Bug on WebFlux When Headers Arrive With Spanish Words

[juacamilo99]

• Description: Currently, I am working on a Spanish project using Spring Web Flux where some requests´ headers contain accents, for example, "Fármaco". We notice some requests are arriving with an unusual format ISO on the header´s value. Taking the previous example, we are getting "Fármaco" only when the request comes with a body, no matter what kind of body, even with an empty body. When the request comes without body and header values with accents, it works fine. This is an example of the way we are managing the requests:

package local.demo;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.reactive.function.server.RouterFunction;
import org.springframework.web.reactive.function.server.ServerResponse;

import static org.springframework.web.reactive.function.server.RequestPredicates.all;
import static org.springframework.web.reactive.function.server.RouterFunctions.route;

@Configuration
public class DemoController {

    @Bean
    public RouterFunction<ServerResponse> routesRouterFunction(RoutesHandler routesHandler) {
        return route(all(), routesHandler::processRequest);
    }
}

package local.demo;

import org.springframework.stereotype.Component;
import org.springframework.web.reactive.function.server.ServerRequest;
import org.springframework.web.reactive.function.server.ServerResponse;
import reactor.core.publisher.Mono;

@Component
public class RoutesHandler {

    public Mono<ServerResponse> processRequest(ServerRequest serverRequest) {
        return ServerResponse.ok().bodyValue("Request processed");
    }
}

• Versions:

  • Srping Boot 3.3.11
  • Spring Webflux 6.1.19

• How to Reproduce: Create a simple service using Web Flux to manage the requests like the above example, to make it easier to reproduce, create a handler method with the objective of making debugging and capturing the request headers and their values. Throw a post request with any kind of body and headers using words in Spanish that use accents like "Fármaco" and see the headers value while debugging the code, like this:

[bclozel]

Thanks for reaching out, but I believe the issue lies with the HTTP client sending the request.
In general, only ASCII characters should be used in field values but historically we do support ISO_8859_1 encoded characters.

I have tried the following successfully:

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
class HeadersApplicationTests {

	@LocalServerPort
	int port;

	@Test
	void sendIsoHeaderValue() {
		WebClient webClient = WebClient.create("http://localhost:" + port);
		String result = webClient.get()
				.uri("/headers")
				.header("Test", "Héàder")
				.retrieve()
				.bodyToMono(String.class)
				.block();

		assertThat(result).isEqualTo("Héàder");
	}

}

@Component
public class RoutesHandler {

	public static final Logger log = LoggerFactory.getLogger(RoutesHandler.class);

	public Mono<ServerResponse> processRequest(ServerRequest serverRequest) {
		String value = serverRequest.headers().firstHeader("Test");
		log.info("header value: " + value);
		return ServerResponse.ok().bodyValue(value);
	}
}

The client is probably not sending ISO-encoded characters. I would encourage only using ASCII characters anyway as this is bound to create problems with proxies and network appliances along the way.

Note that Spring Boot 3.3 and Spring Framework 6.1 are only supported commercially at this point, so please upgrade to the latest OSS supported version as soon as possible (or consider our commercial subscription). This version of Spring Framework is vulnerable to multiple CVEs (https://spring.io/security/cve-2025-41242, https://spring.io/security/cve-2025-41249).