form-data: CRLF injection in form-data via unescaped multipart field names and filenames

Summary

form-data builds multipart/form-data request bodies. Through v4.0.5, the field name passed to FormData#append and the filename option are concatenated directly into the Content-Disposition header with no escaping of CR (\r), LF (\n), or ". An application that uses untrusted input as a field name or filename therefore lets an attacker terminate the header line and either inject additional headers or smuggle whole additional multipart parts into the request the application forwards to a backend.

This is CWE-93 (CRLF injection). It is a divergence from how browsers and the WHATWG HTML spec serialize form-data (they escape these characters), so the fix is to match that behavior. Severity is conditional: it depends on the consuming application passing attacker-controlled data as a field name or filename. Applications that only use fixed/trusted field names are not affected.

Details

In lib/form_data.js, _multiPartHeader builds the part header as:

'Content-Disposition': ['form-data', 'name="' + field + '"'].concat(contentDisposition || [])

and _getContentDisposition builds filename="' + filename + '"'. Neither escapes control characters, so a \r\n in field/filename ends the header line. The same applies to ", which can break out of the quoted parameter.

Proof of concept

const FormData = require('form-data');
const form = new FormData();
form.append('email"\r\nX-Injected: true\r\nfake="', 'user@example.com');
console.log(form.getBuffer().toString());

Before the fix this emits an injected X-Injected: true header line. A field name that also includes --<boundary> sequences can introduce additional parts (e.g. an extra name="is_admin" field), which a downstream parser accepts as legitimate.

Impact

For an application that uses untrusted field names/filenames:

• Field injection / override (integrity). Inject or override fields the backend trusts (e.g. is_admin, role) — the primary demonstrated impact.
• Header injection into the generated multipart part.

Claims of guaranteed privilege escalation, authentication bypass, high confidentiality impact, and availability impact are application-dependent downstream consequences, not properties of form-data itself, and are not demonstrated by the PoC.

Severity

The demonstrated, library-attributable impact is integrity (field/header injection); there is no demonstrated confidentiality disclosure or availability impact in form-data itself, and exploitation requires the consuming app to feed untrusted data into field names/filenames. A Moderate (≈5.3, I:L) rating is also defensible given that precondition.

Patch

Fixed in 4.0.6, 3.0.5, and 2.5.6. Users on older 0.x/1.x/2.x releases should upgrade to 2.5.6 or later.

The fix escapes \r, \n, and " as %0D, %0A, and %22 in field names and filenames, matching the WHATWG HTML multipart/form-data encoding algorithm that browsers implement. This neutralizes the injection while leaving ordinary field names (including name[0], dotted, and unicode names) unchanged.

Workaround

Until upgrading, validate or reject field names/filenames that contain control characters before calling append:

if (/[\r\n]/.test(field)) { throw new Error('invalid field name'); }

Credit

Reported by yueyueL.