Hotfix: disable autoescape by default when rendering Jinja2 templates

changelog.d/8394.bugfix

@@ -0,0 +1 @@
+Fix URLs being accidentally escaped in Jinja2 templates. Broke in v1.20.0.

synapse/config/_base.py

@@ -194,7 +194,10 @@ def read_file(cls, file_path, config_name):
             return file_stream.read()
 
     def read_templates(
-        self, filenames: List[str], custom_template_directory: Optional[str] = None,
+        self,
+        filenames: List[str],
+        custom_template_directory: Optional[str] = None,
+        autoescape: bool = False,
     ) -> List[jinja2.Template]:
         """Load a list of template files from disk using the given variables.
 
@@ -210,6 +213,9 @@ def read_templates(
             custom_template_directory: A directory to try to look for the templates
                 before using the default Synapse template directory instead.
 
+            autoescape: Whether to autoescape variables before inserting them into the
+                template.
+
         Raises:
             ConfigError: if the file's path is incorrect or otherwise cannot be read.
 
@@ -233,7 +239,7 @@ def read_templates(
             search_directories.insert(0, custom_template_directory)
 
         loader = jinja2.FileSystemLoader(search_directories)
-        env = jinja2.Environment(loader=loader, autoescape=True)
+        env = jinja2.Environment(loader=loader, autoescape=autoescape)
 
         # Update the environment with our custom filters
         env.filters.update(

synapse/config/saml2_config.py

@@ -169,8 +169,10 @@ def read_config(self, config, **kwargs):
             saml2_config.get("saml_session_lifetime", "15m")
         )
 
+        # We enable autoescape here as the message may potentially come from a
+        # remote resource
         self.saml2_error_html_template = self.read_templates(
-            ["saml_error.html"], saml2_config.get("template_dir")
+            ["saml_error.html"], saml2_config.get("template_dir"), autoescape=True
         )[0]
 
     def _default_saml_config_dict(