Dealing with Request Data

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2023-12-01

The most important rule about web development is “Do not trust the user”. This is especially true for incoming request data on the input stream. With WSGI this is actually a bit harder than you would expect. Because of that Werkzeug wraps the request stream for you to save you from the most prominent problems with it.

Missing EOF Marker on Input Stream

The input stream has no end-of-file marker. If you would call the read() method on the wsgi.input stream you would cause your application to hang on conforming servers. This is actually intentional however painful. Werkzeug solves that problem by wrapping the input stream in a special LimitedStream. The input stream is exposed on the request objects as stream. This one is either an empty stream (if the form data was parsed) or a limited stream with the contents of the input stream.

When does Werkzeug Parse?

Werkzeug parses the incoming data under the following situations:

you access either form, files, or stream and the request method was POST or PUT.
if you call parse_form_data().

These calls are not interchangeable. If you invoke parse_form_data() you must not use the request object or at least not the attributes that trigger the parsing process.

This is also true if you read from the wsgi.input stream before the parsing.

General rule: Leave the WSGI input stream alone. Especially in WSGI middlewares. Use either the parsing functions or the request object. Do not mix multiple WSGI utility libraries for form data parsing or anything else that works on the input stream.

How does it Parse?

The standard Werkzeug parsing behavior handles three cases:

input content type was multipart/form-data. In this situation the stream will be empty and form will contain the regular POST / PUT data, files will contain the uploaded files as FileStorage objects.
input content type was application/x-www-form-urlencoded. Then the stream will be empty and form will contain the regular POST / PUT data and files will be empty.
the input content type was neither of them, stream points to a LimitedStream with the input data for further processing.

Special note on the get_data method: Calling this loads the full request data into memory. This is only safe to do if the max_content_length is set. Also you can either read the stream or call get_data().

Limiting Request Data

To avoid being the victim of a DDOS attack you can set the maximum accepted content length and request field sizes. The BaseRequest class has two attributes for that: max_content_length and max_form_memory_size.

The first one can be used to limit the total content length. For example by setting it to 1024 * 1024 * 16 the request won’t accept more than 16MB of transmitted data.

Because certain data can’t be moved to the hard disk (regular post data) whereas temporary files can, there is a second limit you can set. The max_form_memory_size limits the size of POST transmitted form data. By setting it to 1024 * 1024 * 2 you can make sure that all in memory-stored fields is not more than 2MB in size.

This however does not affect in-memory stored files if the stream_factory used returns a in-memory file.

How to extend Parsing?

Modern web applications transmit a lot more than multipart form data or url encoded data. Extending the parsing capabilities by subclassing the BaseRequest is simple. The following example implements parsing for incoming JSON data:

from werkzeug.utils import cached_property
from werkzeug.wrappers import Request
from simplejson import loads

class JSONRequest(Request):
    # accept up to 4MB of transmitted data.
    max_content_length = 1024 * 1024 * 4

    @cached_property
    def json(self):
        if self.headers.get('content-type') == 'application/json':
            return loads(self.data)