CVE-2026-25577: Emmett Framework DoS Vulnerability

CVE-2026-25577 Overview

CVE-2026-25577 is a Denial of Service vulnerability in the Emmett web framework that allows unauthenticated attackers to trigger HTTP 500 errors by sending malformed Cookie headers. The vulnerability exists in the cookies property of emmett_core.http.wrappers.Request, which fails to properly handle CookieError exceptions when parsing malformed Cookie headers.

Emmett is a Python framework designed to simplify web development processes. Prior to version 1.3.11, the framework's cookie parsing logic did not implement proper exception handling, allowing attackers to crash application instances by sending specially crafted HTTP requests with malformed cookies.

Critical Impact

Unauthenticated attackers can cause denial of service by triggering unhandled CookieError exceptions, resulting in HTTP 500 errors and potential application instability.

Affected Products

• Emmett Framework versions prior to 1.3.11
• Applications using emmett_core.http.wrappers.Request cookie parsing functionality

Discovery Timeline

• 2026-02-10 - CVE-2026-25577 published to NVD
• 2026-02-11 - Last updated in NVD database

Technical Details for CVE-2026-25577

Vulnerability Analysis

The vulnerability stems from improper exception handling in the cookie parsing mechanism. When the cookies property is accessed, the framework iterates through Cookie header values and calls cookies.load(cookie) without any try-catch wrapper. If a malformed cookie value triggers a CookieError (or any other exception), the entire request handling fails with an unhandled exception, resulting in an HTTP 500 response.

This weakness is classified under CWE-248 (Uncaught Exception), indicating that the application fails to catch exceptions that propagate to an outer call layer. The vulnerability is particularly concerning because it requires no authentication and can be exploited remotely over the network with low complexity.

Root Cause

The root cause is the absence of exception handling around the SimpleCookie.load() method call in emmett_core/http/wrappers/__init__.py. Python's http.cookies.SimpleCookie class can raise CookieError exceptions when encountering malformed cookie syntax, invalid characters, or improperly encoded values. Without proper try-except blocks, these exceptions propagate up the call stack and crash the request handler.

Attack Vector

The attack vector is network-based and requires no privileges or user interaction. An attacker can exploit this vulnerability by:

1. Crafting HTTP requests with malformed Cookie headers
2. Sending these requests to any endpoint of an affected Emmett application
3. The malformed cookie triggers a CookieError during parsing
4. The unhandled exception results in an HTTP 500 error

Repeated exploitation can lead to sustained denial of service, impacting application availability for legitimate users.

     def cookies(self) -> SimpleCookie:
         cookies: SimpleCookie = SimpleCookie()
         for cookie in self.headers.get("cookie", "").split(";"):
-            cookies.load(cookie)
+            try:
+                cookies.load(cookie)
+            except Exception:
+                continue
         return cookies

     @property

Source: GitHub Commit Update

Detection Methods for CVE-2026-25577

Indicators of Compromise

• Sudden increase in HTTP 500 error responses from Emmett-based applications
• Application logs showing unhandled CookieError or similar exceptions in request handling
• Unusual patterns of requests with malformed or oversized Cookie headers
• Service availability degradation without corresponding increase in legitimate traffic

Detection Strategies

• Monitor application error logs for CookieError exceptions originating from emmett_core.http.wrappers
• Implement web application firewall rules to detect and block requests with malformed Cookie header syntax
• Set up alerting for abnormal rates of HTTP 500 responses on Emmett endpoints
• Analyze incoming request patterns for cookie headers containing invalid characters or encoding

Monitoring Recommendations

• Enable detailed exception logging in Emmett applications to capture the full stack trace of cookie parsing failures
• Configure rate limiting on endpoints to mitigate the impact of repeated exploitation attempts
• Implement health check monitoring to detect service degradation caused by DoS attacks
• Review web server access logs for suspicious patterns in Cookie header values

How to Mitigate CVE-2026-25577

Immediate Actions Required

• Upgrade Emmett Framework to version 1.3.11 or later immediately
• Review application logs for evidence of prior exploitation attempts
• Implement input validation at the web server or WAF level to filter malformed cookies
• Consider implementing rate limiting on all public-facing endpoints as a defense-in-depth measure

Patch Information

The vulnerability has been fixed in Emmett Framework version 1.3.11. The patch adds exception handling around the cookies.load() call, catching any exception and continuing to the next cookie rather than allowing the exception to propagate. Organizations should upgrade to 1.3.11 or later as soon as possible.

For detailed patch information, refer to the GitHub Commit Update and the GitHub Security Advisory.

Workarounds

• Deploy a reverse proxy or WAF to validate and sanitize Cookie headers before they reach the application
• Implement custom middleware to catch exceptions in cookie parsing before they trigger 500 errors
• Use rate limiting to reduce the impact of potential DoS attacks while preparing for the patch deployment
• Consider temporarily disabling cookie-dependent functionality if it is not critical to operations

# Upgrade Emmett Framework to patched version
pip install --upgrade emmett-core>=1.3.11

# Verify installed version
pip show emmett-core | grep Version