CVE-2024-38809 - ETag Header Parsing Leads to DoS – What You Need to Know

CVE-2024-38809 is a freshly discovered vulnerability that targets applications parsing ETags from the If-Match or If-None-Match HTTP headers. This issue can allow malicious users to exhaust your server's resources and trigger a Denial of Service (DoS) attack. In this post, we’ll break down what’s happening, why it matters, how it works, and how you can fix or mitigate it — including practical code snippets.

What is CVE-2024-38809?

This vulnerability affects web applications that read and parse ETags from the If-Match and If-None-Match headers — common in RESTful APIs and HTTP caching mechanisms.

Problem:
When these headers are not properly validated or limited, attackers can send them with extremely long or complex values. The application then tries to parse these huge headers, potentially burning CPU and memory, slowing down, or even crashing.

Why is This Dangerous?

- Denial of Service (DoS): Attackers don’t need special access. Just by sending large and cleverly crafted headers, they can bog down your server, making your application slow or unavailable.
- Low Barrier to Exploit: This does not require guessing passwords or sophisticated techniques. Anyone knowing about this hole can hammer your endpoints with big headers.

Let’s look at a simple HTTP request

GET /some-resource HTTP/1.1
Host: example.com
If-None-Match: "<VERY-LONG-ETAG-HERE>"

Applications try to parse ETag values from these headers, often looping through a comma-separated list. Attackers can abuse this by sending gigabyte-sized headers or a million ETags.

Here’s how an attacker might automate it, using Python

import requests

very_long_etag = ",".join(['"ETAG_TOO_LONG_%d"' % i for i in range(100000)])
headers = {
    "If-None-Match": very_long_etag
}
response = requests.get("https://victim.com/api/data";, headers=headers)
print(response.status_code)

This code generates a header with 100,000 ETags. When the server tries to parse it, your CPU and memory usage spike. Multiply that by many requests per second, and your server is in trouble.

Who is Affected?

- Any app or API that reads/parses ETag values from incoming HTTP headers.
- Especially dangerous if you use popular Java or JavaScript frameworks that process these headers for conditional requests, caching, etc.

1. Upgrade Your Dependencies

Most major frameworks are releasing patches for this issue. Check your vendor’s page and update to patched versions immediately.

- Spring Framework
- Apache Tomcat
- [Nginx/Caddy/Node.js] – Check your stack!

2. Implement a Size Limit on Headers

If you’re stuck with an older or unpatched version, enforce a reasonable size limit on these headers.

Add a filter to check the length of incoming header values

@WebFilter("/*")
public class ETagHeaderLimitFilter implements Filter {
    private static final int MAX_ETAG_HEADER_SIZE = 1024; // 1KB example

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest) request;
        String ifMatch = req.getHeader("If-Match");
        String ifNoneMatch = req.getHeader("If-None-Match");

        if ((ifMatch != null && ifMatch.length() > MAX_ETAG_HEADER_SIZE) ||
            (ifNoneMatch != null && ifNoneMatch.length() > MAX_ETAG_HEADER_SIZE)) {
            ((HttpServletResponse)response).sendError(400, "ETag header too large");
            return;
        }
        chain.doFilter(request, response);
    }
}

Node.js Example: Express Middleware

app.use(function(req, res, next) {
  const maxLen = 1024;
  if (
    (req.headers['if-match'] && req.headers['if-match'].length > maxLen) ||
    (req.headers['if-none-match'] && req.headers['if-none-match'].length > maxLen)
  ) {
    return res.status(400).send('ETag header too large');
  }
  next();
});

Pick a limit that makes sense for your application. Most normal ETags are less than 128 characters.

Reference Links

- Original Advisory: GitHub
- Spring Blog on CVE-2024-38809
- Common Vulnerabilities and Exposures: CVE-2024-38809
- OWASP: HTTP Header Attacks
- Example Exploit Scripts

Apply header size limits on any app where you cannot immediately upgrade.

4. Monitor your server metrics for unusual CPU/memory spikes from incoming requests with large headers.

Bottom Line

CVE-2024-38809 proves that sometimes the simplest input – like a too-long HTTP header – can have outsized consequences. Patch, limit, and monitor – and you’ll be much safer against this class of problems!

Timeline

Published on: 09/27/2024 17:15:12 UTC
Last modified on: 11/21/2024 09:26:51 UTC