diff --git a/endpoints/trigger.py b/endpoints/trigger.py
index ffab858df..a45588feb 100644
--- a/endpoints/trigger.py
+++ b/endpoints/trigger.py
@@ -8,6 +8,7 @@ import json
 
 from github import Github, UnknownObjectException, GithubException
 from tempfile import SpooledTemporaryFile
+from jsonschema import validate
 
 from app import app, userfiles as user_files, github_trigger
 from util.tarfileappender import TarfileAppender
@@ -547,6 +548,43 @@ class GithubBuildTrigger(BuildTrigger):
     return None
 
 class CustomBuildTrigger(BuildTrigger):
+  payload_schema = {
+    'type': 'object',
+    'properties': {
+      'commits': {'type': 'string'},
+      'ref': {'type': 'string'},
+      'default_branch': {'type': 'string'},
+      'commit_info': {
+        'type': 'object',
+        'properties': {
+          'url': {'type': 'string'},
+          'message': {'type': 'string'},
+          'date': {'type': 'string'},
+          'author': {
+            'type': 'object',
+            'properties': {
+              'username': {'type': 'string'},
+              'url': {'type': 'string'},
+              'avatar_url': {'type': 'string'},
+            },
+            'required': ['username', 'url', 'avatar_url'],
+          },
+          'committer': {
+            'type': 'object',
+            'properties': {
+              'username': {'type': 'string'},
+              'url': {'type': 'string'},
+              'avatar_url': {'type': 'string'},
+            },
+            'required': ['username', 'url', 'avatar_url'],
+          },
+        },
+        'required': ['url', 'message', 'date'],
+      },
+    },
+    'required': ['commits', 'ref', 'default_branch'],
+  }
+
   @classmethod
   def service_name(cls):
     return 'custom'
@@ -554,51 +592,12 @@ class CustomBuildTrigger(BuildTrigger):
   def is_active(self, config):
     return 'public_key' in config
 
-  @staticmethod
-  def _metadata_from_payload(payload):
+  def _metadata_from_payload(self, payload):
     try:
-      metadata = {
-        'commit_sha': payload['commit'],
-        'ref': payload['ref'],
-        'default_branch': payload.get('default_branch', 'master'),
-      }
-    except KeyError:
+      validate(json.loads(payload), self.payload_schema)
+    except:
       raise InvalidPayloadException()
-
-    commit_info = payload['commit_info']
-    if commit_info is not None:
-      try:
-        metadata['commit_info'] = {
-            'url': commit_info['url'],
-            'message': commit_info['message'],
-            'date': commit_info['date'],
-        }
-      except KeyError:
-        raise InvalidPayloadException()
-
-      author = commit_info['author']
-      if author is not None:
-        try:
-          metadata['commit_info']['author'] = {
-            'username': author['username'],
-            'avatar_url': author['avatar_url'],
-            'url': author['url'],
-          }
-        except KeyError:
-          raise InvalidPayloadException()
-
-      committer = commit_info['committer']
-      if committer is not None:
-        try:
-          metadata['commit_info']['committer'] = {
-            'username': committer['username'],
-            'avatar_url': committer['avatar_url'],
-            'url': committer['url'],
-          }
-        except KeyError:
-          raise InvalidPayloadException()
-
-    return metadata
+    return payload
 
   def handle_trigger_request(self, request, trigger):
     payload = request.get_json()