x86: Out-of-bounds read in vRTC emulation

Accesses to the CMOS memory contents are done using an indirect IO port

pair. Therefore Xen needs to cache the guest chosen index, and one of

the usages of the index didn't take the necessary locking to avoid

concurrent changes. As a result, a guest could change the index after

it being checked, causing a subsequent out-of-bound read access to the

contents of an array.

Affected products

Xen

Xen

consult Xen advisory XSA-503 - UNKNOWN

References

xenbits.xenproject.org

https://xenbits.xenproject.org/xsa/advisory-503.html

GitHub Security Advisories

GHSA-w4qm-qcx6-wmw8

Accesses to the CMOS memory contents are done using an indirect IO port pair. Therefore Xen...

https://github.com/advisories/GHSA-w4qm-qcx6-wmw8

Accesses to the CMOS memory contents are done using an indirect IO port pair. Therefore Xen needs to cache the guest chosen index, and one of the usages of the index didn't take the necessary locking to avoid concurrent changes. As a result, a guest could change the index after it being checked, causing a subsequent out-of-bound read access to the contents of an array.

nvd.nist.gov

https://nvd.nist.gov/vuln/detail/CVE-2026-62430

xenbits.xenproject.org

https://xenbits.xenproject.org/xsa/advisory-503.html

xenbits.xen.org

http://xenbits.xen.org/xsa/advisory-503.html

openwall.com

http://www.openwall.com/lists/oss-security/2026/07/28/19

github.com

https://github.com/advisories/GHSA-w4qm-qcx6-wmw8

JSON source

https://cveawg.mitre.org/api/cve/CVE-2026-62430

Click to expand

{
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "cveMetadata": {
    "cveId": "CVE-2026-62430",
    "assignerOrgId": "23aa2041-22e1-471f-9209-9b7396fa234f",
    "assignerShortName": "XEN",
    "dateUpdated": "2026-07-28T13:34:12.099Z",
    "dateReserved": "2026-07-14T10:28:12.655Z",
    "datePublished": "2026-07-28T12:32:33.150Z",
    "state": "PUBLISHED"
  },
  "containers": {
    "cna": {
      "providerMetadata": {
        "orgId": "23aa2041-22e1-471f-9209-9b7396fa234f",
        "shortName": "XEN",
        "dateUpdated": "2026-07-28T12:32:33.150Z"
      },
      "datePublic": "2026-07-28T12:00:00.000Z",
      "title": "x86: Out-of-bounds read in vRTC emulation",
      "descriptions": [
        {
          "lang": "en",
          "value": "Accesses to the CMOS memory contents are done using an indirect IO port\npair.  Therefore Xen needs to cache the guest chosen index, and one of\nthe usages of the index didn't take the necessary locking to avoid\nconcurrent changes.  As a result, a guest could change the index after\nit being checked, causing a subsequent out-of-bound read access to the\ncontents of an array."
        }
      ],
      "affected": [
        {
          "vendor": "Xen",
          "product": "Xen",
          "defaultStatus": "unknown",
          "versions": [
            {
              "version": "consult Xen advisory XSA-503",
              "status": "unknown"
            }
          ]
        }
      ],
      "references": [
        {
          "url": "https://xenbits.xenproject.org/xsa/advisory-503.html"
        }
      ],
      "impacts": [
        {
          "descriptions": [
            {
              "lang": "en",
              "value": "An attacker can read out-of-bounds, this is limited to Xen data, and not\ndata belonging to other guests."
            }
          ]
        }
      ],
      "configurations": [
        {
          "lang": "en",
          "value": "All Xen versions from 3.2 onwards are vulnerable.  Xen versions 3.1 and\nearlier are not vulnerable."
        }
      ],
      "workarounds": [
        {
          "lang": "en",
          "value": "Running only PV or PVH guests will avoid the vulnerability."
        }
      ]
    },
    "adp": [
      {
        "providerMetadata": {
          "orgId": "af854a3a-2127-422b-91ae-364da2661108",
          "shortName": "CVE",
          "dateUpdated": "2026-07-28T13:34:12.099Z"
        },
        "title": "CVE Program Container",
        "references": [
          {
            "url": "http://xenbits.xen.org/xsa/advisory-503.html"
          }
        ]
      }
    ]
  }
}