Summary
Flask vulnerable to possible disclosure of permanent session cookie due to missing Vary: Cookie header
When all of the following conditions are met, a response containing data intended for one client may be cached and subsequently sent by a proxy to other clients. If the proxy also caches Set-Cookie headers, it may send one client's session cookie to other clients. The severity depends on the application's use of the session, and the proxy'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 strip cookies or ignore responses with cookies.
- The application sets
session.permanent = True. - The application does not access or modify the session at any point during a request.
SESSION_REFRESH_EACH_REQUESTis enabled (the default).- The application does not set a
Cache-Controlheader to indicate that a page is private or should not be cached.
This happens because vulnerable versions of Flask only set the Vary: Cookie header when the session is accessed or modified, not when it is refreshed (re-sent to update the expiration) without being accessed or modified.
Impact
CVE-2023-30861 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 (2.3.2, 2.2.5); upgrading removes the vulnerable code path.
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.
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flask to 2.3.2 or later; flask to 2.2.5 or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2023-30861? CVE-2023-30861 is a high-severity security vulnerability in flask (pip), affecting versions >= 2.3.0, < 2.3.2. It is fixed in 2.3.2, 2.2.5.
- How severe is CVE-2023-30861? CVE-2023-30861 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.
- Which versions of flask are affected by CVE-2023-30861? flask (pip) versions >= 2.3.0, < 2.3.2 is affected.
- Is there a fix for CVE-2023-30861? Yes. CVE-2023-30861 is fixed in 2.3.2, 2.2.5. Upgrade to this version or later.
- Is CVE-2023-30861 exploitable, and should I be worried? Whether CVE-2023-30861 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-2023-30861 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-2023-30861?
- Upgrade
flaskto 2.3.2 or later - Upgrade
flaskto 2.2.5 or later
- Upgrade