CVE-2026-12143: form-data does not escape CR/LF/quote in multipart field names and filenames (CRLF injection)

Vulnerability summary

CVE-2026-12143 is a high (CVSS 8.7) vulnerability in form-data. EPSS estimates a 0.67% chance of exploitation in the next 30 days, higher than 52% of scored vulnerabilities.

Timeline

1. 12-06-2026
   PublishedVulnerability first disclosed.
2. 11-09-2026
   Last modifiedVulnerability information updated.

Description

form-data is a library for creating readable multipart/form-data streams. In versions through 4.0.5, the `field` argument to `FormData#append` and the `filename` option are concatenated verbatim into the `Content-Disposition` header without escaping carriage return (CR), line feed (LF), or double-quote (") characters. An application that passes attacker-controlled data as a field name or filename (for example, an API gateway that turns JSON object keys into multipart field names) allows the attacker to terminate the header line and inject additional headers, or to smuggle entire additional multipart parts, into the request the application forwards to a backend. This can let the attacker add or override form fields (e.g. set `is_admin=true`) seen by the downstream parser. This is an instance of CWE-93 (CRLF injection). The fix escapes CR, LF, and `"` as `%0D`, `%0A`, and `%22` in field names and filenames, matching the serialization browsers use per the WHATWG HTML multipart/form-data encoding algorithm. Exploitation requires the consuming application to use untrusted input as a field name or filename; applications that use only fixed/trusted field names are not affected. Fixed in 2.5.6, 3.0.5, and 4.0.6.

• CWE-93Improper Neutralization of CRLF SequencesThe product uses CRLF (carriage return line feeds) as a special element, e.g. to separate lines or records, but it does not neutralize or incorrectly neutralizes CRLF sequences from inputs.