CVE-2026-25577: Crumbs in the Gearbox: Crashing Emmett Framework with Malformed Cookies

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Crumbs in the Gearbox: Crashing Emmett Framework with Malformed Cookies

Vulnerability ID: CVE-2026-25577
CVSS Score: 7.5
Published: 2026-02-10

A classic input validation oversight in the Emmett Python web framework allows unauthenticated attackers to trigger unhandled exceptions by sending malformed HTTP Cookie headers. By leveraging the strict parsing logic of Python's standard library http.cookies, an attacker can force the application to crash (HTTP 500) on every request containing specific illegal characters. While not a Remote Code Execution (RCE) vector, this vulnerability presents a trivial, low-cost method for Denial of Service (DoS) attacks against any application running affected versions of Emmett.

TL;DR

Emmett < 1.3.11 crashes when parsing malformed cookies. An attacker can send a request with a cookie key containing characters like '(' or '[' to trigger an unhandled CookieError, resulting in a 500 Internal Server Error. Trivial to exploit for DoS.

⚠️ Exploit Status: POC

Technical Details

CWE: CWE-248 (Uncaught Exception)
CVSS v3.1: 7.5 (High)
Attack Vector: Network (Remote)
Impact: Denial of Service (DoS)
Exploit Complexity: Low (Trivial)
Status: Patched

Affected Systems

Emmett Framework Core < 1.3.11
Python web applications using Emmett
emmett-core: < 1.3.11 (Fixed in: 1.3.11)

Code Analysis

Commit: c126757

Fix cookie parsing logic to catch SimpleCookie errors

@@ -62,7 +62,10 @@ 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

Exploit Details

Manual: Trivial curl command: curl -H 'Cookie: bad(=cookie' http://target

Mitigation Strategies

Upgrade Emmett Core to version 1.3.11+
Implement WAF rules to block non-RFC compliant Cookie headers
Implement stricter header validation at the Reverse Proxy level (Nginx/Apache)

Remediation Steps:

Check current version: pip show emmett-core
Update package: pip install --upgrade emmett-core>=1.3.11
Restart application services to apply changes
Verify fix by attempting the PoC curl command against a staging environment

References

Read the full report for CVE-2026-25577 on our website for more details including interactive diagrams and full exploit analysis.

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