AWS Network ACL Restricts SSH

Severity: high
Tags: AWS, Panther, Lateral Movement:Remote Services
Reference: https://docs.aws.amazon.com/vpc/latest/userguide/vpc-recommended-nacl-rules.html
Source: github.com/panther-labs/panther-analysis

SSH access should only be granted from protected network CIDR ranges.

MITRE ATT&CK coverage

Tactic	Techniques
Lateral Movement	`T1021` Remote Services

Rule body yaml

AnalysisType: policy
Filename: aws_network_acl_restricted_ssh.py
PolicyID: "AWS.NetworkACL.RestrictedSSH"
DisplayName: "AWS Network ACL Restricts SSH"
Enabled: true
ResourceTypes:
  - AWS.EC2.NetworkACL
Tags:
  - AWS
  - Panther
  - Lateral Movement:Remote Services
Reports:
  MITRE ATT&CK:
    - TA0008:T1021
Severity: High
Description: >
  SSH access should only be granted from protected network CIDR ranges.
Runbook: >
  Remove the NACL rule granting unprotected SSH access.
Reference: https://docs.aws.amazon.com/vpc/latest/userguide/vpc-recommended-nacl-rules.html
Tests:
  - Name: Secure VPC SSH Access
    ExpectedResult: true
    Resource:
      {
        "AccountId": "123456789012",
        "Region": "ap-southeast-2",
        "ARN": "arn:aws:ec2:ap-southeast-2:123456789012:network-acl/acl-111222333",
        "ID": "acl-111222333",
        "Tags": {},
        "ResourceID": "arn:aws:ec2:ap-southeast-2:123456789012:network-acl/acl-111222333",
        "ResourceType": "AWS.EC2.NetworkACL",
        "TimeCreated": null,
        "Associations":
          [
            {
              "NetworkAclAssociationId": "aclassoc-111222333",
              "NetworkAclId": "acl-111222333",
              "SubnetId": "subnet-111222333",
            },
          ],
        "Entries":
          [
            {
              "CidrBlock": "8.8.8.8/32",
              "Egress": false,
              "IcmpTypeCode": null,
              "Ipv6CidrBlock": null,
              "PortRange": { "From": 22, "To": 22 },
              "Protocol": "-1",
              "RuleAction": "allow",
              "RuleNumber": 99,
            },
            {
              "CidrBlock": "0.0.0.0/0",
              "Egress": true,
              "IcmpTypeCode": null,
              "Ipv6CidrBlock": null,
              "PortRange": { "From": 22, "To": 22 },
              "Protocol": "-1",
              "RuleAction": "allow",
              "RuleNumber": 100,
            },
          ],
        "IsDefault": true,
        "NetworkAclId": "acl-111222333",
        "OwnerId": "123456789012",
        "VpcId": "vpc-6aa60b12",
      }
  - Name: Global VPC SSH Access
    ExpectedResult: false
    Resource:
      {
        "AccountId": "123456789012",
        "Region": "ap-southeast-2",
        "ARN": "arn:aws:ec2:ap-southeast-2:123456789012:network-acl/acl-111222333",
        "ID": "acl-111222333",
        "Tags": {},
        "ResourceID": "arn:aws:ec2:ap-southeast-2:123456789012:network-acl/acl-111222333",
        "ResourceType": "AWS.EC2.NetworkACL",
        "TimeCreated": null,
        "Associations":
          [
            {
              "NetworkAclAssociationId": "aclassoc-111222333",
              "NetworkAclId": "acl-111222333",
              "SubnetId": "subnet-111222333",
            },
          ],
        "Entries":
          [
            {
              "CidrBlock": "0.0.0.0/0",
              "Egress": false,
              "IcmpTypeCode": null,
              "Ipv6CidrBlock": null,
              "PortRange": { "From": 22, "To": 22 },
              "Protocol": "-1",
              "RuleAction": "allow",
              "RuleNumber": 100,
            },
          ],
        "IsDefault": true,
        "NetworkAclId": "acl-111222333",
        "OwnerId": "123456789012",
        "VpcId": "vpc-6aa60b12",
      }
  - Name: Global IPv6 VPC SSH Access
    ExpectedResult: false
    Resource:
      {
        "AccountId": "123456789012",
        "Region": "ap-southeast-2",
        "ARN": "arn:aws:ec2:ap-southeast-2:123456789012:network-acl/acl-111222333",
        "ID": "acl-111222333",
        "Tags": {},
        "ResourceID": "arn:aws:ec2:ap-southeast-2:123456789012:network-acl/acl-111222333",
        "ResourceType": "AWS.EC2.NetworkACL",
        "TimeCreated": null,
        "Associations":
          [
            {
              "NetworkAclAssociationId": "aclassoc-111222333",
              "NetworkAclId": "acl-111222333",
              "SubnetId": "subnet-111222333",
            },
          ],
        "Entries":
          [
            {
              "CidrBlock": null,
              "Egress": false,
              "IcmpTypeCode": null,
              "Ipv6CidrBlock": "::/0",
              "PortRange": { "From": 22, "To": 22 },
              "Protocol": "-1",
              "RuleAction": "allow",
              "RuleNumber": 100,
            },
          ],
        "IsDefault": true,
        "NetworkAclId": "acl-111222333",
        "OwnerId": "123456789012",
        "VpcId": "vpc-6aa60b12",
      }
  - Name: Global VPC SSH Access From Range
    ExpectedResult: false
    Resource:
      {
        "AccountId": "123456789012",
        "Region": "ap-southeast-2",
        "ARN": "arn:aws:ec2:ap-southeast-2:123456789012:network-acl/acl-111222333",
        "ID": "acl-111222333",
        "Tags": {},
        "ResourceID": "arn:aws:ec2:ap-southeast-2:123456789012:network-acl/acl-111222333",
        "ResourceType": "AWS.EC2.NetworkACL",
        "TimeCreated": null,
        "Associations":
          [
            {
              "NetworkAclAssociationId": "aclassoc-111222333",
              "NetworkAclId": "acl-111222333",
              "SubnetId": "subnet-111222333",
            },
          ],
        "Entries":
          [
            {
              "CidrBlock": "0.0.0.0/0",
              "Egress": false,
              "IcmpTypeCode": null,
              "Ipv6CidrBlock": null,
              "PortRange": { "From": 0, "To": 1024 },
              "Protocol": "-1",
              "RuleAction": "allow",
              "RuleNumber": 100,
            },
          ],
        "IsDefault": true,
        "NetworkAclId": "acl-111222333",
        "OwnerId": "123456789012",
        "VpcId": "vpc-6aa60b12",
      }
  - Name: Global VPC SSH Access From Unrestricted Port Range
    ExpectedResult: false
    Resource:
      {
        "AccountId": "123456789012",
        "Region": "ap-southeast-2",
        "ARN": "arn:aws:ec2:ap-southeast-2:123456789012:network-acl/acl-111222333",
        "ID": "acl-111222333",
        "Tags": {},
        "ResourceID": "arn:aws:ec2:ap-southeast-2:123456789012:network-acl/acl-111222333",
        "ResourceType": "AWS.EC2.NetworkACL",
        "TimeCreated": null,
        "Associations":
          [
            {
              "NetworkAclAssociationId": "aclassoc-111222333",
              "NetworkAclId": "acl-111222333",
              "SubnetId": "subnet-111222333",
            },
          ],
        "Entries":
          [
            {
              "CidrBlock": "0.0.0.0/0",
              "Egress": false,
              "IcmpTypeCode": null,
              "Ipv6CidrBlock": null,
              "Protocol": "-1",
              "RuleAction": "allow",
              "RuleNumber": 100,
            },
          ],
        "IsDefault": true,
        "NetworkAclId": "acl-111222333",
        "OwnerId": "123456789012",
        "VpcId": "vpc-6aa60b12",
      }
  - Name: Selective SSH Access
    ExpectedResult: true
    Resource:
      {
        "AccountId": "123456789012",
        "Region": "ap-southeast-2",
        "ARN": "arn:aws:ec2:ap-southeast-2:123456789012:network-acl/acl-111222333",
        "ID": "acl-111222333",
        "Tags": {},
        "ResourceID": "arn:aws:ec2:ap-southeast-2:123456789012:network-acl/acl-111222333",
        "ResourceType": "AWS.EC2.NetworkACL",
        "TimeCreated": null,
        "Associations":
          [
            {
              "NetworkAclAssociationId": "aclassoc-111222333",
              "NetworkAclId": "acl-111222333",
              "SubnetId": "subnet-111222333",
            },
          ],
        "Entries":
          [
            {
              "CidrBlock": "0.0.0.0/0",
              "Egress": false,
              "IcmpTypeCode": null,
              "Ipv6CidrBlock": null,
              "PortRange": null,
              "Protocol": "-1",
              "RuleAction": "allow",
              "RuleNumber": 100,
            },
            {
              "CidrBlock": "1.1.1.1/32",
              "Egress": false,
              "IcmpTypeCode": null,
              "Ipv6CidrBlock": null,
              "PortRange": { "From": 22, "To": 22 },
              "Protocol": "-1",
              "RuleAction": "allow",
              "RuleNumber": 30,
            },
            {
              "CidrBlock": "0.0.0.0/0",
              "Egress": false,
              "IcmpTypeCode": null,
              "Ipv6CidrBlock": null,
              "PortRange": { "From": 22, "To": 22 },
              "Protocol": "-1",
              "RuleAction": "deny",
              "RuleNumber": 50,
            },
          ],
        "IsDefault": true,
        "NetworkAclId": "acl-111222333",
        "OwnerId": "123456789012",
        "VpcId": "vpc-6aa60b12",
      }
  - Name: Selective SSH Access From Port Range
    ExpectedResult: true
    Resource:
      {
        "AccountId": "123456789012",
        "Region": "ap-southeast-2",
        "ARN": "arn:aws:ec2:ap-southeast-2:123456789012:network-acl/acl-111222333",
        "ID": "acl-111222333",
        "Tags": {},
        "ResourceID": "arn:aws:ec2:ap-southeast-2:123456789012:network-acl/acl-111222333",
        "ResourceType": "AWS.EC2.NetworkACL",
        "TimeCreated": null,
        "Associations":
          [
            {
              "NetworkAclAssociationId": "aclassoc-111222333",
              "NetworkAclId": "acl-111222333",
              "SubnetId": "subnet-111222333",
            },
          ],
        "Entries":
          [
            {
              "CidrBlock": "0.0.0.0/0",
              "Egress": false,
              "IcmpTypeCode": null,
              "Ipv6CidrBlock": null,
              "PortRange": null,
              "Protocol": "-1",
              "RuleAction": "allow",
              "RuleNumber": 100,
            },
            {
              "CidrBlock": "1.1.1.1/32",
              "Egress": false,
              "IcmpTypeCode": null,
              "Ipv6CidrBlock": null,
              "PortRange": { "From": 0, "To": 1024 },
              "Protocol": "-1",
              "RuleAction": "allow",
              "RuleNumber": 30,
            },
            {
              "CidrBlock": "0.0.0.0/0",
              "Egress": false,
              "IcmpTypeCode": null,
              "Ipv6CidrBlock": null,
              "PortRange": { "From": 22, "To": 22 },
              "Protocol": "-1",
              "RuleAction": "deny",
              "RuleNumber": 50,
            },
          ],
        "IsDefault": true,
        "NetworkAclId": "acl-111222333",
        "OwnerId": "123456789012",
        "VpcId": "vpc-6aa60b12",
      }

Detection logic

Rule logic imperative Python

import ipaddress
GLOBAL_IPV6 = ipaddress.IPv6Network("::/0")
IPV6_SENTINEL = ipaddress.IPv6Network("::1/128")
def policy(resource):
    ingress_entries = sorted(
        (entry for entry in resource["Entries"] if not entry["Egress"]),
        key=lambda x: x["RuleNumber"],
    )
    for entry in ingress_entries:
        if (
            entry.get("CidrBlock") == "0.0.0.0/0"
            or ipaddress.IPv6Network(entry.get("Ipv6CidrBlock") or IPV6_SENTINEL) == GLOBAL_IPV6
        ) and (
            not entry.get("PortRange")
            or entry["PortRange"]["From"] <= 22 <= entry["PortRange"]["To"]
        ):
            return entry["RuleAction"] == "deny"
    return True

The parser cannot express this rule's logic as a field filter; the imperative Python above is the detection.

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`type:`	`events` / `rules` / `providers`
`vendor:`	`sigma` / `elastic` / `splunk` / `kusto` / `chronicle` (vendor name alone also works: `sigma:`, `kql:`, `secops:`…)
`tactic:`	TA-id, slug, or name: `credential_access`, `TA0006`
`technique:`	technique or sub-technique ID: `T1003`, `T1003.001` (alias `tech:`)
`severity:`	`critical` / `high` / `medium` / `low` / `informational` (alias `sev:`)
`risk_score`	Numeric comparison on the Elastic risk score (0 to 100): `risk_score>50`, `risk_score<=20`, `risk_score=99` (alias `risk`; Elastic rules only)
`stages:`	Rules with exactly N pipeline stages
`correlation:`	`single_event` / `sequence` / `alternatives` / `alternatives_cross_log` / `all_required` / `correlated`
`with:`	Co-occurrence event-id; stacks (`with:4624 with:4769`) to require all, while a comma list in one occurrence (`with:4624,4769`) is an either-or group. Implies multi-event
`like:`	Structural neighbors of a rule slug (equivalents + subsumption stricter / broader): `like:comsvcs_lsass_memory_dump-splunk-sysmon`
`groupby:`	Entity-grouping substring match against `group_by_keys`: `groupby:user`, `groupby:host`
`uses:`	Rules whose predicate tree touches the field (any kind, any value): `uses:CommandLine`
`excludes:`	Rules with top-level `not()` clauses on the field (FP whitelists): `excludes:ParentImage`
`field:` / `value:`	Predicate search; narrows rule cards to those with a matching leaf and drives the indicator tier. Unquoted = substring, wildcards allowed (`value:mimikatz`)
`indicator:`	Shorthand for `field:F value:V`: `indicator:Image=*\powershell.exe`
`kind:`	Filter by predicate kind. Narrows rule cards to those carrying a matching predicate leaf (`vendor:elastic kind:cidr_match`) and drives the indicator tier: `contains` / `starts_with` / `ends_with` / `regex` / `cidr` / `eq` / `in` … (operator aliases `op:`/`match:`)
`has:` / `no:`	`sample`, `field`, `notes`, `refs`, `trace`, `thirdparty`, `rule`, `pattern`, `timewindow`, `threshold`, `newterms`, `sigma`/`elastic`/`splunk`/`kusto`/`chronicle`
`-op:val`	Exclude matches; works on most operators but not `type:`/`like:`/`has:`/`no:` (use `no:<flag>` to exclude a rule flag): `tactic:execution -vendor:splunk`. Standalone `-kind:`/`-field:`/`-value:` drop every rule carrying a matching predicate leaf (`type:rules -kind:is_null`)
`field:"…"` / `value:"…"`	Quoted value = anchored exact match (also allows spaces): `value:"net user"`
`a,b`	Comma = OR inside one operator (`vendor:sigma,elastic`, `severity:high,critical`); repeating a facet merges the same way. `field:`/`value:` never split (literal commas)
`vendors:` / `stage:`	Singular and plural spellings fold to the canonical operator and value: `tactics:` = `tactic:`, `type:event` = `type:events`, `correlation:sequences` = `correlation:sequence`, `has:thresholds` = `has:threshold`
`"quoted phrase"`	Exact-match a multi-word phrase (free text)

AWS Network ACL Restricts SSH

MITRE ATT&CK coverage

Rule body yaml

Detection logic

Rule logic imperative Python

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