Axios: CRLF Injection in multipart/form-data body via unsanitized blob.type in formDataToStream
Date: 2026-05
Affected: npm/axios
Attack vector: other
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-42037
CWE: CWE-93
CVSS: 5.3 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N)

### Summary The `FormDataPart` constructor in `lib/helpers/formDataToStream.js` interpolates `value.type` directly into the `Content-Type` header of each multipart part without sanitizing CRLF (`\r\n`) sequences. An attacker who controls the `.type` property of a Blob/File-like object (e.g., via a user-uploaded file in a Node.js proxy service) can inject arbitrary MIME part headers into the multipart form-data body. This bypasses Node.js v18+ built-in header protections because the injection targets the multipart body structure, not HTTP request headers. ### Details In `lib/helpers/formDataToStream.js` at line 27, when processing a Blob/File-like value, the code builds per-part headers by directly embedding value.type: ``` if (isStringValue) { value = textEncoder.encode(String(value).replace(/\r?\n|\r\n?/g, CRLF)); } else { // value.type is NOT sanitized for CRLF sequences headers += `Content-Type: ${value.type || 'application/octet-stream'}${CRLF}`; } ``` Note that the string path (line above) explicitly sanitizes CRLF, but the binary/blob path does not. This inconsistency confirms the sanitization was intended but missed for `value.type`. ### Attack chain: 1. Attacker uploads a file to a Node.js proxy service, supplying a crafted MIME type containing `\r\n` sequences 2. The proxy appends the file to a FormData and posts it via `axios.post(url, formData)` 3. axios calls `formDataToStream()`, which passes `value.type` unsanitized into the multipart body 4. The downs


Mitigations:

Architectural layers (MAESTRO):