Security Advisory: Improper header validation in Slim-Psr7

A security issue has recently been reported in Slim-Psr7 with the handling of new lines in headers.


An attacker could sneak in a newline (\n) into both the header names and values. While the specification states that \r\n\r\n is used to terminate the header list, many servers in the wild will also accept \n\n. An attacker that is able to control the header names that are passed to Slilm-Psr7 would be able to intentionally craft invalid messages, possibly causing application errors or invalid HTTP requests being sent out with an PSR-18 HTTP client. The latter might present a denial of service vector if a remote service’s web application firewall bans the application due to the receipt of malformed requests.

Affected versions

All versions less than 1.6.1.


The issue is patched in version 1.6.1.


In Slim-Psr7 1.6.0 and below, validate HTTP header keys and/or values, and if using user-supplied values, filter them to strip off leading or trailing newline characters before calling withHeader().

For example:

$request = $request->withHeader(trim($headerName), trim($headerValue));


We are very grateful to and thank Graham Campbell for reporting this issue, providing a patch and working with us on this issue.

Further information