x86/hyperv: Avoid using per-cpu output page in hv_apicid_to_vp_index()
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 6.12.115, 6.16
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: 6.12.115, 6.16.
- Exploitation?/40Exploitation: unknown (data gap). No CISA SSVC exploitation rating yet
- Exposure?/25Exposure: unknown (data gap). No advisory context yet
- Reach20/20Affects Linux kernel (software on nearly every server, desktop or phone)
- Severity?/10Severity: unknown (data gap). No CVSS score or vendor rating
- No patch0/5No signal
What would change the score
- Would score about 75 if added to CISA KEV
- Would score about 70 if CISA's SSVC rates exploitation active
- Would score about 42 if advisories confirm default setups are exposed
CVE-2026-99258 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It affects Linux; fixed in 6.12.115 and 6.16.
- Last modified
- Where the record stands: published, rejected by its publisher, or superseded by another id.What do these mean? →
- Published
- Whether CISA lists it in its Known Exploited Vulnerabilities catalog: proof that attackers are using it.What do these mean? →
- Not listed in CISA KEV
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 20/100 · provisional
- Fix
- Fixed in 6.12.115, 6.16
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
The Linux kernel's Hyper-V support for x86 systems is affected, specifically when running as a VTL2 guest or as an AMD SEV-SNP confidential VM without a paravisor. The flaw lets the hypervisor write output data into memory the guest never allocated for that purpose, which could corrupt memory or cause a crash.
Read the original description
In the Linux kernel, the following vulnerability has been resolved: x86/hyperv: Avoid using per-cpu output page in hv_apicid_to_vp_index() hv_apicid_to_vp_index() currently uses the per-cpu hypercall input and output pages. This function is called when running in VTL2 and when running in an SEV-SNP CoCo VM with no paravisor. In the former case, the output page is allocated, but in the latter case it is not, so the hypervisor stores the output VP index in memory that has not been allocated by the guest. Fix this by using the input page for both input and output. The hypercall has very small input and output, so sharing the same page for both is straightforward. An alternative fix is to allocate the per-cpu output page when running in an SEV-SNP CoCo guest, but this uses significantly more memory, particularly with larger vCPUs counts.
What to do
Upgrade to a fixed version: 6.12.115, 6.16.
Timeline
- CVE published
- First seen by cve.now
- Record last updated
Affected versions
- Linux Linux
From 6.12.41 · fixed in 6.12.115; From 6.15.3 · fixed in 6.16
- Linux Linux
6.16
Evidence 4
- Link
- Link
- Link
- Link
Corrections
None so far.
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Questions about CVE-2026-99258
Is CVE-2026-99258 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
Which versions are affected by CVE-2026-99258?
Linux: from 6.12.41 before 6.12.115; from 6.15.3 before 6.16. Linux: version 6.16.Source: cve.now feed data
How do I fix CVE-2026-99258?
Upgrade to a fixed version: 6.12.115, 6.16.Source: cve.now feed data