CVE-2026-27205Low▾ SunlitFlask session does not add `Vary: Cookie` header when accessed in some ways
▾ Sunlit zone — Low / medium · no exploitation signal
impact 13.8 · likelihood 0.1 · exploitation 0
Need a working PoC? Pro members can cast a request and our team develops one — it lands right here.
Exploit-prediction probability, daily snapshots since Jul 13.
Disclosure to exploitation, from the record and what we observed since indexing it.
Disclosed via OSV
0.4%
Last analysed / modified upstream
When the session object is accessed, Flask should set the Vary: Cookie header. This instructs caches not to cache the response, as it may contain information specific to a logged in user. This is handled in most cases, but some forms of access such as the Python in operator were overlooked.
The severity depends on the application's use of the session, and the cache's behavior regarding cookies. The risk depends on all these conditions being met.
Cache-Control header to indicate that a page is private or should not be cached.flask < 3.1.3Upgrade to a patched release:
flask 3.1.3Connected by shared product, vendor, weakness, or advisory.