utls 2026.9.21

BoringSSL-backed TLS for CPython, with Chrome impersonation

Drop-in replacement for the ssl stdlib with a Fingerprint API for Chrome impersonation. Built on stock BoringSSL through boring-sys - no curl-impersonate, no Go runtime at import time. Use it with any known Python http client you like or used to.

Supports CPython 3.7 onward, including freethreaded builds.

Why this exists

We sat on this for a long time, kept it private as we needed to think longer about whether the world really needed one more TLS library.

The initial thinking was: why do I have to install yet whole another http client just because we need to look like a real browser? The Python ecosystem already ships perfectly fine http clients - urllib3, niquests, httpx, aiohttp - and all of them ride on top of ssl. If the impersonation lives one layer down, in the SSL layer itself, every one of them gets it for free. No fork of curl, no Go runtime imported at module load, no second http client to teach your codebase, no patched BoringSSL to maintain across CVE cycles.

That is what utls is: the ClientHello machinery, plugged in exactly where the rest of Python already plugs in.

It was private until now. Enjoy it, but with care - browser impersonation has legitimate uses (interop testing, anti-bot researches, making sure your own service still answers a real Chrome correctly) and it has illegitimate ones. The license forbids nothing; your judgment does.

Contributions are welcome. New Chrome versions are one profile module plus one registry line; platform fixes, bug reports and CI cleanups are read and merged.

Getting Started

Install from PyPI (pre-built wheels for Linux, macOS, and Windows):

pip install utls

Then swap ssl for utls anywhere in your code:

import utls as ssl

ctx = ssl.create_default_context()

Or use it alongside the stdlib:

from utls import SSLContext, PROTOCOL_TLS_CLIENT

ctx = SSLContext(PROTOCOL_TLS_CLIENT)
ctx.load_default_certs()

import socket
sock = socket.create_connection(("example.com", 443))
ssock = ctx.wrap_socket(sock, server_hostname="example.com")
ssock.sendall(b"GET / HTTP/1.1\r\nHost: example.com\r\n\r\n")
print(ssock.recv(4096).decode())

It works with asyncio out of the box:

import asyncio
import utls as ssl

async def main():
    ctx = ssl.create_default_context()
    reader, writer = await asyncio.open_connection("example.com", 443, ssl=ctx)
    writer.write(b"GET / HTTP/1.1\r\nHost: example.com\r\n\r\n")
    await writer.drain()
    print((await reader.read(4096)).decode())
    writer.close()

asyncio.run(main())

Browser impersonation

import utls, socket

ctx = utls.create_default_context()
ctx.set_fingerprint("chrome:stable")

with socket.create_connection(("www.google.com", 443)) as raw:
    with ctx.wrap_socket(raw, server_hostname="www.google.com") as s:
        s.sendall(b"GET / HTTP/1.1\r\nHost: www.google.com\r\nConnection: close\r\n\r\n")
        print(s.recv(4096))

The bundled profile registry can be inspected at runtime:

from utls import Fingerprint, presets

print(presets())
# ['chrome:131', 'chrome:142', 'chrome:146', 'chrome:148', 'chrome:150', 'chrome:152', 'chrome:stable']

fp = Fingerprint.from_preset("chrome:stable")
print(fp.ja3_hash, fp.ja4_hash)

Profiles ship as plain Python modules under utls.profiles.*. utls is Chrome-only by design. Adding a new Chrome version is one .py file plus one registry line.

What impersonation actually rewrites

set_fingerprint controls the TLS ClientHello bytes: cipher suite list and order, extension list and order, named groups, key shares, signature algorithms, ALPN/ALPS payloads, GREASE placement, certificate compression, explicit ECH GREASE or real config, and so on. Everything that goes into a JA3 or JA4 hash, utls drives.

What it does not rewrite, because they live above TLS:

• The HTTP request line, method, path, and body.
• HTTP/2 SETTINGS frames, WINDOW_UPDATE values, HEADERS frame priority flags, and the Akamai HTTP/2 fingerprint that some bot-detection vendors track. Those belong to your HTTP client; pair utls with niquests or urllib3-future if you need them honored end-to-end.

Carrying the canonical Chrome header set

Each profile also publishes the exact HTTP request-header set Chrome sends on a top-level navigation, in the on-the-wire insertion order. The HTTP layer is yours to drive, but the headers are right there if you want matched cosmetics:

from utls import Fingerprint

fp = Fingerprint.from_preset("chrome:stable")
for name, value in fp.http_headers.items():
    print(f"{name}: {value}")
# sec-ch-ua: "Chromium";v="148", "Google Chrome";v="148", "Not/A)Brand";v="99"
# sec-ch-ua-mobile: ?0
# sec-ch-ua-platform: "Linux"
# User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 ...
# Accept: text/html,application/xhtml+xml,...
# ...

Request- or session-dependent headers (Host, Cookie, Referer, Content-Length, Content-Type) are deliberately omitted from the dict; the HTTP client is responsible for emitting those. For HTTP/1.1, prepend Host at position 0 to preserve Chrome's wire order.

Choosing a profile

• chrome:stable always tracks the newest Chrome major shipped in a utls release. Use this if you want to drift with Chrome without code changes.
• chrome:152, chrome:150, chrome:148, chrome:146, chrome:142, chrome:131 pin to specific Chrome majors. Use these when reproducibility matters - e.g. a long-running scraper that should not silently change shape when utls is upgraded.

The JA4 hash is the fingerprint identity that bot-detection vendors and TLS observatories actually index on. chrome:142 / chrome:146 / chrome:148 share the same JA4 because they share the same TLS-layer ClientHello (the deltas are HTTP-layer); chrome:131 differs because it uses the legacy ALPS codepoint (0x4469 vs 0x44cd). chrome:150 also differs: it is the first stable major to advertise post-quantum ML-DSA (FIPS 204) signature algorithms, which changes the signature_algorithms bytes and therefore the JA4. chrome:152 adds the trust_anchors extension (0xCA34), taking the JA4 extension count from 16 to 17.

Why JA3 varies but JA4 stays put

Chrome 110+ permutes its TLS extension order on every connection to defeat order-based fingerprinting. utls mirrors that behavior. As a result:

• JA3 hashes the wire order verbatim, so fp.ja3_hash (and the hash you'd compute from a live capture) shifts per connection. Use the JA3 string for diffing the extension set, not the hash for equality.
• JA4 sorts extension codepoints before hashing, so fp.ja4_hash is stable and the value that matches your live capture.

Capturing a fingerprint from a live ClientHello

If you have raw ClientHello bytes (e.g. captured via tcpdump, mitmproxy, or a custom TLS trap), you can rebuild a Fingerprint from them and feed it back into a context:

from utls import Fingerprint, SSLContext

raw = open("clienthello.bin", "rb").read()
fp = Fingerprint.from_capture(raw)
print(fp.ja4_hash)

ctx = SSLContext()
ctx.set_fingerprint(fp)

Captured fingerprints carry no HTTP headers (fp.http_headers == {}); they only encode the TLS layer. To round-trip a full profile, use fp.to_dict() + Fingerprint.from_dict(...) and add http_headers manually.

Server-side and impersonation

set_fingerprint on a server-side context raises at the Rust level. The ClientHello is, by definition, the client's choice; a server-side fingerprint would be meaningless on the wire. Server contexts can still inspect the peer ClientHello for diagnostics (see the Server-side section below).

Encrypted Client Hello (ECH)

Chrome profiles ship with ECH GREASE enabled by default, matching real Chrome. To offer real ECH - encrypting the inner ClientHello (including the real SNI) under the server's published HPKE config - fork a per-peer context carrying the wire-format ECHConfigList bytes:

import utls, socket

base = utls.SSLContext()
base.load_default_certs()
base.set_fingerprint("chrome:stable")

# ECH configs are peer-specific* - each origin publishes its own in a DNS
# HTTPS RR. `set_ech_configs` is non-mutating: it returns a NEW SSLContext
# that shares the underlying SSL_CTX (and CA store) with `base` via
# SSL_CTX_up_ref, so the fork is cheap. `base` is untouched and can be
# re-forked for other peers.
ech_bytes = ...  # you are responsible to get it yourself!
ctx = base.set_ech_configs(ech_bytes)

with socket.create_connection(("example.com", 443)) as raw:
    ssl_sock = ctx.wrap_socket(raw, server_hostname="example.com")
    ssl_sock.do_handshake()

utls does not perform the DNS HTTPS RR lookup; the caller fetches the ech= bytes (e.g. via dnspython's HTTPS rdata). Pass None to clear the ECH override in the fork.

Niquests or urllib3-future does ECH transparently via a custom resolver (E.g. DNS over HTTPS) No effort required.

Feature parity with stdlib

utls re-exports the public names of the ssl module that matter for client and server code paths. The Python facade is a real subclass of ssl.SSLContext, so isinstance(ctx, ssl.SSLContext) is true and most downstream libraries (urllib3-future, niquests, httpx with custom transports) accept a utls context directly.

What's different from ssl:

• TLS 1.2 is the minimum version. SSLv2, SSLv3, TLS 1.0 and TLS 1.1 are not available - BoringSSL refuses to negotiate them (No decent client out there should try < TLS 1.2).
• Hostname verification uses SAN only, never the Common Name.
• compression() always returns None (TLS compression is disabled, as it should be - BoringSSL doesn't ship it - as anyone should).
• PSK callbacks are not available - BoringSSL deleted them upstream.
• A Fingerprint API exists and is honored on the client side. Server-side set_fingerprint rejects at the Rust level: the ClientHello is the client's choice, not the server's.

Server-side

Server-side TLS is supported for everything BoringSSL still exposes:

• mTLS (client certificate auth via verify_mode=CERT_REQUIRED),
• ALPN selection from a server preference list,
• SNI dispatch via set_servername_callback,
• ECDH curve restriction (set_ecdh_curve),
• Session ticket count (set_num_tickets),
• set_session_id_context,
• Server-side ClientHello fingerprinting (read-only diagnostics).

Things you can't have on the server side because BoringSSL doesn't have them: DTLS, FFDHE parameters, SSLv2/SSLv3, TLS compression, PSK.

urllib3-future / niquests

utls is automatically picked up if installed. enjoy.

Disclaimer

Early/Beta project. The public API is stable; we do not plan to diverge from stdlib for the ssl-compatible subset. Not pure Python - you need either a pre-built wheel or a build environment (Rust + cmake + ninja + Go for BoringSSL).

MIT-licensed (see LICENSE). BoringSSL is permissively licensed; boring-sys is Apache-2.0 - bundled into the wheel.

• FIPS mode is not on the roadmap.
• Chrome only. Firefox / Safari / Edge are unlikely. Thus, contribution are welcomed.
• PyPy is not supported.

Throughput is on par with stdlib ssl: ~343 MiB/s for bulk recv vs. stdlib's 340 MiB/s on a loopback TLS 1.3 / AES-256-GCM connection. (yes, it is faster than rtls!)

Contributions, bug reports and feedback are welcome.

Versioning

This project uses CalVer (YYYY.0M.0D). It aims to be a drop-in replacement for stdlib ssl, so semantic versioning would be misleading - pin a lower bound, not an upper bound.

Prior art

• rtls - same drop-in approach over rustls. utls's MemoryBIO-first design and read-batching pass are directly inspired by it; many of the compatibility tests were ported from there.
• uTLS - the reference Go-language TLS fingerprinting library. The spec data encoded in our Chrome profiles is checked against uTLS's published ClientHellos.
• curl-impersonate - the original "browser-on-the-wire" project. utls aims to give Python the same capability without a non-native Python http client or a patched BoringSSL.

JA4 pinning

The JA4 fingerprint algorithm is pinned to FoxIO-LLC/ja4 spec revision 0.18.8. The constant is duplicated in python/utls/_fingerprint.py (JA4_SPEC_VERSION) and crates/utls-core/src/fingerprint/ja4.rs, and a dedicated CI job asserts they agree.

Documentation

For the ssl-compatible surface, the stdlib documentation at https://docs.python.org/3/library/ssl.html applies almost verbatim - the notable exceptions are listed under Feature parity with stdlib.