Bug 2441596 (CVE-2026-27205)

Summary: CVE-2026-27205 flask: Flask: Information disclosure via improper caching of session data
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: anthomas, aprice, bdettelb, caswilli, doconnor, ehelms, ggainey, jdobes, jkoehler, jsamir, juwatts, jwong, kaycoth, kgaikwad, kshier, lphiri, mhulan, nmoumoul, oezr, omaciel, orabin, osousa, pcreech, rchan, smallamp, stcannon, teagle, tmalecek, ttakamiy, yguenane
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in Flask, a Web Server Gateway Interface (WSGI) web application framework. When a Flask application accesses the session object using certain methods, it may fail to set the `Vary: Cookie` header. This oversight can cause sensitive, user-specific information to be improperly cached by a caching proxy, potentially leading to information disclosure to unauthorized parties.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Description OSIDB Bzimport 2026-02-21 06:01:38 UTC 
Flask is a web server gateway interface (WSGI) web application framework. In versions 3.1.2 and below, when the session object is accessed, Flask should set the Vary: Cookie header., resulting in a Use of Cache Containing Sensitive Information vulnerability. The logic instructs caches not to cache the response, as it may contain information specific to a logged in user. This is handled in most cases, but some forms of access such as the Python in operator were overlooked. The severity and risk depend on the application being hosted behind a caching proxy that doesn't ignore responses with cookies, not setting a Cache-Control header to mark pages as private or non-cacheable, and accessing the session in a way that only touches keys without reading values or mutating the session. The issue has been fixed in version 3.1.3.