Aardvark.Cef.Process.Core Shared Memory Handler: Unvalidated memcpy

Affected Versions

Introduction

The SharedMemoryV8Handler::Execute function in Aardvark.Cef.Process.Core bridges JavaScript and native shared memory operations through Chromium Embedded Framework's V8 bindings. A critical flaw in this bridge allowed any JavaScript code—particularly in a compromised renderer process—to specify arbitrary copy lengths that could exceed the actual allocated buffer.

The vulnerability sits at the intersection of two dangerous assumptions: that JavaScript-provided length values are trustworthy, and that memcpy operations are inherently safe when the source pointer is valid. Neither assumption holds when an attacker controls one end of the inter-process communication channel.

The Vulnerability Explained

The shared memory handler exposes the openMapping function to JavaScript, which returns a handle containing data, length, and buffer members. When executing a mapping operation, the native code retrieves the ArrayBuffer data pointer and immediately copies handle->length bytes from handle->data:

void* ptr = handle->buffer->GetArrayBufferData();
if (ptr != nullptr)
{
    memcpy(ptr, handle->data, handle->length);
}

The critical defect: handle->length comes directly from JavaScript arguments with no validation that it fits within the destination buffer's actual allocation. An attacker in a compromised renderer can simply pass length: 0xFFFFFFFF or any value larger than the ArrayBuffer backing store.

Exploitation Scenario

Consider a legitimate page that uses openMapping to share a 4KB configuration block between JavaScript and native code. An attacker who compromises the renderer through a separate vulnerability (such as a V8 JIT bug or UAF) can:

1. Call openMapping with a valid buffer reference but an attacker-controlled length value
2. Supply handle->length = 0x100000 while the actual ArrayBuffer is only 4096 bytes
3. Trigger memcpy that writes 1MB of attacker-controlled data from handle->data into a 4KB buffer

This overflows into adjacent heap chunks, corrupting metadata, vtables, or other sensitive structures. On platforms without strong heap isolation, this reliably leads to code execution in the browser process—escaping the renderer sandbox entirely.

The Fix

The patch introduces explicit bounds validation before the memcpy operation:

void* ptr = handle->buffer->GetArrayBufferData();
if (ptr != nullptr)
{
    size_t bufferSize = handle->buffer->GetArrayBufferByteLength();
    if (handle->length > bufferSize)
    {
        exception = "Shared memory length exceeds destination buffer size.";
        return true;
    }
    memcpy(ptr, handle->data, handle->length);
}

The fix leverages CefV8Value::GetArrayBufferByteLength() to obtain the actual allocated size of the V8 ArrayBuffer backing store. This value is authoritative—it reflects the true memory boundary regardless of what JavaScript claims.

Key aspects of this specific change:

• Source of truth: GetArrayBufferByteLength() queries the V8 engine's internal state, not attacker-controlled data
• Fail-closed: On validation failure, the function sets an exception and returns immediately, preventing any copy operation
• No truncation: The fix rejects oversized operations entirely rather than silently truncating, ensuring callers receive clear error feedback

Key Takeaways

• V8 ArrayBuffer objects have authoritative size information via GetArrayBufferByteLength()—always query this before native memory operations rather than trusting JavaScript-provided lengths
• The SharedMemoryV8Handler pattern of accepting length alongside data pointers from JavaScript creates inherent trust boundary violations; treat all JavaScript numeric parameters as potentially malicious
• memcpy with a length parameter originating from inter-process communication requires explicit upper-bound validation against the destination allocation, even when the source pointer appears controlled
• Browser process code handling renderer-provided data must assume the renderer is compromised—the sandbox model depends on this defensive posture

How Orbis AppSec Detected This

Source: The length parameter from JavaScript arguments passed to the openMapping function, stored in handle->length

Sink: The memcpy(ptr, handle->data, handle->length) call where ptr points to ArrayBuffer data obtained via GetArrayBufferData()

Missing control: No comparison between handle->length and the actual ArrayBuffer byte length before the memory copy operation

CWE: CWE-119 — Improper Restriction of Operations within the Bounds of a Memory Buffer

Fix: Added validation that handle->length does not exceed handle->buffer->GetArrayBufferByteLength() before executing memcpy, with exception generation on failure

Orbis AppSec automatically detected this vulnerability and opened a pull request with the fix. Try Orbis AppSec on your repositories to find and fix issues like this automatically.

Conclusion

This vulnerability exemplifies a recurring pattern in CEF-based applications: the temptation to treat JavaScript-to-native bridges as trusted channels. The SharedMemoryV8Handler assumed that because the length parameter was "just a number," it would be reasonable. In reality, every numeric parameter crossing the V8 boundary is an attack surface.

The fix is minimal but precisely targeted: query the authoritative buffer size, validate, and fail safely. For developers building similar inter-process memory sharing mechanisms, this pattern—verify against allocation metadata, not caller claims—should be mandatory.