VulnerabilityPublished 19 Jul 2026

CVE-2026-63941: Linux kernel · KVM: arm64: Correctly cap ZCR_EL2 provided by a guest hypervisor

Source GCVE Database

Authoritative record:CVE-2026-63941· High 8.8

Not known to be exploited · CVSS 8.8 High · fixed in 83726330748981372bde86ed5411d7b306612991 : patch Linux kernel soon.

Data as of 26 Oct 2026, 13:47 UTC

CVSS 3.1 base score

8.8High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.1% chance of exploitation in 30 days – higher than 4% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 83726330748981372bde86ed5411d7b306612991, 2090c1a2368339eab0e14dcde12fdf6a5ea6359d, 742a9b5ccd8c46caf983ca90c94f855466968e34

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

Attack Vector: Local
Attack Complexity: Low
Privileges Required: Low
User Interaction: None
Scope: Changed
Confidentiality: High
Integrity: High
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Local: The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Either: the attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or remotely (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., tricking a legitimate user into opening a malicious document).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 19 Jul 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

ghsa-9mxh-6wp4-9v6p

What to do

Upgrade Linux kernel to 83726330748981372bde86ed5411d7b306612991, 2090c1a2368339eab0e14dcde12fdf6a5ea6359d or 742a9b5ccd8c46caf983ca90c94f855466968e34.

Affected software

Product	Affected	Fixed in
LinuxLinux	Affected ≥ b3d29a8230998b36afecf494b199211d26052785, < 83726330748981372bde86ed5411d7b306612991	Fixed in 83726330748981372bde86ed5411d7b306612991
LinuxLinux	Affected ≥ b3d29a8230998b36afecf494b199211d26052785, < 2090c1a2368339eab0e14dcde12fdf6a5ea6359d	Fixed in 2090c1a2368339eab0e14dcde12fdf6a5ea6359d
LinuxLinux	Affected ≥ b3d29a8230998b36afecf494b199211d26052785, < 742a9b5ccd8c46caf983ca90c94f855466968e34	Fixed in 742a9b5ccd8c46caf983ca90c94f855466968e34
LinuxLinux	Affected =6.11	Fixed in–

AI-suggested

The vulnerability description indicates that a fix has been implemented in the Linux kernel source code. Users should monitor official Linux kernel releases and apply updates that include this fix once available.

Description

In the Linux kernel, the following vulnerability has been resolved: KVM: arm64: Correctly cap ZCR_EL2 provided by a guest hypervisor ZCR_EL2 can be updated by a VHE guest hypervisor either using ZCR_EL2 (which traps) or ZCR_EL1 (which does not trap). KVM handles both in different way: - on ZCR_EL2 trap, ZCR_EL2.LEN is immediately capped at the VM's own VL limit. This has the potential to break existing SW that relies on the full LEN field to be stateful. - on ZCR_EL1 access, we do absolutely nothing. On restoring the SVE context for an L2 guest, we directly restore the guest hypervisor's view of ZCR_EL2 into the physical ZCR_EL2. If the guest's view of the register was updated using the ZCR_EL2 accessor, the value has already been sanitised (with the caveat mentioned above). But if the guest used ZCR_EL1, the raw value is written into the HW, and the L2 guest can now access VLs that it shouldn't. Fix all the above by moving the VL capping to the restore points, ensuring that: - the HW is always programmed with a capped value, irrespective of the accessor being used, - the ZCR_EL2.LEN field is always completely stateful, irrespective of the accessor being used. Additionally, move ZCR_EL2 to be a sanitised register, ensuring that only the LEN field is actually stateful. This requires some creative construction of the RES0 mask, as the sysreg generation script does not yet generate RAZ/WI fields. [maz: rewrote commit message, tidy up access_zcr_el2()]

Impact

An L2 guest hypervisor could exploit this vulnerability to access vector lengths (VLs) that exceed its permitted limits, potentially leading to unauthorized access or manipulation of hardware vector registers. This could affect the isolation guarantees between nested guests in a virtualized environment on arm64 architectures using KVM.

Summary

The vulnerability in the Linux kernel KVM arm64 implementation involved incorrect capping of the ZCR_EL2 register's LEN field when updated by a guest hypervisor. Updates via ZCR_EL2 traps were capped immediately, potentially breaking software relying on the full LEN field statefulness, while updates via ZCR_EL1 were not trapped or capped. Upon restoring the SVE context for an L2 guest, the raw ZCR_EL2 value could be written directly to hardware, allowing the L2 guest to access vector lengths beyond its allowed limit. The fix moves the capping logic to restore points to ensure consistent capping regardless of the accessor and sanitizes ZCR_EL2 so only the LEN field is stateful.

References

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