CVE-2026-63318

CVE Details

Release Date:	2026-09-03
Impact:	Moderate	What is this?

Description

QEMU's 9pfs readonly check at line 2108-2109 tests mode & O_TRUNC and mode & O_APPEND against the raw 9P open mode byte. For the 9P2000.u protocol, mode uses 9P flag values (Otrunc=0x10, Oappend=0x80), but the check compares against Linux constants (O_TRUNC=0x200, O_APPEND=0x400). Since 0x10 & 0x200 = 0 and 0x80 & 0x400 = 0, both flags bypass the readonly guard. The mode is then correctly converted to host flags via omode_to_uflags(), so open() is called with O_TRUNC -- truncating host files on readonly shares. 9P2000.L is NOT affected (guest passes Linux flags directly, so the bit values match).

See more information about CVE-2026-63318 from MITRE CVE dictionary and NIST NVD

NOTE: The following CVSS metrics and score provided are preliminary and subject to review.

CVSS v3 metrics

Base Score:	6.5
Vector String:	CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:H/A:N
Version:	3.1
Attack Vector:	Local
Attack Complexity:	Low
Privileges Required:	Low
User Interaction:	None
Scope:	Changed
Confidentiality Impact:	None
Integrity Impact:	High
Availability Impact:	None

Errata information

Platform	Errata	Release Date
Oracle Linux version 9 (qemu-kvm)	ELSA-2026-500245	2026-09-03