Fixes #3847.

lib/pure/asyncnet.nim

@@ -388,7 +388,7 @@ proc accept*(socket: AsyncSocket,
   return retFut
 
 proc recvLineInto*(socket: AsyncSocket, resString: FutureVar[string],
-    flags = {SocketFlag.SafeDisconn}) {.async.} =
+    flags = {SocketFlag.SafeDisconn}, maxLength = MaxLineLength) {.async.} =
   ## Reads a line of data from ``socket`` into ``resString``.
   ##
   ## If a full line is read ``\r\L`` is not
@@ -401,13 +401,14 @@ proc recvLineInto*(socket: AsyncSocket, resString: FutureVar[string],
   ## is read) then line will be set to ``""``.
   ## The partial line **will be lost**.
   ##
+  ## The ``maxLength`` parameter determines the maximum amount of characters
+  ## that can be read before a ``ValueError`` is raised. This prevents Denial
+  ## of Service (DOS) attacks.
+  ##
   ## **Warning**: The ``Peek`` flag is not yet implemented.
   ##
   ## **Warning**: ``recvLineInto`` on unbuffered sockets assumes that the
   ## protocol uses ``\r\L`` to delimit a new line.
-  ##
-  ## **Warning**: ``recvLineInto`` currently uses a raw pointer to a string for
-  ## performance reasons. This will likely change soon to use FutureVars.
   assert SocketFlag.Peek notin flags ## TODO:
   assert(not resString.mget.isNil(),
          "String inside resString future needs to be initialised")
@@ -454,6 +455,12 @@ proc recvLineInto*(socket: AsyncSocket, resString: FutureVar[string],
         else:
           resString.mget.add socket.buffer[socket.currPos]
       socket.currPos.inc()
+
+      # Verify that this isn't a DOS attack: #3847.
+      if resString.mget.len > maxLength:
+        let msg = "recvLine received more than the specified `maxLength` " &
+                  "allowed."
+        raise newException(ValueError, msg)
   else:
     var c = ""
     while true:
@@ -475,10 +482,17 @@ proc recvLineInto*(socket: AsyncSocket, resString: FutureVar[string],
         resString.complete()
         return
       resString.mget.add c
+
+      # Verify that this isn't a DOS attack: #3847.
+      if resString.mget.len > maxLength:
+        let msg = "recvLine received more than the specified `maxLength` " &
+                  "allowed."
+        raise newException(ValueError, msg)
   resString.complete()
 
 proc recvLine*(socket: AsyncSocket,
-    flags = {SocketFlag.SafeDisconn}): Future[string] {.async.} =
+    flags = {SocketFlag.SafeDisconn},
+    maxLength = MaxLineLength): Future[string] {.async.} =
   ## Reads a line of data from ``socket``. Returned future will complete once
   ## a full line is read or an error occurs.
   ##
@@ -492,6 +506,10 @@ proc recvLine*(socket: AsyncSocket,
   ## is read) then line will be set to ``""``.
   ## The partial line **will be lost**.
   ##
+  ## The ``maxLength`` parameter determines the maximum amount of characters
+  ## that can be read before a ``ValueError`` is raised. This prevents Denial
+  ## of Service (DOS) attacks.
+  ##
   ## **Warning**: The ``Peek`` flag is not yet implemented.
   ##
   ## **Warning**: ``recvLine`` on unbuffered sockets assumes that the protocol
@@ -501,7 +519,7 @@ proc recvLine*(socket: AsyncSocket,
   # TODO: Optimise this
   var resString = newFutureVar[string]("asyncnet.recvLine")
   resString.mget() = ""
-  await socket.recvLineInto(resString, flags)
+  await socket.recvLineInto(resString, flags, maxLength)
   result = resString.mget()
 
 proc listen*(socket: AsyncSocket, backlog = SOMAXCONN) {.tags: [ReadIOEffect].} =