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.
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.
Werkzeug parses the incoming data under the following situations:
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.
The standard Werkzeug parsing behavior handles three cases:
Special note on the data attribute: Accessing 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 access 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.
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)