iam: take the implicit user name from the verified identity (#11903)

* iam: take the implicit user name from the verified identity

For self-service actions sent without UserName, the implicit user was
resolved by re-parsing the raw Authorization header for a SigV4
credential and looking that access key up. The header is not what
authentication verified: a request can be authenticated by one
mechanism while carrying a header that names another user's access
key, and the action then ran against that other user.

The auth middleware already records the verified identity in the
request context, so use it, on both the standalone iam server and the
embedded IAM surface. When no verified identity is present the name is
left unset rather than guessed.

* iam: test implicit user name resolution

Cover a request authenticated by presigned credentials that also
carries a junk SigV4 header naming another user: the self-service
actions must run as the verified signer. Update the unit tests for the
context-based lookup.

* iam: resolve the implicit user for standalone GetUser too

The embedded surface fills UserName for GetUser; the standalone server
skipped it, so an implicit GetUser on `weed iam` answered NoSuchEntity.
Same verified-identity source as the other self-service actions.

* iam: cover all implicit-username actions in the regression test

weed/s3api/s3api_embedded_iam.go

@@ -2594,53 +2594,19 @@ func (e *EmbeddedIamApi) ListGroupPolicies(s3cfg *iam_pb.S3ApiConfiguration, val
 	return resp, nil
 }
 
-// handleImplicitUsername adds username who signs the request to values if 'username' is not specified.
-// According to AWS documentation: "If you do not specify a user name, IAM determines the user name
-// implicitly based on the Amazon Web Services access key ID signing the request."
-// This function extracts the AccessKeyId from the SigV4 credential and looks up the corresponding
-// identity name in the credential store.
+// handleImplicitUsername adds the name of the authenticated caller to values
+// when 'UserName' is not specified, per AWS: "If you do not specify a user name,
+// IAM determines the user name implicitly based on the Amazon Web Services
+// access key ID signing the request." The name comes from the identity the
+// authentication middleware verified, which the request context carries; the
+// Authorization header itself is attacker-controlled and is never re-parsed.
 func (e *EmbeddedIamApi) handleImplicitUsername(r *http.Request, values url.Values) {
-	if len(r.Header["Authorization"]) == 0 || values.Get("UserName") != "" {
+	if values.Get("UserName") != "" {
 		return
 	}
-	// Log presence of auth header without exposing sensitive signature material
-	glog.V(4).Infof("Authorization header present, extracting access key")
-	// Parse AWS SigV4 Authorization header format:
-	// "AWS4-HMAC-SHA256 Credential=AKIAIOSFODNN7EXAMPLE/20130524/us-east-1/iam/aws4_request, ..."
-	s := strings.Split(r.Header["Authorization"][0], "Credential=")
-	if len(s) < 2 {
-		return
+	if name := s3_constants.GetIdentityNameFromContext(r); name != "" {
+		values.Set("UserName", name)
 	}
-	s = strings.Split(s[1], ",")
-	if len(s) < 1 {
-		return
-	}
-	s = strings.Split(s[0], "/")
-	if len(s) < 1 {
-		return
-	}
-	// s[0] is the AccessKeyId
-	accessKeyId := s[0]
-	if accessKeyId == "" {
-		return
-	}
-	// Nil-guard: ensure iam is initialized before lookup
-	if e.iam == nil {
-		glog.V(4).Infof("IAM not initialized, cannot look up access key")
-		return
-	}
-	// Look up the identity by access key to get the username
-	identity, _, found := e.iam.LookupByAccessKey(accessKeyId)
-	if !found {
-		// Mask access key in logs - show only first 4 chars
-		maskedKey := accessKeyId
-		if len(accessKeyId) > 4 {
-			maskedKey = accessKeyId[:4] + "***"
-		}
-		glog.V(4).Infof("Access key %s not found in credential store", maskedKey)
-		return
-	}
-	values.Set("UserName", identity.Name)
 }
 
 // iamSelfServiceActions are actions that users can perform on their own resources without admin rights.