Session data disclosure via omitted cache variance
Published Feb 21, 2026 · Updated Feb 21, 2026
Cache-based information disclosure in Pallets Flask before 3.1.3 allows remote attackers to receive another user's session-specific response. SessionMixin.accessed remains false when application code performs key-only operations such as in or len, so save_session omits Vary: Cookie. Exposure requires a proxy that caches cookie-bearing responses, no private or non-cacheable Cache-Control directive, and a logged-in user to prime the cache before an unauthenticated request receives the cached content.
Summary
What happened
Cache-based information disclosure in Pallets Flask before 3.1.3 allows remote attackers to receive another user's session-specific response. SessionMixin.accessed remains false when application code performs key-only operations such as in or len, so save_session omits Vary: Cookie. Exposure requires a proxy that caches cookie-bearing responses, no private or non-cacheable Cache-Control directive, and a logged-in user to prime the cache before an unauthenticated request receives the cached content.
The record
- CVE
- CVE-2026-27205
- Published
- Feb 21, 2026
- Updated
- Feb 21, 2026
- Vendor
- Pallets
- Product
- Flask
- Classifications
- CWE-524, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Feb 21, 2026CVE publishedPublication date reported by the CVE source.
- Feb 21, 2026Record 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.
- Affected versionversion=< 3.1.3
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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