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Part 2.5

In the world of embedded systems, the login prompt often stands as a digital fortress, guarding the inner workings of a device. My latest challenge: the RVLink MV2402 roof unit and its web authentication. My objective was clear – to achieve persistent, programmatic control by dismantling this barrier. No more passwords, no more session juggling; just direct access to the core. This isn't about finding a common vulnerability, but about meticulously reverse engineering the firmware and making precise, surgical alterations to bend the system to my will. Join me as I recount the steps to bypass the RVLink's session management and unlock its full potential.

Understanding the Battlefield: RVLink's Session Management

Before charging into battle, every good reverse engineer surveys the terrain. My target, the RVLink MV2402, is built upon a COMFAST CF-E5 (QCA9531 chipset) and its firmware leverages nginx, FastCGI, and a custom webmgnt binary to handle web UI and API requests. This webmgnt process became my primary focus.

Through careful analysis of the C code (thanks Ghidra) and MIPS assembly, I identified three critical gatekeepers of authentication (names are my own):

1. authenticated_COMFAST_session_id: A character buffer normally populated by the cgi_login function via sprintf upon successful login. It's zeroed out by memset during logout or system initialization.
2. is_logged_in: A global character (boolean) flag. This is the primary indicator within cgi_login that a user has successfully authenticated.
3. session_valid: An integer variable residing in webcfg_main. This appeared to be the ultimate gatekeeper, used as the final check for granting access to authenticated sections of the web UI and API.

cgi_login() Conceptual Diagram

Apparently my mermaid.js flow charts aren't rendering properly even though they looked fine in the preview. I'm working on it.

webcfg_main() Session Handling

My strategy was to neutralize these gatekeepers one by one, or find the most efficient way to make them permanently declare a valid, authenticated session.

Target 1: Neutralizing the Session ID (authenticated_COMFAST_session_id)

My first approach was to defang the session ID itself. If cgi_login never set a unique session ID, what would happen?

• The Problem: A unique session ID is generated and expected.
• The Patch: I located the sprintf call responsible for creating the session ID within cgi_login and the subsequent sw (Store Word) instruction that saved the value in v1 back into the stack. By NOPing (No Operation) these MIPS instructions, I prevented any session ID from being set.
• The Result: The authenticated_COMFAST_session_id buffer remained all zeros (as initialized by memset). While initially, I thought a zeroed-out ID would always be invalid, further testing revealed that under certain conditions (like a session already being established or other checks being bypassed), an empty session ID could still permit access. This was an interesting first step, but not the full bypass I sought.

Target 2: Forcing is_logged_in to True – The Direct Approach

Next, I turned my attention to the is_logged_in flag within the cgi_login function. If this flag could be forced to 1 (true), then cgi_login should report success regardless of the credentials provided.

• The Problem: cgi_login performed actual username and password checks before setting is_logged_in.
• The Initial Patch (MIPS32 context for some parts of webmgnt): I identified the instruction that would set a register (e.g., $v1) to the result of the credential check before it was stored into the is_logged_in global variable. I patched this to directly load 1 into the register. The MIPS16e instruction li v1, 0x1 (hex: 6B 01) became my tool of choice here.
• The Result (Partial Victory): This was promising! is_logged_in could be forced to 1.

The "Aha!" Moment & Iterative Refinement in cgi_login

However, the path to true control is rarely linear. My expectation was that patching this single instruction would allow any user+pass combination. I hit a snag: admin:<any_password> worked, but completely dummy usernames still failed.

• The Problem Encountered: The username check (bnez v0, LAB_00405828 at 0x00405800 in MIPS16e) was correctly identifying bad usernames and branching over my beautiful password-forcing patch. The is_logged_in variable (represented by $v1 here) would retain its initial 0 value.

• The Breakthrough Solution: The lightbulb moment! Why try to satisfy the username check when I could simply… remove it? I NOP'd the bnez v0, LAB_00405828 instruction that branched on username failure. This forced execution always into the password check block, where my other patch was waiting to declare success.

• The Glorious Result: With both the username branch NOP'd and the password result forced to true, any username/password combination sailed through cgi_login()!

The Ultimate Target: session_valid – The Centralized Kill Switch

While conquering cgi_login was a significant victory, it still required a call to that endpoint. I yearned for a more passive, permanent bypass. This led me to the session_valid variable within the main webcfg_main logic – the final arbiter of access.

• The Rationale: Targeting session_valid offered the most robust and maintainable solution. It's the final checkpoint; if it says "yes," access is granted.
• The Problem: This variable was set based on session ID comparisons and other checks.
• The Solution: I decided to patch the logic at the point of assignment. Conceptually in C, I wanted to change complex conditional logic to a simple session_valid = 1;. In assembly, this meant finding where session_valid was written to memory and ensuring the value written was always 1.
• The Key Insight: As I'd noted, "Firmware authentication can often be bypassed with minimal, targeted binary patches—if you understand the control flow and key variables, you don’t need to brute-force or exploit vulnerabilities."
• The Result: This was the master stroke. After applying this patch and restarting the webcfg service (a crucial step!), all session checks effectively passed without any prior login or valid session cookie. Direct API calls, previously requiring authentication, now worked flawlessly. I had achieved persistent, universal access.

The control flow for an API request is now rather simple

Patching Philosophy: Minimal Changes, Maximum Impact

Throughout this endeavor, a few principles guided my patching strategy:

• Centralized Patching: Targeting a final, central check like session_valid is generally more robust than patching numerous upstream checks.
• Minimal Binary Changes: Each modification carries risk. By making the smallest possible changes (often just a single instruction or a small block), I reduced the chance of unintended side effects and made future maintenance (like reapplying patches to updated firmware) much easier.

radiff2 webmgnt_original webmgnt_patched

0x00005800 2a13 => 6500 0x00005800 -- Remove branch to error on username CMP FAIL
0x00005826 6778 => 6b01 0x00005826 -- Always load 1 instead of password CMP result
0x00005972 ea40d308 => 65006500 0x00005972 -- Prevent session_id from being written
0x000061c5 00 => 01 0x000061c5 -- Initialize session_valid with 1 not 0
0x00007619 00 => 01 0x00007619 -- Prevent session_valid from being cleared
0x00054bac 01 => 00 0x00054bac -- Supposed "auth" flag in CGI func table for logout
0x00054bc0 01 => 00 0x00054bc0 -- Supposed "auth" flag in CGI func table for mbox_config

• Meticulous Documentation: Keeping detailed notes (I use Obsidian) was invaluable for tracking variables, addresses, patch attempts, and insights.

Conclusion: The Kingdom is Mine

By systematically analyzing the RVLink MV2402's webmgnt binary in the firmware, identifying key authentication variables (authenticated_COMFAST_session_id, is_logged_in, and critically, session_valid), and applying targeted MIPS assembly patches, I successfully achieved a complete and persistent bypass of the web authentication mechanism.

This journey underscores that sometimes the most elegant solutions in reverse engineering aren't about complex exploits, but about a deep understanding of the target's internal logic and the strategic application of minimal, precise changes. The castle gates are now wide open.