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CVE-2025-40026 (GCVE-0-2025-40026)

Vulnerability from cvelistv5 – Published: 2025-10-28 09:32 – Updated: 2026-08-05 12:07
VLAI
Title
KVM: x86: Don't (re)check L1 intercepts when completing userspace I/O
Summary
In the Linux kernel, the following vulnerability has been resolved: KVM: x86: Don't (re)check L1 intercepts when completing userspace I/O When completing emulation of instruction that generated a userspace exit for I/O, don't recheck L1 intercepts as KVM has already finished that phase of instruction execution, i.e. has already committed to allowing L2 to perform I/O. If L1 (or host userspace) modifies the I/O permission bitmaps during the exit to userspace, KVM will treat the access as being intercepted despite already having emulated the I/O access. Pivot on EMULTYPE_NO_DECODE to detect that KVM is completing emulation. Of the three users of EMULTYPE_NO_DECODE, only complete_emulated_io() (the intended "recipient") can reach the code in question. gp_interception()'s use is mutually exclusive with is_guest_mode(), and complete_emulated_insn_gp() unconditionally pairs EMULTYPE_NO_DECODE with EMULTYPE_SKIP. The bad behavior was detected by a syzkaller program that toggles port I/O interception during the userspace I/O exit, ultimately resulting in a WARN on vcpu->arch.pio.count being non-zero due to KVM no completing emulation of the I/O instruction. WARNING: CPU: 23 PID: 1083 at arch/x86/kvm/x86.c:8039 emulator_pio_in_out+0x154/0x170 [kvm] Modules linked in: kvm_intel kvm irqbypass CPU: 23 UID: 1000 PID: 1083 Comm: repro Not tainted 6.16.0-rc5-c1610d2d66b1-next-vm #74 NONE Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 RIP: 0010:emulator_pio_in_out+0x154/0x170 [kvm] PKRU: 55555554 Call Trace: <TASK> kvm_fast_pio+0xd6/0x1d0 [kvm] vmx_handle_exit+0x149/0x610 [kvm_intel] kvm_arch_vcpu_ioctl_run+0xda8/0x1ac0 [kvm] kvm_vcpu_ioctl+0x244/0x8c0 [kvm] __x64_sys_ioctl+0x8a/0xd0 do_syscall_64+0x5d/0xc60 entry_SYSCALL_64_after_hwframe+0x4b/0x53 </TASK>
Severity
7.9 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < a908eca437789589dd4624da428614c1275064dc (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < 00338255bb1f422642fb2798ebe92e93b6e4209b (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < e0ce3ed1048a47986d15aef1a98ebda25560d257 (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < ba35a5d775799ce5ad60230be97336f2fefd518e (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < 3d3abf3f7e8b1abb082070a343de82d7efc80523 (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < e7177c7e32cb806f348387b7f4faafd4a5b32054 (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < 3a062a5c55adc5507600b9ae6d911e247e2f1d6e (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < 7366830642505683bbe905a2ba5d18d6e4b512b8 (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < e750f85391286a4c8100275516973324b621a269 (git)
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Linux Linux Affected: 3.0
Unaffected: 0 , < 3.0 (semver)
Unaffected: 5.4.301 , ≤ 5.4.* (semver)
Unaffected: 5.10.246 , ≤ 5.10.* (semver)
Unaffected: 5.15.195 , ≤ 5.15.* (semver)
Unaffected: 6.1.157 , ≤ 6.1.* (semver)
Unaffected: 6.6.111 , ≤ 6.6.* (semver)
Unaffected: 6.12.52 , ≤ 6.12.* (semver)
Unaffected: 6.16.12 , ≤ 6.16.* (semver)
Unaffected: 6.17.2 , ≤ 6.17.* (semver)
Unaffected: 6.18 , ≤ * (original_commit_for_fix)
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{
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      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: x86: Don\u0027t (re)check L1 intercepts when completing userspace I/O\n\nWhen completing emulation of instruction that generated a userspace exit\nfor I/O, don\u0027t recheck L1 intercepts as KVM has already finished that\nphase of instruction execution, i.e. has already committed to allowing L2\nto perform I/O.  If L1 (or host userspace) modifies the I/O permission\nbitmaps during the exit to userspace,  KVM will treat the access as being\nintercepted despite already having emulated the I/O access.\n\nPivot on EMULTYPE_NO_DECODE to detect that KVM is completing emulation.\nOf the three users of EMULTYPE_NO_DECODE, only complete_emulated_io() (the\nintended \"recipient\") can reach the code in question.  gp_interception()\u0027s\nuse is mutually exclusive with is_guest_mode(), and\ncomplete_emulated_insn_gp() unconditionally pairs EMULTYPE_NO_DECODE with\nEMULTYPE_SKIP.\n\nThe bad behavior was detected by a syzkaller program that toggles port I/O\ninterception during the userspace I/O exit, ultimately resulting in a WARN\non vcpu-\u003earch.pio.count being non-zero due to KVM no completing emulation\nof the I/O instruction.\n\n  WARNING: CPU: 23 PID: 1083 at arch/x86/kvm/x86.c:8039 emulator_pio_in_out+0x154/0x170 [kvm]\n  Modules linked in: kvm_intel kvm irqbypass\n  CPU: 23 UID: 1000 PID: 1083 Comm: repro Not tainted 6.16.0-rc5-c1610d2d66b1-next-vm #74 NONE\n  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015\n  RIP: 0010:emulator_pio_in_out+0x154/0x170 [kvm]\n  PKRU: 55555554\n  Call Trace:\n   \u003cTASK\u003e\n   kvm_fast_pio+0xd6/0x1d0 [kvm]\n   vmx_handle_exit+0x149/0x610 [kvm_intel]\n   kvm_arch_vcpu_ioctl_run+0xda8/0x1ac0 [kvm]\n   kvm_vcpu_ioctl+0x244/0x8c0 [kvm]\n   __x64_sys_ioctl+0x8a/0xd0\n   do_syscall_64+0x5d/0xc60\n   entry_SYSCALL_64_after_hwframe+0x4b/0x53\n   \u003c/TASK\u003e"
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 7.9,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - Exploitation requires either `KVM_RUN`/`ioctl()` access to `/dev/kvm` on the host or code execution inside a guest running a nested hypervisor; there is no network-reachable path to `x86_emulate_insn()`.\nAC:L - The syzkaller reproducer deterministically toggles the port-I/O bitmap while servicing the `KVM_EXIT_IO` exit, and the guest-side variant only needs a second L1 vCPU to write the bitmap page \u2014 the attacker controls both sides of the window, and nested virt is enabled by default on kvm_intel/kvm_amd.\nPR:L - The attacker needs only unprivileged access to `/dev/kvm` (commonly granted via the `kvm` group or a VMM process), or ordinary code execution inside a guest; no root or CAP_SYS_ADMIN in the init namespace is required.\nUI:N - The vCPU thread reaches the buggy re-check on the normal `KVM_RUN` re-entry path after a userspace I/O exit; no victim action is needed.\nS:C - A guest (L1/L2) triggers corrupted state and a WARN/panic in the host kernel\u0027s KVM module, crossing the guest-to-host security boundary, and the aborted emulation also crosses the L1/L2 authority boundary by delivering an I/O intercept L1 never legitimately received.\nC:L - The stale `pio.count`/`pio.size` makes the next emulated `in` take the completion path and hand back data captured during the earlier I/O read on L2\u0027s behalf, leaking bounded `pio_data` contents across the L1/L2 boundary; the copy is capped at the 1024-byte `read_cache`, so it is not an arbitrary read.\nI:L - KVM injects a nested IOIO VM-exit to L1 for an access it already performed against the host device model (double execution) and subsequently substitutes stale `pio_data` for a real port read, corrupting guest I/O state in a limited, non-arbitrary way.\nA:H - The un-cleared `vcpu-\u003earch.pio.count` trips `WARN_ON_ONCE()` in `emulator_pio_in_out()` in the host kernel \u2014 a full host panic on the widely deployed `panic_on_warn=1` configuration \u2014 and leaves the vCPU with permanently desynchronized emulation state."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-05T12:07:08.793Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/a908eca437789589dd4624da428614c1275064dc"
        },
        {
          "url": "https://git.kernel.org/stable/c/00338255bb1f422642fb2798ebe92e93b6e4209b"
        },
        {
          "url": "https://git.kernel.org/stable/c/e0ce3ed1048a47986d15aef1a98ebda25560d257"
        },
        {
          "url": "https://git.kernel.org/stable/c/ba35a5d775799ce5ad60230be97336f2fefd518e"
        },
        {
          "url": "https://git.kernel.org/stable/c/3d3abf3f7e8b1abb082070a343de82d7efc80523"
        },
        {
          "url": "https://git.kernel.org/stable/c/e7177c7e32cb806f348387b7f4faafd4a5b32054"
        },
        {
          "url": "https://git.kernel.org/stable/c/3a062a5c55adc5507600b9ae6d911e247e2f1d6e"
        },
        {
          "url": "https://git.kernel.org/stable/c/7366830642505683bbe905a2ba5d18d6e4b512b8"
        },
        {
          "url": "https://git.kernel.org/stable/c/e750f85391286a4c8100275516973324b621a269"
        }
      ],
      "title": "KVM: x86: Don\u0027t (re)check L1 intercepts when completing userspace I/O",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2025-40026",
    "datePublished": "2025-10-28T09:32:33.075Z",
    "dateReserved": "2025-04-16T07:20:57.152Z",
    "dateUpdated": "2026-08-05T12:07:08.793Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "epss": {
      "cve": "CVE-2025-40026",
      "date": "2026-09-16",
      "epss": "0.00158",
      "percentile": "0.05406"
    },
    "nvd": "{\"cve\":{\"id\":\"CVE-2025-40026\",\"sourceIdentifier\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"published\":\"2025-10-28T10:15:42.167\",\"lastModified\":\"2026-07-30T06:24:06.610\",\"vulnStatus\":\"Deferred\",\"cveTags\":[],\"descriptions\":[{\"lang\":\"en\",\"value\":\"In the Linux kernel, the following vulnerability has been resolved:\\n\\nKVM: x86: Don\u0027t (re)check L1 intercepts when completing userspace I/O\\n\\nWhen completing emulation of instruction that generated a userspace exit\\nfor I/O, don\u0027t recheck L1 intercepts as KVM has already finished that\\nphase of instruction execution, i.e. has already committed to allowing L2\\nto perform I/O.  If L1 (or host userspace) modifies the I/O permission\\nbitmaps during the exit to userspace,  KVM will treat the access as being\\nintercepted despite already having emulated the I/O access.\\n\\nPivot on EMULTYPE_NO_DECODE to detect that KVM is completing emulation.\\nOf the three users of EMULTYPE_NO_DECODE, only complete_emulated_io() (the\\nintended \\\"recipient\\\") can reach the code in question.  gp_interception()\u0027s\\nuse is mutually exclusive with is_guest_mode(), and\\ncomplete_emulated_insn_gp() unconditionally pairs EMULTYPE_NO_DECODE with\\nEMULTYPE_SKIP.\\n\\nThe bad behavior was detected by a syzkaller program that toggles port I/O\\ninterception during the userspace I/O exit, ultimately resulting in a WARN\\non vcpu-\u003earch.pio.count being non-zero due to KVM no completing emulation\\nof the I/O instruction.\\n\\n  WARNING: CPU: 23 PID: 1083 at arch/x86/kvm/x86.c:8039 emulator_pio_in_out+0x154/0x170 [kvm]\\n  Modules linked in: kvm_intel kvm irqbypass\\n  CPU: 23 UID: 1000 PID: 1083 Comm: repro Not tainted 6.16.0-rc5-c1610d2d66b1-next-vm #74 NONE\\n  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015\\n  RIP: 0010:emulator_pio_in_out+0x154/0x170 [kvm]\\n  PKRU: 55555554\\n  Call Trace:\\n   \u003cTASK\u003e\\n   kvm_fast_pio+0xd6/0x1d0 [kvm]\\n   vmx_handle_exit+0x149/0x610 [kvm_intel]\\n   kvm_arch_vcpu_ioctl_run+0xda8/0x1ac0 [kvm]\\n   kvm_vcpu_ioctl+0x244/0x8c0 [kvm]\\n   __x64_sys_ioctl+0x8a/0xd0\\n   do_syscall_64+0x5d/0xc60\\n   entry_SYSCALL_64_after_hwframe+0x4b/0x53\\n   \u003c/TASK\u003e\"}],\"affected\":[{\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"affectedData\":[{\"vendor\":\"Linux\",\"product\":\"Linux\",\"defaultStatus\":\"unaffected\",\"programFiles\":[\"arch/x86/kvm/emulate.c\",\"arch/x86/kvm/kvm_emulate.h\",\"arch/x86/kvm/x86.c\"],\"repo\":\"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\"versions\":[{\"version\":\"8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9\",\"lessThan\":\"a908eca437789589dd4624da428614c1275064dc\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9\",\"lessThan\":\"00338255bb1f422642fb2798ebe92e93b6e4209b\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9\",\"lessThan\":\"e0ce3ed1048a47986d15aef1a98ebda25560d257\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9\",\"lessThan\":\"ba35a5d775799ce5ad60230be97336f2fefd518e\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9\",\"lessThan\":\"3d3abf3f7e8b1abb082070a343de82d7efc80523\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9\",\"lessThan\":\"e7177c7e32cb806f348387b7f4faafd4a5b32054\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9\",\"lessThan\":\"3a062a5c55adc5507600b9ae6d911e247e2f1d6e\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9\",\"lessThan\":\"7366830642505683bbe905a2ba5d18d6e4b512b8\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9\",\"lessThan\":\"e750f85391286a4c8100275516973324b621a269\",\"versionType\":\"git\",\"status\":\"affected\"}]},{\"vendor\":\"Linux\",\"product\":\"Linux\",\"defaultStatus\":\"affected\",\"programFiles\":[\"arch/x86/kvm/emulate.c\",\"arch/x86/kvm/kvm_emulate.h\",\"arch/x86/kvm/x86.c\"],\"repo\":\"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\"versions\":[{\"version\":\"3.0\",\"status\":\"affected\"},{\"version\":\"0\",\"lessThan\":\"3.0\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"5.4.301\",\"lessThanOrEqual\":\"5.4.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"5.10.246\",\"lessThanOrEqual\":\"5.10.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"5.15.195\",\"lessThanOrEqual\":\"5.15.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"6.1.157\",\"lessThanOrEqual\":\"6.1.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"6.6.111\",\"lessThanOrEqual\":\"6.6.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"6.12.52\",\"lessThanOrEqual\":\"6.12.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"6.16.12\",\"lessThanOrEqual\":\"6.16.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"6.17.2\",\"lessThanOrEqual\":\"6.17.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"6.18\",\"lessThanOrEqual\":\"*\",\"versionType\":\"original_commit_for_fix\",\"status\":\"unaffected\"}]}]}],\"metrics\":{\"cvssMetricV31\":[{\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"type\":\"Secondary\",\"cvssData\":{\"version\":\"3.1\",\"vectorString\":\"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H\",\"baseScore\":7.9,\"baseSeverity\":\"HIGH\",\"attackVector\":\"LOCAL\",\"attackComplexity\":\"LOW\",\"privilegesRequired\":\"LOW\",\"userInteraction\":\"NONE\",\"scope\":\"CHANGED\",\"confidentialityImpact\":\"LOW\",\"integrityImpact\":\"LOW\",\"availabilityImpact\":\"HIGH\"},\"exploitabilityScore\":2.0,\"impactScore\":5.3}]},\"references\":[{\"url\":\"https://git.kernel.org/stable/c/00338255bb1f422642fb2798ebe92e93b6e4209b\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/3a062a5c55adc5507600b9ae6d911e247e2f1d6e\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/3d3abf3f7e8b1abb082070a343de82d7efc80523\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/7366830642505683bbe905a2ba5d18d6e4b512b8\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/a908eca437789589dd4624da428614c1275064dc\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/ba35a5d775799ce5ad60230be97336f2fefd518e\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/e0ce3ed1048a47986d15aef1a98ebda25560d257\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/e7177c7e32cb806f348387b7f4faafd4a5b32054\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/e750f85391286a4c8100275516973324b621a269\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"}]}}",
    "redhat_vex": {
      "aggregate_severity": "Moderate",
      "current_release_date": "2026-09-16T15:05:37+00:00",
      "cve": "CVE-2025-40026",
      "id": "CVE-2025-40026",
      "initial_release_date": "2025-10-28T00:00:00+00:00",
      "product_status:fixed": "1708",
      "product_status:known_affected": "127",
      "product_status:known_not_affected": "15",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: KVM: x86: Don\u0027t (re)check L1 intercepts when completing userspace I/O",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2025/cve-2025-40026.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "low",
      "current_release_date": "2026-08-29T01:16:04Z",
      "cve": "CVE-2025-40026",
      "id": "CVE-2025-40026",
      "initial_release_date": "2025-10-29T00:25:45Z",
      "product_status:known_affected": "336",
      "product_status:recommended": "67",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2025-40026",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2025-40026.json",
      "version": "16"
    }
  }
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.


Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.