Improper Privilege Management in shelljs/shelljsValid
Dec 26th 2021
If ShellJS scripts running locally are using ShellJS exec function, local users on the filesystem can read the stdout of the running ShellJS process to disclose sensitive information present in the privileged process. This may leak sensitive information present in the privileged process which may be abused to perform local privilege escalation. Local users can also crash ShellJS scripts running as root via creating a stdout file and triggering an EACCESS error.
Proof of Concept 1 (Leak sensitive information from privileged process [Read stdout])
The paramFiles, stdout and stderr files created will have the following permissions
-rw-r--r-- 1 root root 3135 Dec 16 23:08 shelljs_81cdfe4bdfc9d93323a4 -rw-r--r-- 1 root root 832 Dec 16 23:08 shelljs_a99c412389ba1efa8807 -rw-r--r-- 1 root root 0 Dec 16 23:08 shelljs_f83a2c65d1ff46ca11a0
When these files are created it is possible for other users to access them using a Bash script.
1: So for instance, if root user / cronjob / privileged ShellJS process used shell.exec to generate a secret key (example scenario)
// poc.js var shell = require('shelljs'); var secret = shell.exec("openssl rand -base64 32").stdout;
2: Then it would be possible to obtain the secret by using the following Bash script to obtain the secret from the poc.js as a lower privileged user.
Run this Bash command, then execute the poc.js. (Note this deletes all files and folders in /tmp directory in order to see the output more clearly)
rm -rf /tmp/* while true; do cat /tmp/*; done
This could be abused in other scenarios to obtain sensitive information such as passwords depending on what command is bein executed in shell.exec()
Proof of Concept 2 (Crash privileged ShellJS processes)
As a lower-privileged user, you can crash ShellJS processes by using the following Bash script which creates a stdoutFile before a stdoutFile is even created and run it. When executing execSync on the NodeJS file, the file exits and causes an EACCESS error. Note: This exploit script clears the /tmp directory before executing to identify the paramFile when created
1: Execute the Bash script
rm -rf /tmp/* while [ -z "$paramFile" ]; do paramFile=$(ls /tmp); done; stdoutFile=$(cat $paramFile | jq ".stdoutFile" | tr -d '"'); touch $stdoutFile;
2: Run the poc.js previously
When ShellJS is used to create shell scripts which may be running as root, users with low-level privileges on the system can leak sensitive information such as passwords (depending on implementation) from the stdout of the privileged ShellJS process OR shutdown privileged ShellJS processes via the exec function present in ShellJS
The stdout, stderr and the paramFiles created when executing execSync should only have read and write privileges available for the current process. This can be done via paramFile, stderr and stdout file precreation and locking them before any write operation is done. This would solve both issues listed above