Activating Chrome DevTools Protocol in Memory: Bypassing –remote-debugging-port Restrictions

1. DevToolsAgentHost::StartRemoteDebuggingServer  https://source.chromium.org/chromium/chromium/src/+/main:content/browser/devtools/devtools_agent_host_impl.cc;l=215

2. std::unique_ptr<DevToolsSocketFactory> is an abstract C++ class exposing a virtual method called CreateForHttpServer. The shellcode needs to fabricate a convincing fake of this object.

   

3. Internally, Chrome relies on its custom allocator, PartitionAlloc (https://chromium.googlesource.com/chromium/src/+/master/base/allocator/partition_allocator/PartitionAlloc.md), whenever it allocates a DevToolsSocketFactory instance.

chrome.dll does not make our life easy: none of the functions we need are exported. One reliable way to locate them at runtime is by scanning the module’s memory for their byte-level signatures.

Architecture

The post-exploitation module is split into two components:

• A loader
• A shellcode payload

Loader

The loader’s job is to inject the shellcode into the PPID (Parent Process ID) of chrome.exe.

It is critical that the shellcode runs within the PPID’s address space and executes from its main thread: chrome.exe!WinMainCRTStartup.

Hijacking the main thread, however, is a fragile and crash-prone operation. We need a mechanism that lets the thread execute our code without being suspended or interrupted in its normal routine.

Special APCs are exactly what this calls for. Unlike regular APCs, which require the target thread to enter an alertable wait state, Special APCs execute asynchronously on a thread even while it is actively running.

The loader therefore follows these steps:

1. Identify the chrome.exe PPID (or accept it as an argument)
2. Standard injection primitive: OpenProcess(), VirtualAllocEx(), WriteProcessMemory()
3. Locate the main thread inside the PPID (typically the first thread spawned by the process)
4. Resolve NtQueueApcThreadEx and enqueue the Special APC targeting that main thread

https://repnz.github.io/posts/apc/user-apc/

Shellcode

The shellcode begins by resolving at runtime all the modules and functions it depends on (including chrome.dll), then launches the signature-scanning phase.

Example of a signature pattern:

Given a signature and a module base address, a scanning routine resolves the .text section boundaries of chrome.dll and walks forward from the section start until it matches the supplied pattern:

The signature can reside anywhere within the target function; it is not anchored to the prologue or epilogue. Once a match is found, RtlLookupFunctionEntry() is used to recover the function’s actual start address from the address of the matched signature.

After all required addresses have been resolved, the next step is to craft the fake server_socket_factory that StartRemoteDebuggingServer expects as its argument. Including padding, the object is 16 bytes wide, so chrome!malloc must allocate exactly that amount.

Allocating through chrome!malloc is non-negotiable. Using HeapAlloc, the CRT malloc, or any external allocator will crash Chrome, because PartitionAlloc enforces strict metadata checks on every pointer it later frees.

The layout of the allocated object:

+0x00: [vtable_addr]  → points to TCPServerSocketFactory::vftable in .rdata
+0x08: [0x2406]       → 9222 in little-endian
+0x0A: [0x0000...]    → padding (zeroed)

At this point, StartRemoteDebuggingServer can finally be called, and CDP comes alive inside the existing Chrome process, without a single command-line flag having been touched.

Conclusion