CVE-2024-21896

What it actually does

Monkey-patching Buffer.prototype.utf8Write allows an attacker to alter the result of path.resolve() when the permission model converts resolved paths to Buffers, bypassing path traversal protections in the experimental permission model.

Why it matters to the business

Applications relying on Node.js experimental permissions may inadvertently grant access to files outside intended directories, risking data exposure or unauthorized operations in sandboxed environments; low adoption due to experimental status reduces widespread impact but still requires prompt attention for users testing the feature.

How an attack unfolds

An attacker first monkey-patches Buffer.prototype.utf8Write in the target Node.js process using the experimental permission model, then supplies a user-controlled path intended for Buffer conversion; the patched method modifies the resolved path at runtime to traverse outside allowed directories, evading the permission checks.

How to fix it

Update Node.js to a build containing commit 3eccdd1ae61777a381a7ac3bc0ea62443dbb50b8 or later; disable the experimental permission model via --experimental-permission if the feature is not required.

How to confirm the fix worked

Verify Node.js binary or source includes the referenced commit; attempt path traversal inputs under the permission model both before and after the update to confirm enforcement; review application startup flags for permission model usage.

Typical attack path

1. Identify Node.js application using experimental permission model in versions 20 or 21
2. Inject code to monkey-patch Buffer.prototype.utf8Write before permission checks run
3. Provide crafted path input that path.resolve() processes
4. Trigger Buffer.from() conversion on the resolved path to activate the patch
5. Access or manipulate files outside the permitted directory tree

How to detect it

Review application source and runtime for dynamic modifications to Buffer.prototype.utf8Write; inspect Node.js process startup arguments and permission model configuration; examine file access logs for unexpected paths when the permission model is active.

What your security team should do

• Audit all Node.js deployments for use of the experimental permission model
• Apply the fix commit immediately on affected 20.x and 21.x branches
• Disable --experimental-permission in production until the model stabilizes
• Monitor upstream Node.js releases for permission model updates