Web Bot Auth — verifiable bot identity

Web Bot Auth lets a bot prove who it is by signing each HTTP request with a key it controls. Sites can then allow or block specific bots without IP allow-lists, user-agent strings, or guesswork. Built on RFC 9421 HTTP Message Signatures.

What it is #

Web Bot Auth is an emerging convention that lets a bot prove its identity cryptographically on every request, using the standard HTTP Message Signatures mechanism from RFC 9421. Instead of guessing whether a request really comes from OpenAI’s crawler by inspecting the user-agent string and looking up reverse DNS, the server reads a Signature header, fetches the bot’s public key from a published key directory, and verifies the signature.

The work now has an IETF home. In September 2026 the Web Bot Auth working group adopted draft-ietf-webbotauth-httpsig-protocol as its first working-group document, folding in the separate key-directory draft that used to sit alongside it. One document now covers the trust model, the signing rules, the Signature-Agent header used to discover a bot’s keys, and the JWK Set that header points at — served, the draft asks IANA to register, from /.well-known/http-message-signatures-directory. Cloudflare ships verification at the network edge, and a growing list of major crawlers sign their traffic.

Working-group adoption is not publication. The draft is still a draft, its details can and will change before it becomes an RFC, and the well-known URI it names is requested rather than registered. What adoption does tell you is that the mechanism is no longer one vendor’s proposal: it has a chartered group, chairs, and a deliverable date, so building against it is a bet on a process rather than on a company.

Why it matters #

• User-agent strings lie. Anyone can setUser-Agent: GPTBot/1.0 . Signed requests cannot be forged without the bot operator’s private key.
• IP allow-lists rot. Crawler IP ranges change. A signature-based check survives infrastructure migrations on the bot’s side.
• Granular policy. Once you can verify the caller, you can apply different rules — paywall bypass for partner agents, slower rate limits for low-trust crawlers — without bespoke detection.
• Composable with Content Signals and robots.txt for AI crawlers. robots.txt declares the policy; Web Bot Auth proves the identity the policy is about to be applied to.

Treat it as optional for now. The draft is pre-RFC, the verifier ecosystem is small, and most sites will get the benefit transparently via their CDN before they touch any code. But the direction is clear: bot identity is moving from “trust the header” to “verify the signature”.

How to implement #

If you are running a site:

• Use a verifier that supports your signing profile. Follow its documented identity result. If verification happens at a proxy, accept that result only over a trusted proxy-to-origin path; strip client-supplied copies of identity headers.
• Combine, do not replace. Web Bot Auth tells you who is calling.robots.txt andContent Signals tell you what they may do with the response. Both layers are needed.
• Keep authorisation separate. A valid signature establishes a key/identifier association, not permission or good behaviour. Apply your own policy to verified identifiers and retain your existing defaults for unsigned traffic.

If you operate a bot:

• Generate an asymmetric signing keypair. The draft restricts you to algorithms in the RFC 9421 registry and rules out shared-secret HMAC outright — a symmetric key would have to be handed to every site that wants to verify, which defeats the point.
• Publish the public key as a JWK Set. For default directory discovery, serve it athttps://bot.example/.well-known/http-message-signatures-directory with media typeapplication/http-message-signatures-directory+json . SendSignature-Agent: sig1="https://bot.example" : the value is an HTTPSorigin , not the well-known file’s URL. For a direct JWK Set URL instead, sendSignature-Agent: sig1="https://bot.example/keys.json";type=jwks_uri .
• Use the same label (sig1 here) inSignature ,Signature-Input andSignature-Agent . Cover the matching dictionary member with"signature-agent";key="sig1" inSignature-Input , alongside the required target component. Seedraft sections 5.2.1 and 5.5 .
• Sign every request withSignature andSignature-Input per RFC 9421, covering@authority or@target-uri and carrying thecreated ,expires ,keyid , andtag parameters.tag must beweb-bot-auth , which is what lets a verifier tell this profile apart from other uses of message signatures on the same connection.
• Rotate keys without breaking verifiers: allow for cached directories and outstanding signatures when overlapping keys; remove compromised keys promptly.

Common mistakes #

• Blocking unsigned traffic as a default. The standard is opt-in for bots; legitimate non-signing clients (including most browsers) will be locked out.
• Skipping created andexpires , or accepting stale timestamps. Both are mandatory signature parameters in the draft; without a freshness window a captured signature replays forever.
• Verifying only the homepage. Bots fetch internal pages too; the policy has to apply site-wide.
• Assuming a signature authenticates User-Agent . It protects only the components it covers; the profile does not require that header to be signed.

Verification #

• Send an actually signed request using your signing client. Confirm the verifier reports the expected identifier, then change a covered component and confirm verification fails. Placeholder signature values cannot test success.
• For bot operators: feed your signed request into an RFC 9421 verifier and confirm the canonicalised signature base matches what your client constructed.
• Check your access logs for a verified-bot tag on traffic from signing crawlers (OpenAI, Anthropic, Perplexity, and others publish their key sets).

Related topics #

Sources & further reading #

- [RFC 9421 — HTTP Message Signatures](https://www.rfc-editor.org/rfc/rfc9421) — IETF
- [draft-ietf-webbotauth-httpsig-protocol — HTTP Message Signatures for automated traffic](https://datatracker.ietf.org/doc/draft-ietf-webbotauth-httpsig-protocol/) — IETF Web Bot Auth Working Group
- [IETF Web Bot Auth (webbotauth) Working Group](https://datatracker.ietf.org/wg/webbotauth/about/) — IETF
- [Cloudflare — Forget IPs: using cryptography to verify bot and agent traffic](https://blog.cloudflare.com/web-bot-auth/) — Cloudflare