Firefox CVE-2024-9680

CVE-2024-9680

Description

An attacker was able to achieve code execution in the content process by exploiting a use-after-free in Animation timelines. We have had reports of this vulnerability being exploited in the wild. This vulnerability affects Firefox < 131.0.2, Firefox ESR < 128.3.1, Firefox ESR < 115.16.1, Thunderbird < 131.0.1, Thunderbird < 128.3.1, and Thunderbird < 115.16.0.

tl;dr (Video Version)

Credit: Fireship

Use-After-Free Exploit ?!

A "use-after-free" vulnerability is a type of memory corruption issue that occurs when a program continues to use a pointer (or reference) to memory after it has been freed (deallocated). This is a dangerous condition because the memory in question is no longer owned by the program, meaning it might be reassigned for another purpose or be altered by other parts of the program. If the program continues to use this freed memory, it can lead to unpredictable behavior, including crashes, data corruption, or exploitation by attackers.

Breakdown of the Vulnerability:

1. Memory Allocation and Deallocation:

   • In many programming languages, especially those like C and C++, memory is manually managed. Developers allocate memory when they need it (e.g., with functions like malloc() or new) and free it when they are done with it (e.g., with free() or delete).

2. The Problem:

   • A use-after-free issue arises when a pointer still points to memory that has already been freed. If the program later attempts to read from or write to this memory, unexpected things can happen because the memory may now be reallocated to another part of the program, altered, or even no longer available for safe use.

3. Consequences:

   • Crash: The program may attempt to access invalid or corrupt memory, leading to a crash.
   • Data Corruption: The program could unintentionally overwrite or read unintended data, causing corruption.
   • Security Exploitation: Attackers can exploit use-after-free vulnerabilities by manipulating the program’s memory space. For example, if an attacker can allocate controlled data into the freed memory, they could potentially execute arbitrary code, change program execution flow, or gain unauthorized access.

Example Scenario:

Consider a simplified example in C:

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

int main() {
    int *ptr = (int *)malloc(sizeof(int)); // Allocate memory
    *ptr = 42;                             // Use the allocated memory
    free(ptr);                             // Free the memory

    // Use the pointer after freeing the memory (use-after-free)
    printf("%d\n", *ptr);                  // Undefined behavior, potential crash or exploit
    return 0;
}

In the example:

• Memory is allocated for an integer, and the value 42 is assigned to it.
• The memory is then freed with free(ptr).
• After freeing the memory, the pointer ptr is still being used (dereferenced in the printf() statement), which leads to undefined behavior. The memory may have been reassigned, and accessing it could lead to a crash or, in a more dangerous scenario, the program being exploited.

Exploitation Potential:

Attackers can exploit use-after-free vulnerabilities by carefully manipulating memory management. Here’s how an attacker could potentially use this vulnerability:

1. Memory Reallocation: After the memory is freed, if the attacker can control what data gets placed into that freed memory, they can cause the program to use this new data in place of the original data.
2. Code Injection: In certain cases, the attacker could inject malicious code into the memory location, allowing them to hijack the program’s control flow, leading to remote code execution or privilege escalation.

Mitigations:

To prevent use-after-free vulnerabilities, developers can:

• Set pointers to NULL after freeing them: This ensures that if a pointer is accidentally used after being freed, the program will crash more predictably or behave in a controlled way, rather than accessing invalid memory.
• Smart pointers: In languages like C++, using smart pointers (e.g., std::shared_ptr, std::unique_ptr) can help manage memory automatically, reducing the chances of manual memory mismanagement.
• Bounds checking and sanitizers: Use tools like AddressSanitizer (ASan) that help detect use-after-free conditions during development.

How the CSS Animation Timeline Exploited the Flaw

In this specific case, the CSS Animation Timeline—which is responsible for controlling the timing of animations in a web page—was interacting with other browser components in such a way that an object tied to the animation’s timeline was freed, but the browser continued to use it.

1. CSS Animation Timeline Mechanism:

• The CSS Animation Timeline in Firefox is responsible for coordinating the timing and progression of animations. It essentially manages when and how animations start, stop, and transition across keyframes. Each time an animation is updated or queried, the browser has to fetch and manipulate objects associated with the timeline.

2. Incorrect Object Handling:

• Due to a flaw in Firefox's handling of the CSS Animation Timeline, an object (or series of objects) that should have been protected (kept in memory) while still being used in the timeline was instead freed (deallocated) prematurely.
• Despite the memory being freed, the timeline still attempted to reference the object to update or query the animation, triggering a use-after-free condition. In this state, the memory could be reassigned, or the data could be corrupt or controlled by an attacker.

3. Attacker Control:

• The key to exploiting a use-after-free vulnerability is timing and control over the memory state. An attacker could craft a malicious webpage that triggered the CSS animation to manipulate the browser’s internal object states.
• After the memory tied to the timeline is freed, the attacker could potentially fill that memory space with controlled data (e.g., injecting specific data into the freed memory). When the CSS animation timeline tried to reference the freed memory, it could end up accessing this injected data, leading to the possibility of remote code execution (RCE), browser crashes, or other unintended behavior.

4. Exploit Mechanism:

• Triggering the vulnerability: An attacker could craft specific HTML and CSS content to force the browser into freeing memory that is still being used by the CSS animation timeline. This would likely involve manipulating the timing of animations, stopping and starting animations in quick succession, or exploiting edge cases in how animation keyframes are processed.
• Exploiting the freed memory: Once the object is freed, the attacker might be able to carefully control what is placed into the freed memory slot (via memory allocation techniques), which could allow them to overwrite the object with malicious data.
• Execution of malicious code: When the CSS Animation Timeline tries to access this memory, it could mistakenly use the attacker's controlled data, potentially executing code that the attacker has injected.

5. Impact:

• Remote Code Execution (RCE): The ultimate goal for many use-after-free exploits is to achieve RCE, where the attacker can run arbitrary code on the user's machine. This happens when the attacker can manipulate the browser's memory space enough to control what gets executed next.
• Browser Crashes: Even if the exploit doesn’t achieve RCE, use-after-free vulnerabilities often cause the browser to crash when it tries to access invalid memory.

Why the CSS Animation Timeline is Vulnerable:

The CSS Animation Timeline is a complex system that interacts with many different parts of the browser engine, such as the rendering engine, DOM (Document Object Model), and JavaScript execution environment. Managing the lifecycle of objects tied to animations—especially when they are dynamically updated or removed—is tricky, and even small mistakes in memory management can lead to use-after-free bugs.

In the case of CVE-2024-9680, it seems the timeline object was not properly tracked, so it was freed while still in use. If an attacker can repeatedly trigger this bug with carefully timed animation updates or manipulations, they can potentially exploit the vulnerability.

Conceptual Example:

This hypothetical example might involve creating a webpage with a complex set of CSS animations and dynamically manipulating them via JavaScript to trigger a use-after-free condition.

<!DOCTYPE html>
<html>
<head>
    <style>
        @keyframes exampleAnimation {
            from { opacity: 0; }
            to { opacity: 1; }
        }

        .animate {
            animation: exampleAnimation 5s infinite;
        }
    </style>
</head>
<body>
    <div id="targetElement" class="animate">Animating Element</div>

    <script>
        // Example setup: A function that continuously creates and destroys animations
        // The goal here is to simulate rapid, repeated manipulations of the CSS animation timeline
        function triggerVulnerability() {
            const target = document.getElementById('targetElement');

            // Create an animation, then remove it quickly in a loop
            let i = 0;
            const interval = setInterval(() => {
                i++;
                if (i % 2 === 0) {
                    target.classList.add('animate');
                } else {
                    target.classList.remove('animate');
                }

                // Potentially causing a race condition or triggering the vulnerability
                if (i > 1000) { 
                    clearInterval(interval); 
                }
            }, 1); // Rapid manipulation of the animation state
        }

        // Simulating dynamic DOM manipulation and timeline interaction
        triggerVulnerability();
    </script>
</body>
</html>

What This Code Does:

1. The CSS defines an animation (exampleAnimation) that fades in an element’s opacity.
2. JavaScript (triggerVulnerability()) rapidly adds and removes the animate class from an element, causing the browser’s animation timeline to be repeatedly updated and potentially forcing the browser to manage object creation and destruction in quick succession.
3. This rapid manipulation of animations could, in theory, cause the browser to improperly manage memory if there is a flaw in how it tracks animation objects (like in the CSS Animation Timeline).

Conceptual Path to Exploitation:

1. Object Allocation and Deallocation: When an animation is created, the browser allocates memory for managing it. If the animation is stopped or removed, this memory is freed.
2. Triggering Use-After-Free: In the hypothetical case of a vulnerability, rapid creation and destruction of the animation objects could cause the browser to attempt to access an object that has already been deallocated, leading to a use-after-free condition.
3. Exploit Potential: If an attacker can control this memory (perhaps by forcing the browser to allocate controlled data into the freed memory space), they could potentially manipulate the browser’s behavior, leading to remote code execution.