diff --git a/services/clsi/app.coffee b/services/clsi/app.coffee
index fe9ad532fe..44d31575cc 100644
--- a/services/clsi/app.coffee
+++ b/services/clsi/app.coffee
@@ -50,26 +50,9 @@ staticServer = express.static Settings.path.compilesDir, setHeaders: (res, path,
 		# that could be used in same-origin/XSS attacks.
 		res.set("Content-Type", "text/plain")
 
-
-
-app.get "/project/:project_id/output/*", (req, res, next) ->
-	basePath = Path.resolve("#{Settings.path.compilesDir}/#{req.params.project_id}")
-	path = Path.normalize("#{basePath}/#{req.params[0]}")
-	if path.slice(0, basePath.length) != basePath
-		logger.warn path: req.params[0], project_id: req.params.project_id, "trying to leave project directory, aborting"
-		res.send(404)
-		return
-	fs.lstat path, (error, stats) ->
-		if error?
-			if error.code == "ENOENT"
-				error.statusCode = 404
-			return next(error)
-		if stats.isSymbolicLink()
-			error = new Error("file is a symlink")
-			error.statusCode = 404
-			return next(error)
-		req.url = "/#{req.params.project_id}/#{req.params[0]}"
-		staticServer(req, res, next)
+app.get "/project/:project_id/output/*", require("./app/js/SymlinkCheckerMiddlewear"), (req, res, next) ->
+	req.url = "/#{req.params.project_id}/#{req.params[0]}"
+	staticServer(req, res, next)
 
 app.get "/status", (req, res, next) ->
 	res.send "CLSI is alive\n"
diff --git a/services/clsi/app/coffee/SymlinkCheckerMiddlewear.coffee b/services/clsi/app/coffee/SymlinkCheckerMiddlewear.coffee
new file mode 100644
index 0000000000..b7baf1fa77
--- /dev/null
+++ b/services/clsi/app/coffee/SymlinkCheckerMiddlewear.coffee
@@ -0,0 +1,17 @@
+Path = require("path")
+fs = require("fs")
+Settings = require("settings-sharelatex")
+logger = require("logger-sharelatex")
+
+
+module.exports = (req, res, next)->
+	basePath = Path.resolve("#{Settings.path.compilesDir}/#{req.params.project_id}")
+	requestedFsPath = Path.normalize("#{basePath}/#{req.params[0]}")
+	fs.realpath requestedFsPath, (err, realFsPath)->
+		if err?
+			return res.send(500)
+		else if requestedFsPath != realFsPath
+			logger.warn requestedFsPath:requestedFsPath, realFsPath:realFsPath, path: req.params[0], project_id: req.params.project_id, "trying to access a different file (symlink), aborting"
+			return res.send(404)
+		else
+			return next()
diff --git a/services/clsi/test/unit/coffee/SymlinkCheckerMiddlewearTests.coffee b/services/clsi/test/unit/coffee/SymlinkCheckerMiddlewearTests.coffee
new file mode 100644
index 0000000000..50bd4271b5
--- /dev/null
+++ b/services/clsi/test/unit/coffee/SymlinkCheckerMiddlewearTests.coffee
@@ -0,0 +1,60 @@
+should = require('chai').should()
+SandboxedModule = require('sandboxed-module')
+assert = require('assert')
+path = require('path')
+sinon = require('sinon')
+modulePath = path.join __dirname, "../../../app/js/SymlinkCheckerMiddlewear"
+expect = require("chai").expect
+
+describe "SymlinkCheckerMiddlewear", ->
+
+	beforeEach ->
+
+		@settings = 
+			path:
+				compilesDir: "/compiles/here"
+
+		@fs = {}
+		@SymlinkCheckerMiddlewear = SandboxedModule.require modulePath, requires:
+			"settings-sharelatex":@settings
+			"logger-sharelatex": 
+				log:->
+				warn:->
+			"fs":@fs
+		@req = 
+			params:
+				project_id:"12345"
+
+		@res = {}
+		@req.params[0]= "output.pdf"
+
+
+	describe "sending a normal file through", ->
+		beforeEach ->
+			@fs.realpath = sinon.stub().callsArgWith(1, null, "#{@settings.path.compilesDir}/#{@req.params.project_id}/output.pdf")
+
+		it "should call next", (done)->
+			@SymlinkCheckerMiddlewear @req, @res, done
+
+
+	describe "with a symlink file", ->
+		beforeEach ->
+			@fs.realpath = sinon.stub().callsArgWith(1, null, "/etc/#{@req.params.project_id}/output.pdf")
+
+		it "should send a 404", (done)->
+			@res.send = (resCode)->
+				resCode.should.equal 404
+				done()
+			@SymlinkCheckerMiddlewear @req, @res
+
+	describe "with an error from fs.realpath", ->
+
+		beforeEach ->
+			@fs.realpath = sinon.stub().callsArgWith(1, "error")
+
+		it "should send a 500", (done)->
+			@res.send = (resCode)->
+				resCode.should.equal 500
+				done()
+			@SymlinkCheckerMiddlewear @req, @res
+