Bug 2562352 (CVE-2026-116686)

Summary:	CVE-2026-116686 mcp: mcp: Denial of Service via schema reference resolution
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security DevOps Team <prodsec-dev>
Status:	NEW ---	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	CC:	alinfoot, anpicker, anthomas, bbrownin, blitton, dfreiber, dkeler, drow, dschmidt, dtrifiro, ebourniv, ehelms, ggainey, hasun, ikhan, ilpinto, jburrell, jfula, jlanda, jowilson, jpasqual, juwatts, jwong, kaycoth, kshier, ltomasbo, mbarnett, mdellweg, mhayden, mhulan, mwoodson, nmoumoul, nyancey, omaciel, ometelka, osousa, pcreech, ptisnovs, rbobbitt, rchan, rjohnson, sbunciak, sdoran, simaishi, smallamp, stcannon, suppawar, syedriko, thason, tmalecek, vkumar, weaton, xdharmai, yguenane, ykashtan
Target Milestone:	---	Keywords:	Security
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Hardware:	All
OS:	Linux
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Doc Text:	A flaw was found in mcp. Client sessions validate server-declared schema documents using a default resolver that automatically retrieves external network and local filesystem references. A malicious server can exploit this flaw to cause a Denial of Service (DoS) by stalling client sessions synchronously or to trigger unauthorized local file reads and outbound network requests.	Story Points:	---
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oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description OSIDB Bzimport 2026-10-27 19:31:51 UTC

The Model Context Protocol Python SDK is the official Python SDK for Model Context Protocol servers and clients. From 1.10.0 until 1.30.0 and 2.2.0, ClientSession, ClientSessionGroup, and Client tool calls validate server-declared outputSchema documents with jsonschema's default resolver, causing an unresolved $ref value to fetch an HTTP, HTTPS, or file URL from the client's network and filesystem context. A server can trigger blind requests or local-file reads and can stall every session in the process because resolution runs synchronously on the event loop without a timeout. This issue is fixed in versions 1.30.0 and 2.2.0.