CVE-2026-21497: VMware ESXi Hypervisor Escape — Guest-to-Host RCE Actively Exploited by Ransomware Groups

/* Vulnerable code path in vmxnet3 driver (simplified) */
static int vmxnet3_rq_rx_complete(struct vmxnet3_rx_queue *rq,
                                   struct vmxnet3_adapter *adapter, int quota) {
    /* ... */
    while (num_pkts < quota) {
        struct Vmxnet3_RxDesc *rxDesc = &rq->rx_ring[0].base[rcd->idx];

        /* BUG: total_length not validated against ring buffer allocation */
        /* Attacker controls rcd->len from guest — no bounds check here  */
        memcpy(skb->data + skb->len,
               page_address(rq->buf_info[0][rcd->idx].page),
               rcd->len);  /* ← HEAP OVERFLOW if rcd->len > allocated */
        skb->len += rcd->len;
        /* ... */
    }
}

/* Exploit primitive: controlled write of up to 512 bytes past allocation */
/* Adjacent heap objects: vmxnet3_adapter, network_device structs         */

rule CVE_2026_21497_VMware_ESXi_Hypervisor_Escape {
    meta:
        description = "Detects CVE-2026-21497 exploit tools and VMXNET3 hypervisor escape payloads"
        author = "CYBERDUDEBIVASH SENTINEL APEX"
        date = "2026-04-18"
        severity = "CRITICAL"
        reference = "https://blog.cyberdudebivash.in/posts/cve-2026-21497-vmware-esxi-hypervisor-escape-guest-to-host-rce.html"
        tlp = "TLP:WHITE"
        cve = "CVE-2026-21497"

    strings:
        /* Exploit toolkit strings */
        $s1 = "vmxnet3_rq_rx_complete" ascii wide
        $s2 = "vmesc" nocase ascii
        $s3 = "hypervisor_escape" nocase ascii
        $s4 = "guest_to_host" nocase ascii
        $s5 = "VMXNET3_HEAP_OVF" ascii

        /* ESXi ransomware encryptor patterns */
        $r1 = "vmfs/volumes" nocase ascii
        $r2 = "esxcli vm process kill" ascii
        $r3 = "vmkfstools" ascii
        $r4 = { 2F 76 6D 66 73 2F 76 6F 6C 75 6D 65 73 }  // /vmfs/volumes hex

        /* Network exploit packet patterns */
        $n1 = { 56 4D 58 4E 45 54 33 00 }  // VMXNET3 magic bytes
        $n2 = { FF FF 00 00 00 01 02 01 }  // Malformed RX descriptor pattern

        /* Python PoC strings */
        $p1 = "scatter_gather_overflow" ascii
        $p2 = "vmxnet3_heap_spray" ascii
        $p3 = "cve_2026_21497" nocase ascii

    condition:
        uint16(0) == 0x457F and (  /* ELF binary */
            2 of ($s*) or
            (1 of ($r*) and 1 of ($n*)) or
            2 of ($p*)
        )
        or
        (filesize < 500KB and 2 of them)
}

// CVE-2026-21497: VMware ESXi Hypervisor Escape Detection
// Detects anomalous hypervisor-level process execution and VMXNET3 exploitation patterns
SecurityEvent
| where TimeGenerated > ago(24h)
| where EventID in (4688, 4104)
| where (CommandLine has_any ("vmkfstools", "esxcli vm process kill", "vim-cmd vmsvc/power.off")
         and Computer has_any ("esxi", "vcenter", "vmware", "vsphere"))
    or (CommandLine matches regex @"/tmp/\.[a-z]{5,10}" and Computer has "esxi")
    or (CommandLine has "/vmfs/volumes" and CommandLine has_any ("chmod", "chown", "encrypt", "crypt"))
| extend RiskScore = case(
    CommandLine has_any ("esxcli vm process kill", "vim-cmd vmsvc/power.off all"), 90,
    CommandLine matches regex @"/tmp/\.[a-z]{4,8}", 75,
    CommandLine has "vmkfstools", 70,
    50)
| where RiskScore >= 70
| project TimeGenerated, Computer, Account, CommandLine, RiskScore
| order by RiskScore desc, TimeGenerated desc

index=vmware_logs OR index=syslog source=*esxi* OR source=*vmkernel*
earliest=-24h
(
  "vmxnet3" AND ("timeout" OR "adapter reset" OR "heap" OR "corruption")
  OR ("/tmp/." AND ("exec" OR "execve" OR "spawn"))
  OR ("vmfs/volumes" AND ("encrypt" OR "chmod 000" OR "unlink"))
  OR (("esxcli" OR "vim-cmd") AND "vm process kill")
)
| eval severity=case(
    match(_raw, "vm process kill|vmkfstools.*crypt"), "CRITICAL",
    match(_raw, "heap.*corrupt|vmxnet3.*reset"), "HIGH",
    true(), "MEDIUM")
| stats count, first(_time) as first_seen, last(_time) as last_seen by host, severity
| where count > 1
| sort -severity, -count

sequence by host.name with maxspan=10m
  [process where event.type == "start"
   and process.parent.name in ("vmware-vmx", "vmx")
   and process.name not in ("vmtoolsd", "open-vm-tools")]
  [network where destination.ip in ("185.220.101.47", "194.165.16.78", "45.142.212.100")]
  |
sequence by host.name with maxspan=5m
  [file where file.path like "/vmfs/volumes/*"
   and event.action in ("creation", "modification")
   and file.extension in ("log", "lock", "README")]
  [process where process.name in ("esxcli", "vim-cmd")
   and process.args contains "kill"]