CVE-2023-30861Network attack vector

Session cookie disclosure via missing cache variance

Published May 2, 2023 · Updated Feb 13, 2025

Information disclosure in Pallets Flask before 2.2.5 and 2.3.0 through 2.3.1 allows remote attackers to receive other clients' data. SecureCookieSessionInterface.save_session refreshes a permanent session cookie without adding Vary: Cookie when application code neither reads nor changes the session. Disclosure requires a cookie-caching proxy, default per-request session refresh, no private Cache-Control directive, and an affected response; cached data can include another client's session cookie.

CVSS severity8.7
High
EPSS probability1.26%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Information disclosure in Pallets Flask before 2.2.5 and 2.3.0 through 2.3.1 allows remote attackers to receive other clients' data. SecureCookieSessionInterface.save_session refreshes a permanent session cookie without adding Vary: Cookie when application code neither reads nor changes the session. Disclosure requires a cookie-caching proxy, default per-request session refresh, no private Cache-Control directive, and an affected response; cached data can include another client's session cookie.

The record

CVE
CVE-2023-30861
Published
May 2, 2023
Updated
Feb 13, 2025
Vendor
Pallets
Product
Flask
Classifications
CWE-539, CAPEC-31, T1539
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 2, 2023CVE publishedPublication date reported by the CVE source.
  2. Feb 13, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 2.2.5
  2. Affected versionversion=>= 2.3.0, < 2.3.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Pallets · Flaskversion=< 2.2.5; version=>= 2.3.0, < 2.3.2

Published CVSS scores

8.7GitHubCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-539Use of Persistent Cookies Containing Sensitive Information
CAPEC-31Accessing/Intercepting/Modifying HTTP Cookies
T1539Steal Web Session Cookie

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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