VulnerabilityPublished 11 Sep 2025Updated 10 Nov 2026

CVE-2025-40300: Linux kernel · x86/vmscape: Add conditional IBPB mitigation

Affects Linux kernelSource CVE Database V5

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Not known to be exploited · fixed in f866eef8d1c65504d30923c3f14082ad294d0e6d : update Linux kernel when convenient.

Data as of 10 Nov 2026, 15:03 UTC

Severity

Medium

Rated Medium by Threat Radar's AI analysis · No CVSS score from NVD or the CNA

Thin evidence

One scorer or none

Exploitation: No evidence of active exploitation and no public proof of concept
Automatable: Attackers cannot reliably automate steps 1–4 of the kill chain
Technical impact: Limited control over the software or limited information exposure

CISA SSVC assessment, 2026-06-10.

Patch

Available – fixed in f866eef8d1c65504d30923c3f14082ad294d0e6d, d5490dfa35427a2967e00a4c7a1b95fdbc8ede34, d7ddc93392e4a7ffcccc86edf6ef3e64c778db52 +12

Exploitation evidence

Disclosed: evidence found.
CVE published 11 Sep 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: 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 10 Nov 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.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), 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.

What to do

Upgrade to a fixed release listed under Affected software.

Affected software

Product	Affected	Fixed in
LinuxLinux	Affected >=3.16.57 <3.17 >=4.4.168 <4.5	Fixed in f866eef8d1c65504d30923c3f14082ad294d0e6d d5490dfa35427a2967e00a4c7a1b95fdbc8ede34 d7ddc93392e4a7ffcccc86edf6ef3e64c778db52 c08192b5d6730a914dee6175bc71092ee6a65f14 ac60717f9a8d21c58617d0b34274babf24135835 15006289e5c38b2a830e1fba221977a27598176c 893387c18612bb452336a5881da0d015a7e8f4a2 510603f504796c3535f67f55fb0b124a303b44c8 459274c77b37ac63b78c928b4b4e748d1f9d05c8 34e5667041050711a947e260fc9ebebe08bddee5 9c23a90648e831d611152ac08dbcd1283d405e7f 4.5 3.17 2f8f173413f1cbf52660d04df92d0069c4306d25 2f4f2f8f860cb4c3336a7435ebe8dcfded0c9c6e
LinuxLinux	Affected =4.16	Fixed in–

AI-suggested

A fix is available and has been integrated into the Linux kernel in the specified commits. Users should update to the fixed kernel versions to mitigate this vulnerability. The mitigation involves conditional IBPB issuance after VMexit before returning to userspace. Further optimization of IBPB usage is planned post-embargo. No additional user action beyond updating the kernel is required.

Description

CVE-2025-40300 addresses a vulnerability in the Linux kernel related to insufficient branch predictor isolation between a guest and a userspace hypervisor such as QEMU. The mitigation involves conditionally issuing an Indirect Branch Prediction Barrier (IBPB) after a VMexit and before returning to userspace to protect userspace from poisoned branch predictors. This fix targets workloads that frequently switch between hypervisor and userspace, though it may introduce overhead due to doubled IBPB issuance. The vulnerability affects specific Linux kernel versions and has been fixed in later commits.

Impact

The vulnerability allows potential exploitation of branch predictor state leakage between a guest and userspace hypervisor, which could be leveraged for speculative execution side-channel attacks. This impacts the confidentiality of userspace processes interacting with hypervisors. The mitigation reduces this risk by flushing branch predictors conditionally, but may introduce performance overhead for workloads with frequent hypervisor-userspace transitions. There are no known exploits in the wild at this time.

Summary

The Linux kernel vulnerability CVE-2025-40300 involves insufficient isolation of the branch predictor state between a guest virtual machine and a userspace hypervisor, which can lead to speculative execution side-channel attacks. Existing mitigations protect kernel and KVM from malicious guests, but userspace remained vulnerable. The patch adds a conditional IBPB flush after VMexit and before returning to userspace to mitigate this risk. This IBPB is separate from existing IBPB mitigations such as those triggered by the speculation control prctl() interface, resulting in doubled IBPB flushes in some scenarios. The fix is implemented in specific Linux kernel versions and aims to be optimized further after embargo.

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 10 Nov 2026, 15:03 UTC

Technical details

References

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