http-host-header-attacks

SKILL: HTTP Host Header Attacks — Injection & Routing Abuse

AI LOAD INSTRUCTION: Covers Host header injection for password reset poisoning, cache poisoning, SSRF via routing, and virtual host bypass. Includes bypass techniques for Host validation and framework-specific behaviors. Base models often miss the double-Host trick, absolute-URI override, and connection-state attacks.

0. RELATED ROUTING

• web-cache-deception when Host injection is combined with cache behavior
• ssrf-server-side-request-forgery when Host header routes requests to internal services
• open-redirect when Host injection causes redirect to attacker domain
• waf-bypass-techniques when Host manipulation helps bypass WAF routing
• request-smuggling when smuggling enables Host header manipulation past front-end validation
• subdomain-takeover when Host routing exposes internal vhosts resolvable via subdomain

1. ATTACK SURFACE

The Host header is used by web applications and infrastructure for:

2. PASSWORD RESET POISONING

The most common and impactful Host header attack.

How It Works

1. Attacker requests password reset for victim@target.com
2. Attacker modifies Host header in the reset request:
   POST /forgot-password HTTP/1.1
   Host: attacker.com    ← injected
   
   email=victim@target.com

3. Server generates reset link using Host header value:
   "Click here to reset: https://attacker.com/reset?token=SECRET_TOKEN"

4. Victim receives email, clicks link → token sent to attacker
5. Attacker uses token on real target.com to reset password

Testing

Check Burp Collaborator for incoming HTTP request with the reset token.

Variants

• Some apps concatenate: Host: target.com.attacker.com → link becomes https://target.com.attacker.com/reset?token=xxx
• Some apps use only the port portion: Host: target.com:@attacker.com → parsed as attacker.com in some URL parsers

3. WEB CACHE POISONING VIA HOST

Key requirement: Cache must not include Host header in cache key, but application must use Host in response body.

Test by sending two requests with different Host values and checking if the second request returns the first's Host in the response.

4. SSRF VIA HOST ROUTING

When a reverse proxy uses Host header to route to backends:

Common in:

• Nginx proxy_pass based on $host
• Apache ProxyPass with virtual host routing
• Kubernetes Ingress controllers
• Cloud load balancers

5. VIRTUAL HOST BYPASS

Many servers host multiple applications on the same IP via virtual hosting:

Discovery

6. BYPASS TECHNIQUES WHEN HOST IS VALIDATED

6.1 Override Headers

Many frameworks/proxies trust these headers over the Host header:

Test all simultaneously:

6.2 Absolute URL in Request Line

Per HTTP/1.1 spec (RFC 7230): if the request line contains an absolute URI, the Host header SHOULD be ignored. Some servers follow this, some don't — the mismatch between proxy and backend creates the vulnerability.

6.3 Double Host Header

Behavior varies:

• Some proxies validate first Host, app uses second
• Some servers concatenate: target.com, attacker.com
• RFC says: if both differ, return 400. Most servers don't.

6.4 Host with Port / Credentials

URL parsers may extract the "host" portion differently when credentials (@) or fragments (#) are present.

6.5 Trailing Dot

DNS treats target.com. and target.com identically (trailing dot = FQDN). But Host validation may not strip the trailing dot → target.com. ≠ target.com in string comparison → bypass whitelist.

6.6 Tab / Space Injection

Some parsers split on whitespace; the server may use attacker.com portion while validation checks target.com portion.

6.7 Wrap-Around / Enclosed Values

Quoted or bracketed values may be stripped by the app but not by the validator.

7. FRAMEWORK-SPECIFIC BEHAVIOR

8. CONNECTION-STATE ATTACKS

A sophisticated variant exploiting HTTP keep-alive:

This works against proxies that perform Host validation only on the first request of a keep-alive connection.

Testing

9. HOST HEADER ATTACK DECISION TREE

10. TRICK NOTES — WHAT AI MODELS MISS

1. Password reset poisoning doesn't require the victim to be logged in — you request the reset, the victim just clicks the link. The token lands on your server.
2. X-Forwarded-Host is the #1 missed bypass: Most Host validation checks Host header but frameworks silently prefer X-Forwarded-Host when behind a proxy.
3. Double Host header is protocol-valid but behavior-undefined: RFC says reject with 400, but almost no server actually does this. The mismatch between proxy and app is the vulnerability.
4. Absolute URI overrides Host per RFC: GET http://evil.com/path HTTP/1.1\nHost: target.com — the spec says use the request-line URI. But not all implementations agree.
5. Cache poisoning via Host requires the cache to exclude Host from the key: Most CDNs include Host in the cache key. But custom Varnish/Nginx caches may not. Also test with X-Forwarded-Host as cache key differentiator.
6. Connection-state attacks are rarely tested: Automated scanners don't test keep-alive behavior. Manual testing via Burp Repeater's connection reuse is essential.
7. DNS rebinding + Host attacks: If you control DNS, point your domain to the target's IP → your domain resolves to their server → Host header says your domain, but request hits their server. Useful for bypassing IP-based access controls.