Bug 2571250 (CVE-2026-126073)

Summary: CVE-2026-126073 actionpack: Action Pack: Web cache poisoning via improper Vary header generation during content negotiation
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: akostadi, amasferr, dmayorov, ehelms, eshamard, jlledo, jpasqual, juwatts, jvasik, kaycoth, lblicha, mhulan, ogajduse, osousa, pantinor, rblanco, rchan, smallamp, tsedmik, zhunting
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in Action Pack. When determining response formats through content negotiation, Action Pack fails to properly set or append to the HTTP Vary response header under certain request conditions. A remote attacker can exploit this behavior by sending requests through a shared intermediary cache, causing the cache to store an improperly formatted response. This leads to web cache poisoning, where subsequent users receive unexpected response representations, such as HTML instead of JSON or unstyled web pages.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Description OSIDB Bzimport 2026-11-10 20:58:55 UTC 
Action Pack is a framework for handling and responding to web requests. Prior to 8.1.4.1 and 8.0.5.2, Action Pack negotiates the format of a rendered response from the Accept, X-Requested-With and, for XHR requests without Accept, Content-Type request headers, but ActionController::Rendering#_set_vary_header adds Vary: Accept only when the request carries a usable Accept header and the response has no Vary header yet. A response to a request without Accept, or to an XHR request, therefore omits the headers that selected its representation, and a response whose Vary the application set before rendering (for example Vary: Accept-Language) never gains Accept. A shared cache that honours Vary could store such a response and serve it to later clients that request a different representation, possibly leading to cache poisoning in which visitors receive, until the entry expires, a page without its layout or HTML in place of JSON. The attacker cannot choose the content of the response. Applications that render negotiated responses from actions marked public (for example with expires_in ..., public: true) behind a shared cache that honours Vary, such as Thruster or Rack::Cache, are affected. This issue is fixed in versions 8.1.4.1 and 8.0.5.2.