In the fast world of JavaScript task runners, GruntJS stands as one of the early builders of modern workflows. But with great power comes great responsibility — especially when dealing with file operations and operating system permissions.

One major vulnerability creeped into GruntJS and was assigned as CVE-2022-1537. This vulnerability, present in GitHub’s gruntjs/grunt repository before version 1.5.3, can allow attackers to perform arbitrary file writes through a Time-Of-Check to Time-Of-Use (TOCTOU) race condition in the file.copy operation.

Let’s dive into what this means, how it works, and why upgrading is so crucial.

What Is CVE-2022-1537?

A TOCTOU (Time-of-check-to-time-of-use) vulnerability arises when the application checks a resource and then acts on it, but a malicious user is able to change the resource in the blink between those two moments.

In GruntJS’s file.copy method, the program would check if the file to be copied was safe, and then, moments later, perform the copy. This gave attackers an opportunity to substitute a safe file with something more dangerous (like a symlink to a critical system file).

If a lower-privileged user has write access to both the source and destination directories, they could exploit this race condition. With a well-timed attack, they could:

- Write to sensitive files such as /etc/shadow (if GruntJS runs as root!).

Here’s how an attacker could leverage this flaw

1. Lower privileged user creates a symlink named, say, malicious-link, pointing to .bashrc in the GruntJS user’s home directory.

The lower privileged user sets up their own (benign) file as the source file.

3. GruntJS’s task runner (operating as a higher-privileged user) is triggered to copy the source file into the destination directory.
4. Just after GruntJS checks that the destination is fine (but before it performs the copy), the attacker quickly swaps the destination file or changes the symlink to now point to .bashrc or /etc/shadow.

GruntJS, unaware of the swap, writes data into the privileged file.

This allows the attacker to overwrite sensitive files — leading to possible code execution or privilege escalation.

Here’s a simplified Node.js demonstration of the issue using Grunt’s file API

const grunt = require('grunt');

// Imagine 'malicious.txt' was set up to point to a symlink
grunt.file.copy('attacker_supplied.txt', '/home/gruntuser/.bashrc');

At a high level, if an attacker has access to the directories in question, they could update the symlink (used as the destination) between Grunt's internal check and the actual write — letting them modify arbitrary files.

Attacker is bob

- Both users have write access to /tmp/sourcedir and /tmp/destdir

Attack steps

1. Bob sets up: In /tmp/destdir, creates link-to-bashrc, a symlink to /home/gruntuser/.bashrc

`bash

ln -s /home/gruntuser/.bashrc /tmp/destdir/link-to-bashrc

`

2. Bob (or a script) waits: Just after GruntJS checks if /tmp/destdir/link-to-bashrc is OK, Bob replaces the symlink with one targeting /etc/shadow (timing attack).

`bash

rm /tmp/destdir/link-to-bashrc
   ln -s /etc/shadow /tmp/destdir/link-to-bashrc

`

3. GruntJS copies data from innocent.txt to link-to-bashrc, possibly overwriting /etc/shadow or .bashrc.

Result: Privilege escalation if Grunt runs as root — or persistent shell access if .bashrc is hijacked.

Official References

- CVE-2022-1537 NIST entry
- GruntJS security advisory and fix commit
- GruntJS Release 1.5.3

The Fix

The GruntJS team fixed this in version 1.5.3. After the fix, the file.copy method performs extra checks and does the final write operation more securely to block TOCTOU attacks.

Upgrade fix

npm install grunt@1.5.3

Where possible, make sure lower-privileged users do not have write access to any directories GruntJS’s file copy operations touch.

Conclusion

CVE-2022-1537 is a critical example of why basic file operations (like copy) must use secure patterns to avoid TOCTOU race conditions. If you use GruntJS (especially in automated build/deploy processes), upgrade ASAP and review your file permissions to keep your projects — and your users — safe.


Stay safe!🌐  
For more details, check out the GruntJS repository and read the security advisory.

Timeline

Published on: 05/10/2022 14:15:00 UTC
Last modified on: 05/16/2022 17:08:00 UTC