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Vulnerabilities

Vulnerability · CVE-2023-30861

Possible disclosure of permanent session cookie due to missing Vary: Cookie header

Published May 1, 2023 · catalog updated May 1, 2023 · Possible disclosure of permanent

Last checked Nov 5, 2026. A connected source was read within the last 7 days.

Check if you run it

No confirmed exploitation is on record for CVE-2023-30861. EPSS is 1.25%. Act only if a listed version of Possible disclosure of permanent is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (Possible disclosure of permanent).
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
High7.5
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
1.25%
Remediation
Version ranges on file
Affected products
pip flask

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 headers, it may send one client's 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. 1. The application must be hosted behind a caching proxy that does not strip cookies or ignore responses with cookies. 2. The application sets [ ](https://flask.palletsprojects.com/en/2.3.x/api/#flask.session.permanent). 2. The application does not access or modify the session at any point during a request. 4. [ ](https://flask.palletsprojects.com/en/2.3.x/config/#SESSION_REFRESH_EACH_REQUEST) is enabled (the default). 5. The application does not set a header to indicate that a page is private or should not be cached. This happens because vulnerable versions of Flask only set the header when the session is accessed or modified, not when it is refreshed (re-sent to update the expiration) without being accessed or modified.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • pip flask<2.3.1

How to fix CVE-2023-30861

  • Upgrade

    CVE-2023-30861 lists a fix boundary at 2.3.1. Compare the installed build with that boundary before you change production.

  • Version check

    Affected <2.3.1

  • Advisory

    Open the cve_org record and follow its fix.

  • Official advisories

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2023-30861. EPSS is 1.25%. Act only if a listed version of Possible disclosure of permanent is installed.

Which versions are affected?

Compare what you have installed with <2.3.1. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

No weakness class is stored. EPSS 1.25%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Technology hubs

Flask security

Code

The project this record applies to. Exploit listings stay in References.

No public source repository is identified for this product.

Timeline

Detection, updates, and resolution for this record.

  1. May 1, 2023 · Maintainer security tabs

    Detected

    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 headers, it may send one client's 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

  2. May 1, 2023 · Maintainer security tabs

    Updated

    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 headers, it may send one client's 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

Sources

  • CVE.orgVulnerability recordCVE-2023-30861

    Official CVE identifier

    cve.org
  • Maintainer security tabsVendor advisoryGHSA-m2qf-hxjv-5gpq

    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 headers, it may send one client's 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

    github.com

Freshness

Last verified in this catalog Nov 5, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.

  • CVE.orgNov 4, 2026
  • Maintainer security tabsNov 2, 2026