CVE-2022-31799: Bottlepy Bottle Error Handling Flaw

CVE-2022-31799 Overview

CVE-2022-31799 is a critical vulnerability in the Bottle web framework for Python that mishandles errors during early request binding. Bottle is a lightweight WSGI micro web-framework widely used for building web applications and RESTful APIs in Python. The vulnerability exists in versions prior to 0.12.20, where improper exception handling during the request binding phase can lead to unexpected application behavior and potential security implications.

The flaw stems from improper exception handling (CWE-755) where errors occurring during the early stages of request processing are not gracefully managed. This can allow attackers to trigger error conditions that may expose sensitive information or cause application instability through crafted HTTP requests.

Critical Impact

Network-accessible vulnerability allowing unauthenticated attackers to potentially compromise confidentiality, integrity, and availability of affected web applications without user interaction.

Affected Products

• Bottlepy Bottle (versions prior to 0.12.20)
• Debian Linux 9.0, 10.0, 11.0
• Fedora 35, 36

Discovery Timeline

• June 2, 2022 - CVE-2022-31799 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2022-31799

Vulnerability Analysis

The vulnerability resides in the _handle method of the Bottle web framework's core request processing logic. In vulnerable versions, the PATH_INFO environment variable was processed and validated before the request and response objects were properly bound to the current environment. This ordering created a window where exceptions during path processing could occur without proper error handling context.

When a malformed request with an invalid path encoding is received, the framework attempts to decode the PATH_INFO from Latin-1 to UTF-8. If this conversion fails, an exception is raised. In the vulnerable code path, this exception could occur before error handling mechanisms were properly initialized, potentially leading to unhandled exceptions, information disclosure through error messages, or denial of service conditions.

Root Cause

The root cause is improper exception handling (CWE-755) in the request binding sequence. The vulnerable code performed path validation before establishing the error handling context through request.bind(environ) and response.bind(). This meant that any errors during path processing could bypass the framework's normal error handling mechanisms, potentially exposing raw exception details or causing the application to crash.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by sending HTTP requests with specially crafted path strings containing invalid UTF-8 encoding sequences. When the Bottle framework attempts to process these malformed paths before the error handling context is established, it can trigger unhandled exceptions.

The patch reorders the initialization sequence to ensure request and response binding occurs first, wrapping all subsequent path processing within the try-except block:

         return tob(template(ERROR_PAGE_TEMPLATE, e=res))
 
     def _handle(self, environ):
-        path = environ['bottle.raw_path'] = environ['PATH_INFO']
-        if py3k:
-            try:
-                environ['PATH_INFO'] = path.encode('latin1').decode('utf8')
-            except UnicodeError:
-                return HTTPError(400, 'Invalid path string. Expected UTF-8')
-
         try:
+
             environ['bottle.app'] = self
             request.bind(environ)
             response.bind()
+
+            path = environ['bottle.raw_path'] = environ['PATH_INFO']
+            if py3k:
+                try:
+                    environ['PATH_INFO'] = path.encode('latin1').decode('utf8')
+                except UnicodeError:
+                    return HTTPError(400, 'Invalid path string. Expected UTF-8')
+
             try:
                 self.trigger_hook('before_request')
                 route, args = self.router.match(environ)

Source: GitHub Commit e140e1b

Detection Methods for CVE-2022-31799

Indicators of Compromise

• Unusual HTTP requests with malformed or invalid UTF-8 encoded path strings
• Application error logs showing unhandled UnicodeError exceptions in the Bottle framework
• Unexpected 500 Internal Server Error responses to requests with non-standard path encodings
• Stack traces in application logs referencing bottle.py and _handle method

Detection Strategies

• Monitor web application logs for UnicodeError exceptions originating from Bottle's request handling code
• Implement Web Application Firewall (WAF) rules to detect and block requests with suspicious path encodings
• Use dependency scanning tools to identify applications running Bottle versions prior to 0.12.20
• Review application error rates for spikes that may indicate exploitation attempts

Monitoring Recommendations

• Enable detailed logging for your Bottle-based applications to capture exception details
• Set up alerts for unusual patterns of 400 or 500 HTTP error responses
• Monitor for requests containing non-ASCII characters in URL paths that may indicate probing activity
• Implement application performance monitoring to detect denial of service conditions

How to Mitigate CVE-2022-31799

Immediate Actions Required

• Upgrade Bottle to version 0.12.20 or later immediately
• Review application logs for signs of exploitation attempts
• Implement input validation at the reverse proxy or load balancer level as a defense-in-depth measure
• Consider deploying a WAF with rules to filter malformed path requests

Patch Information

The vulnerability has been addressed in Bottle version 0.12.20. The fix reorders the request handling sequence to ensure proper error handling context is established before any path processing occurs. Security patches are available through:

• GitHub Commit a2b0ee6 - Version bump to 0.12.20
• GitHub Commit e140e1b - Error handling fix

Distribution-specific patches are available:

• Debian Security Advisory DSA-5159
• Debian LTS Announcement

Workarounds

• If immediate patching is not possible, deploy a reverse proxy or WAF to filter requests with invalid path encodings
• Implement custom middleware to validate and sanitize request paths before they reach the Bottle application
• Monitor applications closely for signs of exploitation while planning upgrade
• Consider temporarily restricting network access to affected applications until patching is complete

# Upgrade Bottle using pip
pip install --upgrade bottle>=0.12.20

# Verify installed version
pip show bottle | grep Version