What is Use-After-Free?

A use-after-free (UAF) vulnerability occurs when a program continues to use a pointer after the memory it references has been freed. The freed memory may be reallocated for a different purpose, and subsequent use of the stale pointer can read or write to unexpected data.

How Does it Work?

When memory is freed, the allocator returns it to the free pool. If the program continues using the old pointer, an attacker can allocate new data to fill the freed region, then trigger the use of the stale pointer. This allows reading attacker-controlled data or overwriting function pointers.

// VULNERABLE: Use-after-free pattern
struct User *user = malloc(sizeof(struct User));
user->name = "admin";
free(user);  // Memory is freed

// ... later in the code ...
// DANGEROUS: 'user' still points to freed memory
printf("Name: %s\n", user->name);  // Use-after-free!

// Attacker can allocate new data to fill the freed region
// and control what user->name points to

// SECURE FIX: Set pointer to NULL after free
free(user);
user = NULL;  // Prevents accidental reuse

Real-World Examples

Windows OLE RCE (CVE-2025-21298) is a zero-click UAF exploitable via Outlook preview. Chrome 0-day CVE-2024-0519 was a UAF in V8 JavaScript engine. Many browser and kernel exploits leverage UAF as the primitive for arbitrary code execution.

Security Impact

Use-after-free enables arbitrary code execution, information disclosure, and denial of service. It is the most common vulnerability class in modern browsers, operating systems, and firmware — accounting for 36% of Chrome security bugs.

Prevention & Mitigation

Use memory-safe languages (Rust, Go). In C/C++: set pointers to NULL after free, use smart pointers, enable AddressSanitizer (ASan), apply MiraclePtr/raw_ref in Chromium-style projects.

How Precogs AI Stops Use-After-Free

Precogs AI Binary SAST traces heap allocation/deallocation patterns in compiled binaries to detect use-after-free conditions — critical for automotive ECU firmware, medical devices, and embedded systems where memory-safe languages cannot be used.