CVE-2024-7721: 'MemFray' - The Stack Overflow That Broke the 'Secure' Gateway

Let's talk about 'Fortress'. It's that shiny, expensive VPN gateway enterprise admins love because it promises 'zero-latency' packet inspection. To achieve this mythical speed, the developers decided that standard C libraries like strncpy were for chumps. They wrote their own.

Specifically, they implemented a custom HTTP header parser in a module called fast_parse.c. The goal? To strip headers and forward traffic faster than you can say 'segmentation fault'. Unfortunately, in their quest for optimization, they optimized away the safety rails. This isn't just a bug; it's a monument to hubris. The component meant to filter out bad traffic is exactly what lets the bad traffic in.

The vulnerability lives in a function responsible for parsing custom authentication headers. The function allocates a fixed-size buffer of 512 bytes on the stack to temporarily hold header values before validation.

The flaw is deceptively simple: the loop responsible for copying the header value checks for a null terminator but fails to strictly enforce the 512-byte boundary when handling 'continued' header lines (lines starting with whitespace).

If an attacker sends a header that technically spans multiple lines but logically concatenates to 2000+ bytes, the custom parser happily keeps copying bytes past the buffer's end. It bulldozes right through the saved frame pointer and the return address. It's 1990s-style exploitation in 2024.

Here is the logic that doomed the Fortress Gateway. The developers wrote a while loop that trusted the input too much.

Vulnerable Code (fast_parse.c)

void parse_auth_header(char *input_stream) {
    char temp_buf[512];
    int idx = 0;
    
    // The loop checks for null, but implicit length check is flawed
    while (*input_stream != '\0' && *input_stream != '\r') {
        // CRITICAL BUG: No check if idx >= 512 here
        temp_buf[idx++] = *input_stream++;
        
        // Handle multi-line headers (RFC 7230 deprecated this, but Fortress supports it)
        if (*input_stream == '\n' && (*(input_stream+1) == ' ' || *(input_stream+1) == '\t')) {
             input_stream += 2; // Skip newline and indent
        }
    }
}

The Fix

In the patch, they finally introduced a hard limit check. It’s not rocket science, folks.

    while (*input_stream != '\0' && *input_stream != '\r') {
        // PATCH: The bounds check saves the day
        if (idx >= sizeof(temp_buf) - 1) {
            log_error("Header too long");
            return;
        }
        temp_buf[idx++] = *input_stream++;
        // ...
    }

Since this is a stack overflow on a modern Linux-based appliance, we have to deal with NX (No-Execute) bits. We can't just throw shellcode on the stack and jump to it. We need ROP (Return-Oriented Programming).

Here is the attack chain:

1. Trigger: Send an HTTP POST with X-Custom-Auth: AAAA... (padding) followed by specific ROP gadgets.
2. Overwrite: We fill the 512-byte buffer, then overwrite the saved Return Instruction Pointer (RIP) with the address of a pop rdi; ret gadget found in the libc loaded in memory.
3. Payload: We put the address of the string /bin/sh (also found in libc) into the RDI register.
4. Execution: The next return address in our chain points to system().

Because the Fortress daemon runs as root to handle raw network interfaces, popping this shell gives us total control over the device. We are now the admin.

This is a CVSS 9.8 for a reason. It requires no authentication. It requires no user interaction.

If you have a Fortress Gateway exposed to the internet (which is its entire purpose), you are vulnerable. An attacker can pivot from the gateway into the internal network, decrypt VPN traffic (since they now own the keys in memory), or just brick the device for fun. It is a complete compromise of the network perimeter.