Chrome Under Attack: UAF Process Flaws Exploited in Active Campaigns

In an advancement for browser security, Google Chrome’s latest protection mechanism, MiraclePtr, has effectively neutralized two newly discovered use-after-free (UAF) vulnerabilities that previously posed a critical risk for sandbox escapes.

The vulnerabilities, identified by SSD Labs Korea, targeted the browser process—a frequent vector for attackers seeking to break out of Chrome’s security sandbox and execute arbitrary code.

Understanding the Threat: Use-After-Free Vulnerabilities

Use-after-free (UAF) bugs occur when a program continues to use a pointer to memory after it has been freed.

In the context of browsers like Chrome, UAF vulnerabilities are particularly dangerous.

They can allow attackers to corrupt memory, hijack control flow, and ultimately execute code outside the browser’s sandbox, bypassing vital security boundaries.

The two vulnerabilities disclosed centered around Chrome’s password manager and sync features. Both involved asynchronous callbacks in the SyncServiceImpl SyncHandler classes, where raw pointers were used in a way that could result in accessing freed memory if certain objects were destroyed during task execution.

In the past, such flaws would have been prime targets for exploitation using heap spraying and vTable corruption techniques, enabling attackers to achieve code execution with elevated privileges.

How MiraclePtr Works: Technical Overview

MiraclePtr is Chrome’s innovative response to the persistent threat of UAF exploits.

At its core, MiraclePtr leverages a reference-counting mitigation technique called BackupRefPtr (BRP), tightly integrated with Chrome’s custom heap allocator, PartitionAlloc.

Key technical features include:

• Reference Counting: Each allocated object is accompanied by a hidden reference counter. When a pointer references an object, the counter is incremented; when a pointer is destroyed, the counter is decremented.
• Deferred Deallocation: If an object’s reference count is greater than zero when delete or free It is called the memory is not immediately reclaimed. Instead, the object is moved to a quarantine area, making it unavailable for reallocation.
• Memory Quarantine and Poisoning: Memory regions that are still referenced but scheduled for deletion are filled with specific bit patterns (e.g., 0xcc). This ensures that any accidental or malicious access results in a crash, not code execution.
• Safe Pointer Semantics: The BackupRefPtr template class ensures that memory is only actually freed when the last reference is destroyed, preventing dangling pointers.

Pseudo-code from the MiraclePtr implementation illustrates this process:

cppvoid* Alloc(size_t size) {
  void* ptr = ActuallyAlloc(size);
  if (isSupportedAllocation(ptr)) {
    int& ref_count = *(cast<int*>(ptr) - 1);
    ref_count = 1;
  }
  return ptr;
}

void Free(void* ptr) {
  if (isSupportedAllocation(ptr)) {
    atomic_int& ref_count = *(cast<atomic_int*>(ptr) - 1);
    if (ref_count != 1)
      memset(ptr, 0xcc, getAllocationSize(ptr));
    if (--ref_count != 0)
      return;
  }
  ActuallyFree(ptr);
}

Real-World Impact: Blocking Exploits at the Root

The two UAF vulnerabilities uncovered by SSD Labs Korea were demonstrated using proof-of-concept attacks that previously would have allowed attackers to trigger a crash and potentially execute code by corrupting the vTable of a freed object.

However, with MiraclePtr fully enabled, these attacks are rendered non-exploitable.

The reference counting and memory quarantine mechanisms ensure that even if an attacker tries to reclaim and manipulate freed memory, the result is at most a crash, never successful code execution.

Conclusion: A New Era for Browser Security

MiraclePtr represents a major leap forward in defending against a class of vulnerabilities that has long plagued browsers.

By combining reference counting, custom heap management, and aggressive memory quarantine, Chrome now offers robust protection against UAF-based sandbox escapes.

As MiraclePtr becomes standard across browser components, the window for attackers to exploit these critical flaws is rapidly closing, setting a new benchmark for browser security.

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