Vulnerability

Styx is vulnerable to CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Response Splitting').

Vulnerable Component

The vulnerable component is the com.hotels.styx.api.HttpHeaders.Builder due to disabling the HTTP Header validation built into Netty in these locations:

https://github.com/HotelsDotCom/styx/blob/e1d578e9b9c38df9cd19c21dc2eb9b949d85b558/components/api/src/main/java/com/hotels/styx/api/HttpHeaders.java#L145

https://github.com/HotelsDotCom/styx/blob/e1d578e9b9c38df9cd19c21dc2eb9b949d85b558/components/api/src/main/java/com/hotels/styx/api/HttpHeaders.java#L145

new DefaultHttpHeaders(false) disables the built-in validation in Netty. Either use the default constructor or new DefaultHttpHeaders(true instead.

Additionally, another vulnerable component is the StyxToNettyResponseTranslator due to also disabling the HTTP Header validation built into netty in this location.

https://github.com/HotelsDotCom/styx/blob/8d60e5493e65d0d536afc0b350dcb02d24e0f7a7/components/server/src/main/java/com/hotels/styx/server/netty/connectors/StyxToNettyResponseTranslator.java#L30

DefaultHttpResponse nettyResponse = new DefaultHttpResponse(version, httpResponseStatus, false); new DefaultHttpResponse(version, httpResponseStatus, false); disables the built-in validation in Netty. Please use the constructor new DefaultHttpResponse(version, httpResponseStatus, true);

Proof of Concept

The following test plugin proves that there is no header validation occurring.

    static class VulnerablePlugin implements Plugin {

        @Override
        public Eventual<LiveHttpResponse> intercept(LiveHttpRequest request, Chain chain) {
            String header = request.queryParam("header-value").get();
            LiveHttpRequest newRequest = request.newBuilder()
                .header("myRequestHeader", header)
                .build();
            return chain.proceed(newRequest).map(response ->
                response.newBuilder().header("myResponseHeader", header).build()
            ) ;
        }

    }

    @Test
    public void simpleHeaderInjectionVulnerabilityPOC() {
        Plugin vulnerablePlugin = new VulnerablePlugin();
        // a simple way to mock the downstream system
        HttpInterceptor.Chain chain = request -> {
            assertThat(request.header("myRequestHeader").orElse(null), is("test\r\nAnother: CRLF_Injection"));
            return Eventual.of(response(OK).build());
        };

        // an example request you expect your plugin to receive
        String encodedGet = URLEncoder.encode("test\r\nAnother: CRLF_Injection");
        LiveHttpRequest request = get("/foo?header-value=" + encodedGet)
            .build();

        // since this is a test, we want to wait for the response
        LiveHttpResponse response = Mono.from(vulnerablePlugin.intercept(request, chain)).block();

        assertThat(response.header("myResponseHeader").orElse(null), is("test\r\nAnother: CRLF_Injection"));
    }