diff --git a/specs/gloas/builder.md b/specs/gloas/builder.md
index d7105497..5418e41c 100644
--- a/specs/gloas/builder.md
+++ b/specs/gloas/builder.md
@@ -106,7 +106,8 @@ containing:
   `auth.message.slot` is the proposal slot the preferences apply to. The builder
   MUST verify the BLS signature against the `proposer_pubkey` path parameter and
   MUST check that `auth.message.data` matches the value it agreed with the
-  proposer, so that an unauthenticated or replayed request cannot skew a
+  proposer, or the [default auth data][default-auth-data] of its own URL when
+  none was agreed, so that an unauthenticated or replayed request cannot skew a
   proposer's preferences away from the value the proposer chose. If the
   signature fails to verify, the builder MUST return a 401 response; if the
   `auth.message.data` check fails, the builder MUST return a 400 response. The
@@ -161,8 +162,9 @@ the builder MAY serve a bid with any `execution_payment`.
 
 Builders MUST verify the `SignedBuilderRequestAuth` signature against the
 `proposer_pubkey` path parameter, and MUST check that `auth.message.data`
-matches the value they agreed with the proposer and that `auth.message.slot`
-matches the proposal `slot` path parameter (see
+matches the value they agreed with the proposer, or the
+[default auth data][default-auth-data] of their own URL when none was agreed,
+and that `auth.message.slot` matches the proposal `slot` path parameter (see
 [Constructing the `BuilderRequestAuth`][constructing-builder-request-auth]). The
 signature is verified with [`verify_builder_request_auth_signature`](#signing).
 If the signature fails to verify, the builder MUST return a 401 response; if the
@@ -257,6 +259,7 @@ documented in the [Gloas consensus specs][gloas-builder-specs].
 
 [bls]: https://github.com/ethereum/consensus-specs/blob/master/specs/phase0/beacon-chain.md#bls-signatures
 [constructing-builder-request-auth]: ./validator.md#constructing-the-builderrequestauth
+[default-auth-data]: ./validator.md#default-auth-data
 [get-execution-payload-bid-api]: ./../../apis/builder/execution_payload_bid.yaml
 [gloas-builder-specs]: https://github.com/ethereum/consensus-specs/blob/master/specs/gloas/builder.md
 [gloas-consensus-specs]: https://github.com/ethereum/consensus-specs/blob/master/specs/gloas
diff --git a/specs/gloas/validator.md b/specs/gloas/validator.md
index 5a7646b3..18b3dbbd 100644
--- a/specs/gloas/validator.md
+++ b/specs/gloas/validator.md
@@ -14,6 +14,7 @@
     - [`max_execution_payment`](#max_execution_payment)
   - [Bid Request](#bid-request)
     - [Constructing the `BuilderRequestAuth`](#constructing-the-builderrequestauth)
+      - [Default auth data](#default-auth-data)
   - [Proposer Preferences](#proposer-preferences)
   - [Validating a `SignedExecutionPayloadBid`](#validating-a-signedexecutionpayloadbid)
   - [Block proposal](#block-proposal)
@@ -145,9 +146,9 @@ The validator constructs a `BuilderRequestAuth` with the following fields:
   meaning is left to the two parties. It is not tied to an endpoint, so one
   `SignedBuilderRequestAuth` can authenticate the proposer for both
   `getExecutionPayloadBid` and `submitBuilderPreferences`. When no value has
-  been agreed out of band, the validator SHOULD use the UTF-8 bytes of the
-  builder's own advertised URL, exactly as advertised. A zero-length `data` is
-  invalid.
+  been agreed out of band, the validator SHOULD use the
+  [default auth data](#default-auth-data) derived from the builder URL. A
+  zero-length `data` is invalid.
 - `slot`: The proposal slot this request is authorized for, not the slot at
   which the request is signed or sent.
 
@@ -159,6 +160,38 @@ The validator then constructs the `SignedBuilderRequestAuth` by signing the
 validator and discard requests from other parties (e.g. DDOS or replay attempts
 from competing builders).
 
+#### Default auth data
+
+The default `data` for a builder URL is the builder's hostname: the lowercased
+ASCII hostname of the URL, an IPv6 literal written in its compressed form
+([RFC 5952][rfc-5952]) inside brackets. Scheme, userinfo, port, path, query and
+fragment are not part of the builder's identity and are dropped, so differences
+in how the URL is written, such as a trailing `/` or an explicit default port,
+do not change the signed bytes. An internationalized hostname MUST be given in
+its punycode form.
+
+```python
+def get_default_auth_data(url: str) -> bytes:
+    host = urlsplit(url).hostname  # lowercased, userinfo and port removed
+    assert host is not None and host.isascii()
+    if ":" in host:  # IPv6 literal
+        host = f"[{IPv6Address(host).compressed}]"
+    return host.encode("ascii")
+```
+
+| URL                                        | `data`                |
+| ------------------------------------------ | --------------------- |
+| `https://builder.example.com/`             | `builder.example.com` |
+| `HTTPS://Builder.Example.com:443/bids?x=1` | `builder.example.com` |
+| `https://builder.example.com:8080`         | `builder.example.com` |
+| `https://user:pw@builder.example.com/`     | `builder.example.com` |
+| `https://10.0.0.5:18550/eth/v1/builder`    | `10.0.0.5`            |
+| `https://[0:0:0:0:0:0:0:1]:8443/`          | `[::1]`               |
+
+A builder that needs a finer identity than its hostname, for example one host
+serving several builders on different paths or ports, agrees `data` with its
+proposers out of band instead.
+
 ## Proposer Preferences
 
 *Note*: Validator registrations (`ValidatorRegistrationV1`) are **deprecated**
@@ -260,6 +293,7 @@ block on top of a beacon `state` must take the following actions:
 [is-active-builder]: https://github.com/ethereum/consensus-specs/blob/master/specs/gloas/beacon-chain.md#new-is_active_builder
 [proposer-preferences]: https://github.com/ethereum/consensus-specs/blob/master/specs/gloas/p2p-interface.md#new-proposerpreferences
 [proposer-preferences-topic]: https://github.com/ethereum/consensus-specs/blob/master/specs/gloas/p2p-interface.md#new-proposer_preferences
+[rfc-5952]: https://www.rfc-editor.org/rfc/rfc5952
 [signed-execution-payload-bid]: https://github.com/ethereum/consensus-specs/blob/master/specs/gloas/beacon-chain.md#signedexecutionpayloadbid
 [signed-execution-payload-envelope]: https://github.com/ethereum/consensus-specs/blob/master/specs/gloas/beacon-chain.md#signedexecutionpayloadenvelope
 [submit-builder-preferences-api]: ./../../apis/builder/builder_preferences.yaml
diff --git a/types/gloas/request_auth.yaml b/types/gloas/request_auth.yaml
index e5e5e242..f901be00 100644
--- a/types/gloas/request_auth.yaml
+++ b/types/gloas/request_auth.yaml
@@ -11,10 +11,10 @@ Gloas:
           band. The meaning of the up to `MAX_BUILDER_AUTH_DATA_SIZE` (4096) bytes is left
           to the two parties; the builder checks the exact bytes when it
           verifies. When no value has been agreed out of band, implementations
-          SHOULD default to the UTF-8 bytes of the builder's own advertised
-          URL, exactly as advertised, so proposers with no prior relationship
-          can construct an identical `data` deterministically. A zero-length
-          `data` is invalid.
+          SHOULD default to the builder's hostname derived from its URL, see
+          [default auth data](https://github.com/ethereum/builder-specs/blob/master/specs/gloas/validator.md#default-auth-data),
+          so proposers with no prior relationship can construct an identical
+          `data` deterministically. A zero-length `data` is invalid.
         example: "0x1234567890abcdef"
         pattern: "^0x(?:[a-fA-F0-9]{2}){1,4096}$"
       slot:
diff --git a/wordlist.txt b/wordlist.txt
index f57d5419..29bbcd60 100644
--- a/wordlist.txt
+++ b/wordlist.txt
@@ -7,6 +7,7 @@ capella
 cli
 dev
 eth
+hostname
 http
 https
 json
@@ -16,6 +17,7 @@ nodejs
 npm
 oapi
 openapi
+punycode
 ssz
 tbh
 ui
@@ -23,6 +25,7 @@ uint
 unblind
 unblinded
 url
+userinfo
 validators
 validator's
 vc
@@ -34,7 +37,7 @@ fulu
 Gloas
 gloas
 Gwei
+IPv
 PTC
-UTF
 submitBlindedBlockV
 ValidatorRegistrationsV
\ No newline at end of file