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. Noti…

Affected: Packagist: bref/bref

## Impacted Resources

bref/src/Event/Http/Psr7Bridge.php:94-125 multipart-parser/src/StreamedPart.php:383-418

## Description

When Bref 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`](https://github.com/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`](https://www.php.net/manual/en/function.mb-convert-encoding.php) function is used with the first (`$string`) and third (`$from_encoding`) parameters read from the header value.

## Impact

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. - The Lambda should use the `RequestHandlerInterface` handler. - The Lambda 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). - If the Lambda uses a PHP…