Security

SSRF protection

Server-Side Request Forgery (SSRF) is an attack where an attacker can induce a server-side application to make HTTP requests to an unintended destination. A common vector is open redirects: a request to a public URL returns a 3xx redirect pointing to an internal or private IP address (e.g. 127.0.0.1, 10.x.x.x, 169.254.169.254). If the client blindly follows the redirect, the attacker gains access to internal services.

By default, curl_cffi follows redirects (allow_redirects=True) without restricting the target, which mirrors the behavior of requests and httpx. This is fine for desktop applications and scripts, but dangerous in server-side contexts where user-supplied URLs are fetched.

Example: vulnerable FastAPI app

Consider a preview service that fetches a URL on behalf of the user:

from fastapi import FastAPI
from curl_cffi import requests

app = FastAPI()

@app.get("/preview")
def preview(url: str):
    # VULNERABLE: follows redirects to any destination
    r = requests.get(url)
    return {"body": r.text}

An attacker can exploit this by supplying a URL that redirects to an internal service:

GET /preview?url=https://evil.com/redirect

Where https://evil.com/redirect returns:

HTTP/1.1 302 Found
Location: http://169.254.169.254/latest/meta-data/iam/security-credentials/

The application follows the redirect and returns AWS instance credentials (or any other internal resource) to the attacker.

The fix is to use CurlFollow.SAFE:

from fastapi import FastAPI
from curl_cffi import CurlFollow, requests

app = FastAPI()

@app.get("/preview")
def preview(url: str):
    # SAFE: rejects redirects to internal/private IPs
    r = requests.get(url, allow_redirects=CurlFollow.SAFE)
    return {"body": r.text}

Using CurlFollow.SAFE

curl_cffi exposes a CurlFollow.SAFE option backed by curl-impersonate that rejects redirects to internal/private IP addresses:

from curl_cffi import CurlFollow, requests

# Recommended for server-side code that fetches user-supplied URLs
r = requests.get(url, allow_redirects=CurlFollow.SAFE)

# The string "safe" is also accepted
r = requests.get(url, allow_redirects="safe")

# Session-level setting
s = requests.Session(allow_redirects=CurlFollow.SAFE)

With CurlFollow.SAFE, redirects to the following address ranges are rejected:

• Loopback (127.0.0.0/8, ::1)

• Private networks (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16)

• Link-local (169.254.0.0/16, fe80::/10)

allow_redirects values

Value	Behavior
True (default)	Follow all redirects (same as CurlFollow.ALL)
False	Do not follow any redirects
CurlFollow.SAFE or "safe"	Follow redirects, but reject redirects to internal/private IPs
CurlFollow.ALL	Follow all redirects
CurlFollow.OBEYCODE	Follow redirects, reset custom method on 301/302/303
CurlFollow.FIRSTONLY	Only use custom method for the first request, reset for subsequent redirects

Recommendation

If your application fetches URLs provided by users or external systems, always use CurlFollow.SAFE:

from curl_cffi import CurlFollow, requests

session = requests.Session(allow_redirects=CurlFollow.SAFE)

This protects against SSRF without disabling redirects entirely.