# CVE-2023-30861: Flask vulnerable to possible disclosure of permanent session cookie due to missing Vary: Cookie header > Attackers can steal session cookies via caching proxies if Flask sessions are permanent and not accessed, leading to potential account takeover. - **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 2 May 2023 - **Updated:** 13 Feb 2025 - **Also known as:** AZL-43798, AZL-44718, CLSA-2026-1772410250, CLSA-2026-1772410251, DEBIAN-CVE-2023-30861, DLA-3536-1, DSA-5442-1, GHSA-m2qf-hxjv-5gpq, MGASA-2023-0193, OESA-2023-1922, OESA-2023-1936, PYSEC-2023-62, RHSA-2023:3440, RHSA-2023:3444, RHSA-2023:3446, RHSA-2023:3525, ROOT-APP-PYPI-CVE-2023-30861, SUSE-SU-2023:2263-1, SUSE-SU-2023:2263-2, SUSE-SU-2023:2264-1, UBUNTU-CVE-2023-30861, USN-6111-1, openSUSE-SU-2024:12923-1, openSUSE-SU-2026:11902-1 ## What is CVE-2023-30861? CVE-2023-30861 is a high-severity vulnerability in pallets flask, published on 2 May 2023. An attacker can exploit it over the internet without logging in, with no action from the victim. ### Official description Flask is a lightweight WSGI web application framework. When all of the following conditions are met, a response containing data intended for one client may be cached and subsequently sent by the 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. 1. The application must be hosted behind a caching proxy that does not strip cookies or ignore responses with cookies. 2. The application sets `session.permanent = True` 3. The application does not access or modify the session at any point during a request. 4. `SESSION_REFRESH_EACH_REQUEST` enabled (the default). 5. The application does not set a `Cache-Control` header 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. This issue has been fixed in versions 2.3.2 and 2.2.5. Weakness: CWE-539 (Use of Persistent Cookies Containing Sensitive Information) ## Which versions are affected by CVE-2023-30861? 5 open source packages ship an affected version, including flask and Flask. It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `flask` | PyPI | All versions | 2.2.5, 2.3.2 | | `flask` | PyPI | All versions | 2.2.5 | | `flask` | PyPI | From 2.3.0 | 2.3.2 | | `Flask` | PyPI | All versions | 2.2.5 | | `Flask` | PyPI | From 2.3.0 | 2.3.2 | | `python-flask` | Red Hat:openstack:16.1::el8 | All versions | 1:1.0.2-8.el8ost | | `python3-flask` | Red Hat:openstack:16.1::el8 | All versions | 1:1.0.2-8.el8ost | | `python-flask` | Red Hat:openstack:16.2::el8 | All versions | 1:1.0.2-8.el8ost | | `python3-flask` | Red Hat:openstack:16.2::el8 | All versions | 1:1.0.2-8.el8ost | | `python-flask` | Red Hat:openstack:17.0::el9 | All versions | 1:1.1.2-6.el9ost | | `python3-flask` | Red Hat:openstack:17.0::el9 | All versions | 1:1.1.2-6.el9ost | | `python-flask` | Red Hat:rhel_extras_other:7 | All versions | 1:0.10.1-7.el7_9 | | `python-Flask` | SUSE:Enterprise Storage 7 | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Enterprise Storage 7.1 | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Linux Enterprise High Performance Computing 15 SP1-LTSS | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSS | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Linux Enterprise High Performance Computing 15 SP3-ESPOS | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Linux Enterprise High Performance Computing 15 SP3-LTSS | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Linux Enterprise Module for Basesystem 15 SP4 | All versions | 1.0.4-150400.3.3.1 | | `python-Flask` | SUSE:Linux Enterprise Module for Basesystem 15 SP5 | All versions | 1.0.4-150400.3.3.1 | | `python-Flask` | SUSE:Linux Enterprise Module for Package Hub 15 SP4 | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Linux Enterprise Module for Package Hub 15 SP5 | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Linux Enterprise Real Time 15 SP3 | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Linux Enterprise Server 15 SP1-LTSS | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Linux Enterprise Server 15 SP2-LTSS | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Linux Enterprise Server 15 SP3-LTSS | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Linux Enterprise Server for SAP Applications 15 SP1 | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Linux Enterprise Server for SAP Applications 15 SP2 | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Linux Enterprise Server for SAP Applications 15 SP3 | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Manager Proxy 4.2 | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | SUSE:Manager Server 4.2 | All versions | 1.0.2-150100.6.3.1 | | `python-Flask` | openSUSE:Leap 15.4 | All versions | 1.0.4-150400.3.3.1 | | `python-Flask` | openSUSE:Leap 15.5 | All versions | 1.0.4-150400.3.3.1 | | `python-Flask` | openSUSE:Tumbleweed | All versions | 3.1.3-3.1 | Products: Palletsprojects Flask. ## How do I fix CVE-2023-30861? Upgrade flask to a fixed version for your release line: 2.2.5 or 2.3.2. ## Is CVE-2023-30861 being exploited? There is no known exploitation of CVE-2023-30861 in the wild and no public exploit code. Public exploit code: - Flask CVE-2023-30861 Poc 환경구축 (github_poc): https://github.com/fromitive/cve-2023-30861-poc ## Other vulnerabilities in pallets flask - [CVE-2019-1010083](https://insights.sbomify.com/CVE-2019-1010083/): Pallets Project Flask is vulnerable to Denial of Service via Unexpected memory usage (high 7.5) - [CVE-2026-27205](https://insights.sbomify.com/CVE-2026-27205/): Flask session does not add `Vary: Cookie` header when accessed in some ways (low 2.3) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2023-30861 and fix it if it is affected. What it is: CVE-2023-30861 is a high-severity vulnerability in pallets flask, published on 2 May 2023. An attacker can exploit it over the internet without logging in, with no action from the victim. Exploitation: There is no known exploitation of CVE-2023-30861 in the wild and no public exploit code. Affected packages and fixed versions: - flask (PyPI): affected all versions; fixed in 2.2.5, 2.3.2 - flask (PyPI): affected all versions; fixed in 2.2.5 - flask (PyPI): affected from 2.3.0; fixed in 2.3.2 - Flask (PyPI): affected all versions; fixed in 2.2.5 - Flask (PyPI): affected from 2.3.0; fixed in 2.3.2 Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2023-30861/ (Markdown: https://insights.sbomify.com/CVE-2023-30861/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2023-30861 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-30861 - OSV: https://osv.dev/vulnerability/CVE-2023-30861 --- From sbomify Insights: https://insights.sbomify.com/CVE-2023-30861/ Summaries are generated and may contain mistakes; the official description above is the record.