Bug 2536496 (CVE-2026-90203)

Summary:	CVE-2026-90203 kernel: Squashfs: check block offset is not negative
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:	akhatavk, aos-team-art-private, asdas, dpaolell, jdelft, jupierce, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, sghai, sidsharm, suppawar, vlaad, watson-tool-maintainers
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:		Doc Type:	---
Doc Text:	A flaw was found in Squashfs, a compressed read-only filesystem within the Linux kernel. A local attacker with administrative privileges could create a specially crafted Squashfs filesystem that contains a negative block offset. If this malicious filesystem is mounted, an unprivileged user could then read a crafted file within it, leading to an out-of-bounds memory access. This vulnerability could result in information disclosure or a denial of service.	Story Points:	---
Clone Of:		Environment:
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Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description OSIDB Bzimport 2026-09-17 18:13:35 UTC

In the Linux kernel, the following vulnerability has been resolved:

Squashfs: check block offset is not negative

If a negative offset is read off disk (for example the offset into the
decompressed fragment block), this will cause squashfs_copy_data() to
perform an out of bounds access.

Fix by checking if offset is negative, and returning 0.  This matches
existing behaviour where an offset beyond the block returns 0 bytes
copied.

To trigger this out of bounds access requires a crafted Squashfs
filesystem and CAP_SYS_ADMIN to mount it.  Unprivileged users will not be
able to mount such a filesystem, but once mounted, an unprivileged user
can trigger the out of bounds access by reading the crafted file with the
negative offset.

Comment 1 Keith Grant 2026-09-23 23:00:15 UTC

Upstream advisory:
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-90203.mbox