HTTP Response Smuggling is a technique that attackers use to manipulate and inject malicious content in the form of unauthorized HTTP responses, into a single HTTP response from a vulnerable or compromised back-end HTTP agent.
In the maliciously manipulated HTTP response, an attacker can add duplicate header fields that HTTP agents interpret as belonging to separate responses. The combined HTTP response ends up being parsed or interpreted as two or more HTTP responses by the targeted client HTTP agent. This allows malicious HTTP responses to bypass security controls.