CVE-2023-24534 - How Small HTTP and MIME Headers Can Crash Big Servers

Many modern web servers and services rely on efficient, safe parsing of HTTP and MIME headers. But in early 2023, a subtle flaw was discovered: with the right sort of weird HTTP header input, you could make servers, even with tiny requests, tie up huge amounts of computer memory. When that happens, reliable services crash or freeze—opening the door to denial of service (DoS) attacks that require almost no effort from attackers.

Let’s unpack what CVE-2023-24534 means, how it works, and how you can defend your systems against it.

What is CVE-2023-24534?

CVE-2023-24534 identifies a vulnerability in how some HTTP libraries (like Go’s net/http and others) process HTTP and MIME headers. Specifically, the parsers can be fooled into allocating far more memory than is truly needed when they encounter oddly crafted header data. Attackers can exploit this bug to deplete a server’s memory through unusually small HTTP requests.

Impact: Even a low-powered attacker with a basic understanding of HTTP can trigger this, potentially knocking servers offline or degrading services for everyone.

Let’s start simple: HTTP headers are lines like

Host: example.com
User-Agent: browser

When a server receives these, it parses out the keys (Host, User-Agent) and values (example.com, browser). This parsing sometimes needs to do some internal memory allocation, especially if headers are long or repeated.

The vulnerable code tries to be efficient, but certain *strange* header inputs can make it use a lot more memory than it should—for example, when headers repeat many times, include special characters, or are malformed in specific ways.

Suppose an attacker sends this custom HTTP request

GET / HTTP/1.1
Foo: A, ,, , , , , , , , , , , , , , , , , , , , , , , , , , 

The repeated commas and specially placed spaces make the parser mistakenly reserve much more space in memory than needed for the actual small content.

Code Snippet

Here’s a simplified, illustrative snippet in Go-like pseudocode showing where things might go wrong:

func ParseHeader(headerLine string) {
	validParts := strings.Split(headerLine, ",")
	// (Bug) Allocate array space for every comma—even empty ones
	headerArray := make([]string, len(validParts))

	for i, part := range validParts {
		headerArray[i] = strings.TrimSpace(part)
	}
	return headerArray // Only a few non-empty parts, but memory wasted
}

With a header like "Foo: , , , ,", validParts is huge, but almost all entries are empty. Servers will allocate memory for all those entries, even though they are useless.

Real-World Exploitation

Scenario: Suppose a high-traffic API server accepts many parallel HTTP requests. An attacker scripts repeated requests each with a heavily malformed header. Each such connection ties up excessive memory resources. In minutes, memory usage spikes:

Downtime costs can be huge!

Difficult to Detect: These attacks generate very little network traffic but high internal stress—a classic asymmetrical DoS.

The Fix

After the vulnerability was reported and confirmed, maintainers of affected libraries revised their header parsing logic to only allocate memory for actual, meaningful parsed values, not phantom/empty ones.

Fix Example (pseudo-diff)

- headerArray := make([]string, len(validParts))
+ nonEmptyCount := count of non-empty validParts
+ headerArray := make([]string, nonEmptyCount)

Now, only actual header parts that matter get space in memory.

References

- Official CVE Record: CVE-2023-24534
- Go net/http Security Release: Go Blog: Security Release
- Discussion and Details: Go Issue Tracker #56988

How to Protect Yourself

1. Update Your Software: Ensure your HTTP server libraries are fully up to date, especially if you’re using Go or similar platforms.
2. Rate Limit Unusual Requests: Watch for and slow down traffic with large or strange header patterns.
3. Monitor Memory Usage: Set memory alarms to detect sudden surges which might signal a header-based attack.
4. Audit Code: If you have custom parsers, make sure they only allocate memory for actual content, not placeholders or empty parts.

Conclusion

CVE-2023-24534 is a reminder: little details in protocol parsing can have outsized impact on security and reliability. The fix is simple, but the consequences of ignoring it are serious, so take time to patch, monitor, and defend your HTTP endpoints!

Timeline

Published on: 04/06/2023 16:15:00 UTC
Last modified on: 04/18/2023 17:38:00 UTC