Special HTTP headers

Reading time: 10 minutes

Wordlists & Tools

• https://github.com/danielmiessler/SecLists/tree/master/Miscellaneous/Web/http-request-headers
• https://github.com/rfc-st/humble

Headers to Change Location

Rewrite IP source:

• X-Originating-IP: 127.0.0.1
• X-Forwarded-For: 127.0.0.1
• X-Forwarded: 127.0.0.1
• Forwarded-For: 127.0.0.1
• X-Forwarded-Host: 127.0.0.1
• X-Remote-IP: 127.0.0.1
• X-Remote-Addr: 127.0.0.1
• X-ProxyUser-Ip: 127.0.0.1
• X-Original-URL: 127.0.0.1
• Client-IP: 127.0.0.1
• X-Client-IP: 127.0.0.1
• X-Host: 127.0.0.1
• True-Client-IP: 127.0.0.1
• Cluster-Client-IP: 127.0.0.1
• Via: 1.0 fred, 1.1 127.0.0.1
• Connection: close, X-Forwarded-For (Check hop-by-hop headers)

Rewrite location:

• X-Original-URL: /admin/console
• X-Rewrite-URL: /admin/console

Hop-by-Hop headers

A hop-by-hop header is a header which is designed to be processed and consumed by the proxy currently handling the request, as opposed to an end-to-end header.

• Connection: close, X-Forwarded-For

hop-by-hop headers

HTTP Request Smuggling

• Content-Length: 30
• Transfer-Encoding: chunked

HTTP Request Smuggling / HTTP Desync Attack

Cache Headers

Server Cache Headers:

• X-Cache in the response may have the value miss when the request wasn't cached and the value hit when it is cached
  • Similar behaviour in the header Cf-Cache-Status
• Cache-Control indicates if a resource is being cached and when will be the next time the resource will be cached again: Cache-Control: public, max-age=1800
• Vary is often used in the response to indicate additional headers that are treated as part of the cache key even if they are normally unkeyed.
• Age defines the times in seconds the object has been in the proxy cache.
• Server-Timing: cdn-cache; desc=HIT also indicates that a resource was cached

Cache Poisoning and Cache Deception

Local Cache headers:

• Clear-Site-Data: Header to indicate the cache that should be removed: Clear-Site-Data: "cache", "cookies"
• Expires: Contains date/time when the response should expire: Expires: Wed, 21 Oct 2015 07:28:00 GMT
• Pragma: no-cache same as Cache-Control: no-cache
• Warning: The Warning general HTTP header contains information about possible problems with the status of the message. More than one Warning header may appear in a response. Warning: 110 anderson/1.3.37 "Response is stale"

Conditionals

• Requests using these headers: If-Modified-Since and If-Unmodified-Since will be responded with data only if the response header**Last-Modified** contains a different time.
• Conditional requests using If-Match and If-None-Match use an Etag value so the web server will send the content of the response if the data (Etag) has changed. The Etag is taken from the HTTP response.
  • The Etag value is usually calculated based on the content of the response. For example, ETag: W/"37-eL2g8DEyqntYlaLp5XLInBWsjWI" indicates that the Etag is the Sha1 of 37 bytes.

Range requests

• Accept-Ranges: Indicates if the server supports range requests, and if so in which unit the range can be expressed. Accept-Ranges: <range-unit>
• Range: Indicates the part of a document that the server should return. For emxaple, Range:80-100 will return the bytes 80 to 100 of the original response with a status code of 206 Partial Content. Also remember to remove the Accept-Encoding header from the request.
  • This could be useful to get a repsonse with arbitrary reflected javascript code that otherwise could be escaped. But to abuse this you would need to inject this headers in the request.
• If-Range: Creates a conditional range request that is only fulfilled if the given etag or date matches the remote resource. Used to prevent downloading two ranges from incompatible version of the resource.
• Content-Range: Indicates where in a full body message a partial message belongs.

Message body information

• Content-Length: The size of the resource, in decimal number of bytes.
• Content-Type: Indicates the media type of the resource
• Content-Encoding: Used to specify the compression algorithm.
• Content-Language: Describes the human language(s) intended for the audience, so that it allows a user to differentiate according to the users' own preferred language.
• Content-Location: Indicates an alternate location for the returned data.

From a pentest point of view this information is usually "useless", but if the resource is protected by a 401 or 403 and you can find some way to get this info, this could be interesting.
For example a combination of Range and Etag in a HEAD request can leak the content of the page via HEAD requests:

• A request with the header Range: bytes=20-20 and with a response containing ETag: W/"1-eoGvPlkaxxP4HqHv6T3PNhV9g3Y" is leaking that the SHA1 of the byte 20 is ETag: eoGvPlkaxxP4HqHv6T3PNhV9g3Y

Server Info

• Server: Apache/2.4.1 (Unix)
• X-Powered-By: PHP/5.3.3

Controls

• Allow: This header is used to communicate the HTTP methods a resource can handle. For example, it might be specified as Allow: GET, POST, HEAD, indicating that the resource supports these methods.
• Expect: Utilized by the client to convey expectations that the server needs to meet for the request to be processed successfully. A common use case involves the Expect: 100-continue header, which signals that the client intends to send a large data payload. The client looks for a 100 (Continue) response before proceeding with the transmission. This mechanism helps in optimizing network usage by awaiting server confirmation.

Downloads

• The Content-Disposition header in HTTP responses directs whether a file should be displayed inline (within the webpage) or treated as an attachment (downloaded). For instance:

Content-Disposition: attachment; filename="filename.jpg"

This means the file named "filename.jpg" is intended to be downloaded and saved.

Security Headers

Content Security Policy (CSP)

Content Security Policy (CSP) Bypass

Trusted Types

By enforcing Trusted Types through CSP, applications can be protected against DOM XSS attacks. Trusted Types ensure that only specifically crafted objects, compliant with established security policies, can be used in dangerous web API calls, thereby securing JavaScript code by default.

// Feature detection
if (window.trustedTypes && trustedTypes.createPolicy) {
  // Name and create a policy
  const policy = trustedTypes.createPolicy('escapePolicy', {
    createHTML: str => str.replace(/\</g, '&lt;').replace(/>/g, '&gt;');
  });
}

// Assignment of raw strings is blocked, ensuring safety.
el.innerHTML = "some string" // Throws an exception.
const escaped = policy.createHTML("<img src=x onerror=alert(1)>")
el.innerHTML = escaped // Results in safe assignment.

X-Content-Type-Options

This header prevents MIME type sniffing, a practice that could lead to XSS vulnerabilities. It ensures that browsers respect the MIME types specified by the server.

X-Content-Type-Options: nosniff

X-Frame-Options

To combat clickjacking, this header restricts how documents can be embedded in <frame>, <iframe>, <embed>, or <object> tags, recommending all documents to specify their embedding permissions explicitly.

X-Frame-Options: DENY

Cross-Origin Resource Policy (CORP) and Cross-Origin Resource Sharing (CORS)

CORP is crucial for specifying which resources can be loaded by websites, mitigating cross-site leaks. CORS, on the other hand, allows for a more flexible cross-origin resource sharing mechanism, relaxing the same-origin policy under certain conditions.

Cross-Origin-Resource-Policy: same-origin
Access-Control-Allow-Origin: https://example.com
Access-Control-Allow-Credentials: true

Cross-Origin Embedder Policy (COEP) and Cross-Origin Opener Policy (COOP)

COEP and COOP are essential for enabling cross-origin isolation, significantly reducing the risk of Spectre-like attacks. They control the loading of cross-origin resources and the interaction with cross-origin windows, respectively.

Cross-Origin-Embedder-Policy: require-corp
Cross-Origin-Opener-Policy: same-origin-allow-popups

HTTP Strict Transport Security (HSTS)

Lastly, HSTS is a security feature that forces browsers to communicate with servers only over secure HTTPS connections, thereby enhancing privacy and security.

Strict-Transport-Security: max-age=3153600

Header Name Casing Bypass

HTTP/1.1 defines header field‐names as case-insensitive (RFC 9110 §5.1). Nevertheless, it is very common to find custom middleware, security filters, or business logic that compare the literal header name received without normalising the casing first (e.g. header.equals("CamelExecCommandExecutable")). If those checks are performed case-sensitively, an attacker may bypass them simply by sending the same header with a different capitalisation.

Typical situations where this mistake appears:

• Custom allow/deny lists that try to block “dangerous” internal headers before the request reaches a sensitive component.
• In-house implementations of reverse-proxy pseudo-headers (e.g. X-Forwarded-For sanitisation).
• Frameworks that expose management / debug endpoints and rely on header names for authentication or command selection.

Abusing the bypass

1. Identify a header that is filtered or validated server-side (for example, by reading source code, documentation, or error messages).
2. Send the same header with a different casing (mixed-case or upper-case). Because HTTP stacks usually canonicalise headers only after user code has run, the vulnerable check can be skipped.
3. If the downstream component treats headers in a case-insensitive way (most do), it will accept the attacker-controlled value.

Example: Apache Camel exec RCE (CVE-2025-27636)

In vulnerable versions of Apache Camel the Command Center routes try to block untrusted requests by stripping the headers CamelExecCommandExecutable and CamelExecCommandArgs. The comparison was done with equals() so only the exact lowercase names were removed.

# Bypass the filter by using mixed-case header names and execute `ls /` on the host
curl "http://<IP>/command-center" \
  -H "CAmelExecCommandExecutable: ls" \
  -H "CAmelExecCommandArgs: /"

The headers reach the exec component unfiltered, resulting in remote command execution with the privileges of the Camel process.

Detection & Mitigation

• Normalise all header names to a single case (usually lowercase) before performing allow/deny comparisons.
• Reject suspicious duplicates: if both Header: and HeAdEr: are present, treat it as an anomaly.
• Use a positive allow-list enforced after canonicalisation.
• Protect management endpoints with authentication and network segmentation.

References

• CVE-2025-27636 – RCE in Apache Camel via header casing bypass (OffSec blog)
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Disposition
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers
• https://web.dev/security-headers/
• https://web.dev/articles/security-headers