17. HTTP REQUEST SMUGGLING
Lowest dup rate. $5K–$30K. PortSwigger research by James Kettle.
CL.TE (Content-Length front, Transfer-Encoding back)
POST / HTTP/1.1
Content-Length: 13
Transfer-Encoding: chunked
0
SMUGGLED
Detection
1. Burp extension: HTTP Request Smuggler
2. Right-click request → Extensions → HTTP Request Smuggler → Smuggle probe
3. Manual timing: CL.TE probe + ~10s delay = backend waiting for rest of body
Impact Chain
Poison next request → access admin as victim
Steal credentials → capture victim's session
Cache poisoning → stored XSS at scale
Target-Suitability Matrix (2026 reality check)
The classic CL.TE / TE.CL payloads are NOT universally exploitable in 2026. Modern proxies are RFC 9112 strict by default. Fingerprint the front-end BEFORE investing time.
| Front-end | CL.TE | TE.CL | H2.CL | H2.TE | Notes |
|---|---|---|---|---|---|
| Nginx ≥ 1.21 | NO | NO | partial (H2 ingress) | partial | RFC-strict; rejects CL+TE with HTTP 400. Verified locally on Nginx 1.27 — all 9 documented variants killed by front-end (docs/verification/phase2h-smuggling-cachepoison.md). |
| Caddy 2.x | NO | NO | — | — | Hardened by default |
| Envoy ≥ 1.20 | NO | NO | partial | partial | Hardened in most paths |
| HAProxy ≤ 2.4 | ✓ | ✓ | — | — | Vulnerable, see CVE-2021-40346 |
| AWS ALB + specific upstream | partial | partial | ✓ | ✓ | Several disclosed-paid reports 2022-2024 |
| Cloudflare → S3 / Lambda chains | — | — | ✓ | ✓ | H2-downgrade attacks remain viable |
| Older F5 BIG-IP (TMM < 16) | ✓ | — | — | — | Vendor advisories |
| Citrix ADC / NetScaler (older firmware) | ✓ | ✓ | — | — | Disclosed in 2020-2022 |
| Squid 3.x | ✓ | — | — | — | Older deployments |
| Apache Traffic Server (older) | ✓ | ✓ | ✓ | ✓ | PortSwigger research |
| Apache mod_proxy_ajp → Tomcat | — | — | — | — | Cross-protocol HTTP→AJP desync (CVE-2022-26377); smuggled request is opaque to the WAF and reaches internal AJP admin/status paths that lack the external auth controls |
| Custom Python / Go proxies | ✓ | ✓ | — | — | Frequently miss RFC enforcement |
Operator fingerprint quick-check
curl -sI https://target/ | grep -i "Server:"
nginx/1.21+,Caddy,envoy→ CL/TE classic is dead — pivot to H2.CL/H2.TE if the front-end speaks HTTP/2, or look for legacy proxies upstreamHAProxy, header points to AWS/CDN → run the full payload matrix- No Server header → assume hardened, but run a single quick
space-before-colonprobe; if it doesn't 400, dig deeper
H2.CL / H2.TE (the modern dominant vector)
H2-downgrade smuggling attacks rely on the front-end speaking HTTP/2 to the client and HTTP/1.1 to origin. The downgrade introduces CL/TE confusion because HTTP/2's frame-length headers don't survive the conversion cleanly. Most CDN+origin chains in 2024-2026 use this exact topology.
Tools that send HTTP/2 raw frames (Burp Pro's HTTP Request Smuggler extension, h2csmuggler, smuggler.py) are the right starting point against CDN-fronted targets. Avoid HTTP/1.1-only test clients (curl, raw sockets) against H2-front-ended targets — you'll send the wrong protocol entirely.
Mass credential harvesting — the "collector gadget"
The highest-impact smuggling outcome needs no per-victim interaction. Instead of blindly poisoning the queue, smuggle a request aimed at a back-end handler that echoes the full request — a search endpoint that reflects headers, or a redirect that mirrors the request line. The next victim's headers (Cookie, Authorization, X-Access-Token) get attributed to your smuggled request, and the reflecting handler returns them in a response you read. Repeated on a busy keep-alive socket, this harvests live credentials from arbitrary users at scale — and it works even through a CDN (Akamai/Cloudflare) when the CDN↔origin hop desyncs. Chains to hunt-ato.
Related Skills & Chains
hunt-cache-poison— Smuggling + cache is the canonical critical chain; one smuggled request becomes the cached response for every subsequent victim. Chain primitive: CL.TE smuggle a request whose response body contains attacker HTML/JS → front-end cache stores it under a popular URL (/,/login) → de-sync poisoning where the smuggled request becomes the cached response for the next N victims, persisting for the cache TTL.hunt-auth-bypass— Smuggling reaches internal-only routes that the front-end WAF/auth-proxy filters out. Chain primitive: smuggleGET /admin/users HTTP/1.1past the front-end ACL that blocks external/admin/*→ backend processes the smuggled request as if from a trusted internal source → bypass front-end auth by smuggling internal-routed request → admin data in the response queue.hunt-idor— Smuggling attaches the NEXT user's session cookies to an attacker-controlled request path. Chain primitive: smuggleGET /api/me HTTP/1.1with no cookies → backend pairs it with the next legitimate user's incoming connection cookies → victim's session cookie attached to attacker's smuggled request → attacker reads the response containing victim's PII/tokens.hunt-xss— Smuggling injects XSS payloads into the response stream of the next victim without ever appearing in a URL parameter. Chain primitive: smuggled request body contains reflected payload that the backend renders into the next response in the queue → next visitor to/receives attacker HTML inline → reflected XSS at every visitor without any URL parameter visible to them or to logs.security-arsenal— Reach for the smuggling payload bank (CL.TE / TE.CL / TE.TE obfuscations, H2.CL downgrade probes, h2csmuggler one-liners, Burp HTTP Request Smuggler extension config) and the time-delay confirmation template before manual hex-editing.triage-validation— Run the Pre-Severity Gate before claiming Critical: the smuggled-request effect MUST land on a request issued by a different client/session, not your own follow-up. A timing delta in your own browser alone is parser disagreement, not exploitable smuggling.