Summary
Flask session does not add Vary: Cookie header when accessed in some ways
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.
- The application must be hosted behind a caching proxy that does not ignore responses with cookies.
- The application does not set a
Cache-Controlheader to indicate that a page is private or should not be cached. - The application accesses the session in a way that does not access the values, only the keys, and does not mutate the session.
Impact
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-27205? CVE-2026-27205 is a low-severity security vulnerability in flask (pip), affecting versions < 3.1.3. It is fixed in 3.1.3.
- Which versions of flask are affected by CVE-2026-27205? flask (pip) versions < 3.1.3 is affected.
- Is there a fix for CVE-2026-27205? Yes. CVE-2026-27205 is fixed in 3.1.3. Upgrade to this version or later.
- Is CVE-2026-27205 exploitable, and should I be worried? Whether CVE-2026-27205 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
- What actually determines whether CVE-2026-27205 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.
- How do I fix CVE-2026-27205? Upgrade
flaskto 3.1.3 or later.