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CVE-2021-45456: Apache Kylin Command Injection


Command injection in #Apache #Kylin has been found and registered as #CVE-2021-45456 Apache Kylin is an open-source distributed analytics engine designed to provide a SQL interface and multi-dimensional analysis on Hadoop and Alluxio supporting extremely large datasets. It was originally developed by eBay, and is now a project of the Apache Software Foundation.

Background Story

The basic story behind this vulnerability is that the user can create a project, and dump diagnosis information of that project. in order for the solution to dump the diagnosis information it executes a script. Since the project name is controlled by the user, the user can enter the project name as a Linux command but without characters or spaces, after that When the user sends the request of the diagnosis, can modify the project name (i.e. the Linux command) and add spaces and other needed characters but URL-encoded so the command will be a valid command. The solution will process this request, decode the project name, and treat it as a Linux command in the execution process, therefore, it will execute the malicious payload.

Build the lab

I’m using docker on Ubuntu server 20.04

Install docker

  • apt update
  • apt install docker docker-compose

Install Apache Kylin

  • docker pull apachekylin/apache-kylin-standalone:4.0.0  
  • sudo docker run -d \ -m 8G \ -p 7070:7070 \ -p 8088:8088 \ -p 50070:50070 \ -p 8032:8032 \ -p 8042:8042 \ -p 2181:2181 \ -p 1337:1337 \ --name kylin-4.0.0 \ apachekylin/apache-kylin-standalone:4.0.0

Setup the debugger

First, configure the file
  • docker exec -it container_id bash
  • file path /home/admin/apache-kylin-4.0.0-bin-spark2/bin/
  • Under the retrieveStartCommand() function which is the start command function. line number 267
  • Scroll down to line number 307, the line starts with the following $JAVA ${KYLIN_EXTRA_START_OPTS} ${KYLIN_TOMCAT_OPTS}
  • Add the following -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=1337
  • Restart the container docker container restart container_id
  • Login to Kylin, port is 7070. I’m using the docker ip, you can also use the localhost IP.
  • Creds admin:KYLIN
  • Configure the debugger in Intellij IDEA

Reproduce the vulnerability

Based on the advisory, we will create a project with command injected e.g. touchpwned and after that, we will dump the diagnosis information for the project, but while we are doing this we will modify it using burpsuite to trigger the command injection, therefore, triggering the exploit.
  • Once you click “Diagnosis”, intercept the request
  • Change the name touchpawned to %60touch%20pawned%60 which the URL-encoded result of the following:
    `touch pawned`
  • Now, check the container
We demonstrated how you can gain access to the target and leverage this to RCE in the PoC blog from here:

Static Analysis & Debugging

NOTE: to run Kylin solution you run other apache solutions along with it, and this includes spark, Kafka, hbase, hive, spring …etc. therefore the debugging won’t be as detailed as usual because it will take it us into the source code of the other solutions.

Find an entry point

Based on the advisory the vulnerability happens in dumpProjectDiagnosisInfo method, but I want to go through how it handles the request, how the project gets created, how the name got stored, and how the vulnerability gets triggered with the latest request we saw.
  • I searched for “projects” and found the “”. This class here responsible for listing all projects, saving the project, updating the project, deleting the project, updating the project owner, and basically most of the project functions.
  • I set a few breakpoints as you can see and I created a new project called “test1”, you can see this in projectDescData variable the values of the project.

Understand how the project gets created and saved

  • So first time we create a project, the solution will use the saveProject method. Let’s go through this method real quick.
The method handles a POST request to create a new project instance.
  • @RequestMapping(value = "", method = { RequestMethod.POST }, produces = { "application/json" }): This line is an annotation that maps the method to the endpoint for creating a new project instance. It specifies that the endpoint should accept a POST request with an empty URL and that it should produce a JSON response.
  • @ResponseBody: This annotation is used to indicate that the method’s return value should be written directly to the response body.
  • public ProjectInstance saveProject(@RequestBody ProjectRequest projectRequest): This line defines the method signature, which includes a ProjectRequest object as the request body and returns a ProjectInstance object.
  • if (StringUtils.isEmpty(projectDesc.getName())): This line checks whether the name field of the ProjectInstance object is empty.
  • if (!ValidateUtil.isAlphanumericUnderscore(projectDesc.getName())): This line checks whether the name field of the ProjectInstance object contains only alphanumeric characters and underscores.throw new BadRequestException(: If the name field does not contain only alphanumeric characters and underscores, a BadRequestException is thrown.
ProjectInstance createdProj = null;
        try {
            createdProj = projectService.createProject(projectDesc);
        } catch (Exception e) {
            throw new InternalErrorException(e.getLocalizedMessage(), e);
This snippet here creates a new ProjectInstance object named createdProj and sets it initially to null. It then tries to create a new project using a projectService object and the projectDesc parameter passed to the createProject method. If the project creation is successful, the createdProj object will be assigned the newly created project instance. If an exception is thrown during the project creation process, the catch block will be executed.
  • return createdProj;: This line returns the createdProj object, which contains the newly created project instance

How the diagnosis request get proceeded & how the command gets executed

  • It all starts from the dumpProjectDiagnosisInfo method, set the breakpoints.  
  • Now click on “Diagnosis” in the website. you can always see variables and their values right there.  
  • The important line for me is the following String filePath = dgService.dumpProjectDiagnosisInfo(project, diagDir.getFile());
  • We have here the dumpProjectDiagnosisInfo , now follow this and you will find yourself in file
You can see the path here which is supposed to be the path of the diagnosis data.
  • Keep following with the debugger, now this is another interesting
String[] args = { project, exportPath.getAbsolutePath() }; This is an array named args and it contains the project name along with the exportPath which is the diagnosis data path and it’s using the getAbsolutePath() method. The getAbsolutePath() method is a part of the File class. This function returns the absolute pathname of the given file object.
  • After that we see runDiagnosisCLI(args) takes the args array as input.
  • Step-in, and here is the runDiagnosisCLI() method, and we can see the args with the values right there.  
After that we couple of loggers. from there, we go to File script = new File(KylinConfig.getKylinHome() + File.separator + "bin", ""); This line of the method creates a new File object representing a shell script named “” located in the “bin” directory of the Kylin configuration directory. If the script does not exist, the method throws a BadRequestException with a message that indicates the file could not be found.
  • Now, we have diagCmd variable which has the script path and the args.
  • Step-in, and click getCliCommandExecutor()
  • This will take you to getCliCommandExecutor and this method determines if it will get the remote access configuration of a Hadoop cluster or not to execute commands on it, i.e. remote commands. if the value retrieved is null in regards to the remote access configuration of the Hadoop cluster, and this is what happened in our case, the commands will be executed locally.
  • You can see the value of executor returned
We have here kinda two versions of the execute method in the CliCommandExecutor calls. both of the methods execute a shell command and return a Pair object containing the exit code and output of the command. We can see the first execute method takes only one argument: String command. Then, it calls the second execute method with the same command argument, along with a default logAppender of new SoutLogger() and a jobId of null. The second execute method takes the command, a logAppender (which is a logger instance that is used to log the output of the command), and a jobId (which is an optional identifier that can be used to track the execution of the command). The method then checks if a remote host has been specified for the CliCommandExecutor instance. If not, it runs the command locally using the runNativeCommand method, passing in the command, logAppender, and jobId. This method executes the command using a ProcessBuilder and captures the output and exit code of the command. If a remote host has been specified for the CliCommandExecutor instance, the execute method instead runs the command on the remote host using the runRemoteCommand method. Finally, the method checks the exit code of the command. If the exit code is non-zero, the method throws an IOException with an error message containing the exit code, error message, and command itself. Since we know that the command execution will happen locally, I added new breakpoints Step-in to follow runNativeCommand method since it’s the method that will execute the command. Obviously, the code defines a private method runNativeCommand which is called by the execute method in the same class, and it executes a shell command using ProcessBuilder and returns a Pair object containing the exit code and output of the command. The method takes three arguments: command (which is the shell command to be executed), logAppender (which is a logger instance that is used to log the output of the command), and jobId (which is an optional identifier that can be used to track the execution of the command). The method first constructs an array cmd of strings, which contains the command and its arguments. The cmd array is constructed differently depending on the operating system: for Windows, the command is executed using cmd.exe /C, while for other operating systems (such as Linux or macOS), the command is executed using /bin/bash -c. Then, the method constructs a ProcessBuilder instance using the cmd array and sets the redirectErrorStream property to true, which means that any error messages produced by the command will be redirected to the same output stream as the command’s standard output. The method then starts the process using ProcessBuilder.start() and registers it with a JobProcessContext if a jobId is provided. The method then reads the command’s standard output line by line using a BufferedReader, and appends each line to a StringBuilder. For each line, if a logAppender is provided, the line is logged using the Logger.log() method. If the method is interrupted by another thread (as determined by Thread.interrupted()), it destroys the process and returns a Pair object with an exit code of 1 and a message of “Killed”. If the command execution completes successfully, the method waits for the process to exit using Process.waitFor() and returns a Pair object with the exit code and output of the command. Finally, the method checks if the jobId is not null removes the process from the JobProcessContext . You can see from here how the variables get set along the execution of the software. Those are all the variables after the runNativeCommand is done. From here it will return to r = runNativeCommand(command, logAppender, jobId); and now it’s a matter of sending the command output back in the response.

How the execution looks like with an injected malicious payload

Since we understood in-depth how everything gets processed in the previous section, now I will just show screenshots of how it looks like with an injected malicious payload. Follow the same steps in the “Reproduce the vulnerability” section, but instead of sending the request through burpsuite. Send the request from the browser, so you can follow it in the debugger: The basic idea here is that you send the request with the project name edited and encoded. The server behind the solution decodes the payload, so now it’s just a normal Linux command. So, the basic structure of the command as we saw before is “script (” + “project name” + “folder”, and there’s where the injection happens in the project name, so the normal project name is now replaced with the payload. and this is what will be executed.

The root cause

I understood the root cause after the patch diffing. as it’s explained in the patch diffing, they replaced “project” with “projectName” and the reason is when you follow the debugger you will notice that “project” it’s just the name of the project name as it’s submitted (which is controlled by the user) after decoding. so when the attacker submits the malicious payload, the solution decodes it and passes it as it is a payload. The projectName it’s the real name with no characters or spaces. Once you follow
You will notice the projectName variable value This is how it looks like after that

Patch Diffing

The fix link from here: As we can see the project variable was replaced with the projectName variable, and based on what we explained in the root cause of the vulnerable we understand that by replacing the project with projectName we eliminate the danger of the malicious payload injection.


Update Apache to the latest version.

Final Thoughts

This software was a real joy, the dependency between multiple solutions makes it a little bit harder to debug, but I tried my best to make it focus on Apache Kylin only. How the payload gets structured in order to be injected it’s really interesting and fun.


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