CVE-2026-49756

Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in wojtekmach Req allows multipart parameter smuggling via attacker-influenced part metadata.

Req.Utils.encode_form_part/2 in lib/req/utils.ex builds the per-part headers by interpolating the caller-supplied name, filename, and content_type values directly into the content-disposition and content-type lines with no escaping or CRLF stripping. A value containing ", \r, or \n closes the surrounding quoted value and starts a new header line; an additional \r\n--<boundary> terminates the current part and prepends a smuggled part of the attacker's choosing.

This is reachable through every supported way of supplying a part. It is particularly easy when value is a %File.Stream{}, because filename then defaults to Path.basename(stream.path) and POSIX filenames may legitimately contain \r and \n. Any application that forwards user-controlled filenames (or field names / MIME types) through Req.post/2 with form_multipart: lets an attacker inject arbitrary headers into the outgoing multipart body or smuggle additional fields and parts into the request the victim service sends downstream.

This issue affects req: from 0.5.3 before 0.6.0.

Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in wojtekmach Req allows multipart parameter smuggling via attacker-influenced part metadata.

Req.Utils.encode_form_part/2 in lib/req/utils.ex builds the per-part headers by interpolating the caller-supplied name, filename, and content_type values directly into the content-disposition and content-type lines with no escaping or CRLF stripping. A value containing ", \r, or \n closes the surrounding quoted value and starts a new header line; an additional \r\n--<boundary> terminates the current part and prepends a smuggled part of the attacker's choosing.

This is reachable through every supported way of supplying a part. It is particularly easy when value is a %File.Stream{}, because filename then defaults to Path.basename(stream.path) and POSIX filenames may legitimately contain \r and \n. Any application that forwards user-controlled filenames (or field names / MIME types) through Req.post/2 with form_multipart: lets an attacker inject arbitrary headers into the outgoing multipart body or smuggle additional fields and parts into the request the victim service sends downstream.

This issue affects req: from 0.5.3 before 0.6.0.

Summary

Req's multipart form encoder interpolates the per-part name, filename, and content_type directly into the part headers without escaping. An attacker who can influence any of those values can inject CRLF-separated header lines, smuggle additional form fields, or prepend a whole extra part into the request the victim service sends downstream.

Details

Req.Utils.encode_form_part/2 in lib/req/utils.ex builds the per-part header iodata by concatenating the three caller-supplied strings verbatim into content-disposition: form-data; name="<name>"; filename="<filename>" and content-type: <content_type>. There is no CRLF stripping, no quote escaping, and no validation. A value containing "\r\n closes the surrounding quoted value and starts a new header line; an additional \r\n--<boundary> terminates the current part and lets the attacker prepend a smuggled part.

The flaw is reachable through every supported way of supplying a part. It is especially easy to hit when value is a %File.Stream{}, because filename then defaults to Path.basename(stream.path) and POSIX filenames may legitimately contain \r and \n. RFC 7578 / WHATWG form-data requires percent-encoding ", CR, and LF in these fields; the fix adopts that behavior.

PoC

1. Construct a malicious filename such as harmless.txt"\r\nX-Smuggled: marker\r\nContent-Disposition: form-data; name="pwned.
2. Call Req.post!(url, form_multipart: [upload: {"benign body", filename: <malicious>, content_type: "text/plain"}]).
3. The emitted multipart body contains a real X-Smuggled: header line and an extra Content-Disposition for name="pwned", alongside Req's legitimate headers.

Impact

HTTP request smuggling / multipart parameter smuggling in the HTTP client. Any application using Req to send form_multipart requests where any of name, filename, or content_type can be influenced by an untrusted source is affected, most commonly upload proxies and re-uploaders that derive filename from Path.basename/1 on a user-controlled path.

Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in wojtekmach Req allows multipart parameter smuggling via attacker-influenced part metadata.

Req.Utils.encode_form_part/2 in lib/req/utils.ex builds the per-part headers by interpolating the caller-supplied name, filename, and content_type values directly into the content-disposition and content-type lines with no escaping or CRLF stripping. A value containing ", \r, or \n closes the surrounding quoted value and starts a new header line; an additional \r\n--<boundary> terminates the current part and prepends a smuggled part of the attacker's choosing.

This is reachable through every supported way of supplying a part. It is particularly easy when value is a %File.Stream{}, because filename then defaults to Path.basename(stream.path) and POSIX filenames may legitimately contain \r and \n. Any application that forwards user-controlled filenames (or field names / MIME types) through Req.post/2 with form_multipart: lets an attacker inject arbitrary headers into the outgoing multipart body or smuggle additional fields and parts into the request the victim service sends downstream.

This issue affects req: from 0.5.3 before 0.6.0.

Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in wojtekmach Req allows multipart parameter smuggling via attacker-influenced part metadata.

Req.Utils.encode_form_part/2 in lib/req/utils.ex builds the per-part headers by interpolating the caller-supplied name, filename, and content_type values directly into the content-disposition and content-type lines with no escaping or CRLF stripping. A value containing ", \r, or \n closes the surrounding quoted value and starts a new header line; an additional \r\n--<boundary> terminates the current part and prepends a smuggled part of the attacker's choosing.

This is reachable through every supported way of supplying a part. It is particularly easy when value is a %File.Stream{}, because filename then defaults to Path.basename(stream.path) and POSIX filenames may legitimately contain \r and \n. Any application that forwards user-controlled filenames (or field names / MIME types) through Req.post/2 with form_multipart: lets an attacker inject arbitrary headers into the outgoing multipart body or smuggle additional fields and parts into the request the victim service sends downstream.

This issue affects req: from 0.5.3 before 0.6.0.

Summary

Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in wojtekmach Req allows multipart parameter smuggling via attacker-influenced part metadata.

Req.Utils.encode_form_part/2 in lib/req/utils.ex builds the per-part headers by interpolating the caller-supplied name, filename, and content_type values directly into the content-disposition and content-type lines with no escaping or CRLF stripping. A value containing ", \r, or \n closes the surrounding quoted value and starts a new header line; an additional \r\n--<boundary> terminates the current part and prepends a smuggled part of the attacker's choosing.

This is reachable through every supported way of supplying a part. It is particularly easy when value is a %File.Stream{}, because filename then defaults to Path.basename(stream.path) and POSIX filenames may legitimately contain \r and \n. Any application that forwards user-controlled filenames (or field names / MIME types) through Req.post/2 with form_multipart: lets an attacker inject arbitrary headers into the outgoing multipart body or smuggle additional fields and parts into the request the victim service sends downstream.

This issue affects req: from 0.5.3 before 0.6.0.

Workarounds

Sanitize attacker-influenced name, filename, and content_type values before passing them to Req.post/2 with form_multipart:. At minimum, reject (or strip) any value containing \r, \n, or ". When forwarding uploads, derive filename from a normalised string rather than Path.basename/1 on a user-controlled path.

Summary

Req's multipart form encoder interpolates the per-part name, filename, and content_type directly into the part headers without escaping. An attacker who can influence any of those values can inject CRLF-separated header lines, smuggle additional form fields, or prepend a whole extra part into the request the victim service sends downstream.

Details

Req.Utils.encode_form_part/2 in lib/req/utils.ex builds the per-part header iodata by concatenating the three caller-supplied strings verbatim into content-disposition: form-data; name="<name>"; filename="<filename>" and content-type: <content_type>. There is no CRLF stripping, no quote escaping, and no validation. A value containing "\r\n closes the surrounding quoted value and starts a new header line; an additional \r\n--<boundary> terminates the current part and lets the attacker prepend a smuggled part.

The flaw is reachable through every supported way of supplying a part. It is especially easy to hit when value is a %File.Stream{}, because filename then defaults to Path.basename(stream.path) and POSIX filenames may legitimately contain \r and \n. RFC 7578 / WHATWG form-data requires percent-encoding ", CR, and LF in these fields; the fix adopts that behavior.

PoC

1. Construct a malicious filename such as harmless.txt"\r\nX-Smuggled: marker\r\nContent-Disposition: form-data; name="pwned.
2. Call Req.post!(url, form_multipart: [upload: {"benign body", filename: <malicious>, content_type: "text/plain"}]).
3. The emitted multipart body contains a real X-Smuggled: header line and an extra Content-Disposition for name="pwned", alongside Req's legitimate headers.

Impact

HTTP request smuggling / multipart parameter smuggling in the HTTP client. Any application using Req to send form_multipart requests where any of name, filename, or content_type can be influenced by an untrusted source is affected, most commonly upload proxies and re-uploaders that derive filename from Path.basename/1 on a user-controlled path.