{"id":"CVE-2023-30861","aliases":["GHSA-m2qf-hxjv-5gpq","PYSEC-2023-62"],"title":"Flask vulnerable to possible disclosure of permanent session cookie due to missing Vary: Cookie header","summary":"Flask vulnerable to possible disclosure of permanent session cookie due to missing Vary: Cookie header","severity":"high","cvss":7.5,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N","vendor":"flask","product":"flask","ecosystem":"pip","affected":["flask >= 2.3.0, < 2.3.2","flask < 2.2.5"],"patched":["flask 2.3.2","flask 2.2.5"],"published":"2023-05-01","updated":"2026-09-10","sourceUpdated":"2026-09-10T03:50:01.763387498Z","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-m2qf-hxjv-5gpq","references":[{"url":"https://github.com/pallets/flask/security/advisories/GHSA-m2qf-hxjv-5gpq"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2023-30861"},{"url":"https://github.com/pallets/flask/commit/70f906c51ce49c485f1d355703e9cc3386b1cc2b"},{"url":"https://github.com/pallets/flask/commit/afd63b16170b7c047f5758eb910c416511e9c965"},{"url":"https://github.com/pallets/flask"},{"url":"https://github.com/pallets/flask/releases/tag/2.2.5"},{"url":"https://github.com/pallets/flask/releases/tag/2.3.2"},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/flask/PYSEC-2023-62.yaml"},{"url":"https://lists.debian.org/debian-lts-announce/2023/08/msg00024.html"},{"url":"https://security.netapp.com/advisory/ntap-20230818-0006"},{"url":"https://www.debian.org/security/2023/dsa-5442"}],"tags":["osv","pip","exploit-available"],"epss":0.01261,"epssPercentile":0.68357,"exploits":{"github":1,"githubRepos":["https://github.com/fromitive/cve-2023-30861-poc"],"checkedAt":"2026-09-24T07:52:49.879Z"},"exploitAvailable":true,"ingestedAt":"2026-09-12T03:13:01.708Z","slug":"CVE-2023-30861","body":"## Overview\n\nWhen 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.\n\n1. The application must be hosted behind a caching proxy that does not strip cookies or ignore responses with cookies.\n2. The application sets [`session.permanent = True`](https://flask.palletsprojects.com/en/2.3.x/api/#flask.session.permanent).\n2. The application does not access or modify the session at any point during a request.\n4. [`SESSION_REFRESH_EACH_REQUEST`](https://flask.palletsprojects.com/en/2.3.x/config/#SESSION_REFRESH_EACH_REQUEST) is enabled (the default).\n5. The application does not set a `Cache-Control` header to indicate that a page is private or should not be cached.\n\nThis 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.\n\n## Affected packages\n\n- `flask >= 2.3.0, < 2.3.2`\n- `flask < 2.2.5`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `flask 2.3.2`\n- `flask 2.2.5`","depth":"midnight","depthScore":54,"depthScoreParts":{"impact":41.3,"likelihood":0.3,"exploitation":12,"ransomware":0},"changes":[]}