Slow String Operations via MultiPart Requests in Event-Driven Functions

Bref is an open-source project that helps users go serverless on Amazon Web Services with PHP. When Bref prior to version 2.1.17 is used with the Event-Driven Function runtime and the handler is a RequestHandlerInterface, then the Lambda event is converted to a PSR7 object. During the conversion process, if the request is a MultiPart, each part is parsed. In the parsing process, the Content-Type header of each part is read using the Riverline/multipart-parser library.

The library, in the StreamedPart::parseHeaderContent function, performs slow multi-byte string operations on the header value.
Precisely, the mb_convert_encoding function is used with the first ($string) and third ($from_encoding) parameters read from the header value.

An attacker could send specifically crafted requests which would force the server into performing long operations with a consequent long billed duration.

The attack has the following requirements and limitations: The Lambda should use the Event-Driven Function runtime and the RequestHandlerInterface handler and should implement at least an endpoint accepting POST requests; the attacker can send requests up to 6MB long (this is enough to cause a billed duration between 400ms and 500ms with the default 1024MB RAM Lambda image of Bref); and if the Lambda uses a PHP runtime <= php-82, the impact is higher as the billed duration in the default 1024MB RAM Lambda image of Bref could be brought to more than 900ms for each request. Notic…