Http Request Smuggling in Fastapi with Hmac Signatures

HTTP Request Smuggling arises when a frontend (like a load balancer or API gateway) and Fastapi interpret message boundaries differently. With Hmac Signatures, the risk is not in the cryptography itself, but in how signed requests are parsed and validated before application logic runs. If the boundary between the signature metadata and the request body is handled inconsistently—for example, the frontend strips or retransfers headers differently than Fastapi receives them—an attacker can craft requests where the signature validates successfully on one side but the body is interpreted differently on the other.

Consider a setup where a gateway adds or removes headers (such as Content-Length or Transfer-Encoding) before forwarding to Fastapi. An Hmac Signature typically includes selected headers in the signed string; if the gateway modifies those headers after signing, the signature may still appear valid to Fastapi if verification recomputes the hash over the mutated headers. This mismatch allows an attacker to smuggle a request: the signed path and the processed request diverge, enabling request splitting, cache poisoning, or unauthorized access to admin-only endpoints that the signature was intended to protect.

In Fastapi, if you rely on middleware that reads the raw stream to compute the Hmac and then passes the request downstream without strict boundary enforcement, you expose the unauthenticated attack surface that middleBrick tests (e.g., BOLA/IDOR and Unsafe Consumption checks). An attacker can send a request with multiple Content-Length values or mixed Transfer-Encoding and Content-Length, causing Fastapi to parse the body differently than the intended consumer. Because the signature may cover only a subset of headers or an earlier version of the body, the application might process the smuggled segment without detecting inconsistency.

middleBrick’s unauthenticated scans exercise these vectors by probing how Fastapi handles malformed framing when Hmac verification is present but header integrity is not enforced. Findings such as BFLA/Privilege Escalation or Property Authorization can emerge if a smuggled request elevates privileges or accesses properties the signature should have restricted. The scan does not assume internal architecture; it reports whether the endpoint is vulnerable to smuggling-induced logic bypass or data exposure, providing prioritized remediation steps aligned with OWASP API Top 10 and relevant compliance mappings.

To mitigate Request Smuggling when using Hmac Signatures in Fastapi, enforce strict header handling and canonicalization before verification. The core principle is to normalize the request the same way on both sides and to reject requests with ambiguous framing. Below are concrete code examples that implement these safeguards.

First, define a utility to extract and verify the Hmac signature using a consistent set of signed headers. This example signs content-type and x-request-timestamp, and it rejects requests with multiple Content-Length or unexpected Transfer-Encoding values.

import hashlib
import hmac
from fastapi import Fastapi, Request, HTTPException, Header
from typing import List

app = Fastapi()

SECRET_KEY = b"your-secure-secret"
SIGNED_HEADERS = {"content-type", "x-request-timestamp"}

def normalize_signed_headers(headers: dict) -> dict:
    return {k.lower(): v for k, v in headers.items() if k.lower() in SIGNED_HEADERS}

def compute_signature(headers: dict, body: bytes) -> str:
    canonical = "".join(f"{k}:{headers.get(k, '')}" for k in sorted(headers))
    message = canonical.encode() + body
    return hmac.new(SECRET_KEY, message, hashlib.sha256).hexdigest()

@app.middleware("http")
async def verify_hmac_and_validate_framing(request: Request, call_next):
    # Reject ambiguous transfer encodings
    te = request.headers.get("transfer-encoding", "")
    if te.lower() == "chunked":
        raise HTTPException(status_code=400, detail="Transfer-Encoding: chunked not allowed")
    # Ensure single Content-Length interpretation
    content_length = request.headers.get("content-length")
    if content_length is not None:
        try:
            cl_int = int(content_length)
            if cl_int < 0:
                raise HTTPException(status_code=400, detail="Invalid Content-Length")
        except ValueError:
            raise HTTPException(status_code=400, detail="Content-Length must be an integer")
    # Verify Hmac
    received_sig = request.headers.get("x-signature")
    if not received_sig:
        raise HTTPException(status_code=401, detail="Missing signature")
    body = await request.body()
    headers_to_sign = normalize_signed_headers(dict(request.headers))
    expected_sig = compute_signature(headers_to_sign, body)
    if not hmac.compare_digest(expected_sig, received_sig):
        raise HTTPException(status_code=401, detail="Invalid signature")
    response = await call_next(request)
    return response

This middleware ensures that the request body is not processed twice with different interpretations. By rejecting Transfer-Encoding: chunked and enforcing a single valid Content-Length, you reduce the risk of the request being parsed differently at the boundary. The signature is computed over a canonical header set and body, preventing attackers from smuggling via reordered or omitted headers.

Second, if your architecture requires passing the request through a gateway that may modify headers, have the gateway forward the original raw request bytes to Fastapi (e.g., via a dedicated X-Original-Body header or by keeping the connection intact) so Fastapi verifies the signature against the exact bytes the sender intended. Alternatively, use a shared secret and a strict header allowlist so both sides compute the same canonical form. The following snippet shows how Fastapi can recompute the signature when headers might be transformed, by explicitly selecting only the signed headers and rejecting any that are missing or altered.

from fastapi import Header as FastapiHeader

async def get_required_header(name: str, request: Request):
    val = request.headers.get(name)
    if val is None:
        raise HTTPException(status_code=400, detail=f"Missing required header: {name}")
    return val

@app.post("/secure")
async def secure_endpoint(
    content_type: str = FastapiHeader(...),
    x_request_timestamp: str = FastapiHeader(...),
    x_signature: str = FastapiHeader(...),
    body: bytes = None
):
    headers = {
        "content-type": content_type,
        "x-request-timestamp": x_request_timestamp,
    }
    expected = compute_signature(headers, body or b"")
    if not hmac.compare_digest(expected, x_signature):
        raise HTTPException(status_code=401, detail="Invalid signature")
    return {"status": "ok"}

By combining strict framing rules, header canonicalization, and runtime verification, you align the behavior of Fastapi with the expectations of the signer, which is a key remediation pattern tested by middleBrick’s BOLA/IDOR and Property Authorization checks. This approach does not alter the Hmac algorithm but ensures that the boundary between verification and processing remains consistent, reducing the attack surface for smuggling.