FreeRDP versions before 3.32.0 allow an HTTP header injection flaw in the proxy component. The vulnerability arises when a line feed character in the target portion of a routing token named Cookie: msts= is accepted and propagated into the CONNECT request line and Host header by http_proxy_connect. This permits an attacker to inject arbitrary header fragments and alter the framing of requests that are sent through the RDP proxy to an upstream HTTP proxy. The weakness is identified as CWE‑93 and can potentially be used to perform HTTP response splitting, request smuggling, or other side‑channel attacks that depend on manipulation of HTTP headers. The impact is primarily integrity of the HTTP request and potential confidentiality or integrity leaks if the attacker can influence upstream responses. Affected systems include installations of FreeRDP, the open‑source remote desktop protocol implementation, with version numbers 3.0.0 up to, but not including, 3.32.0. Any deployment that uses the RDP client to connect through a proxy gateway and has the FixedTarget option disabled is susceptible. The flaw does not require authentication to the RDP service; any remote client that can initiate a connection to the proxy is able to craft a malicious routing token. Risk and exploitability assessment shows a CVSS score of 6.3, indicating a moderate severity. The EPSS score is not available, and the vulnerability is not currently listed in the CISA KEV catalog. The attack surface is remote, contingent on a client connecting through an upstream HTTP proxy that forwards the CONNECT method. An attacker would craft a request with a lone line feed in the routing token, causing the proxy to forward a malformed request line and manipulated Host header to the upstream proxy, breaking normal request framing. Such exploitation requires the ability to send a custom RDP client or a client with modified connection parameters.

Organizations running FreeRDP 3.0.0 or any earlier version must review their deployments, especially if they employ proxy routing with FixedTarget disabled.

The CVSS score of 6.3 denotes moderate risk, and the lack of an EPSS score suggests that empirical exploitation data is limited. While the flaw is not catalogued by CISA, the remote attack vector and potential for HTTP request manipulation warrant attention.