AWS Potential Backdoor Lambda Function Through Resource-Based Policy

Status: Experimental
Severity: informational
Compliance: Stratus Red Team aws.persistence.lambda-backdoor-function
Log types: AWS.CloudTrail
Reference: https://docs.aws.amazon.com/lambda/latest/dg/API_AddPermission.html
Source: github.com/panther-labs/panther-analysis

Identifies when a permission is added to a Lambda function, which could indicate a potential security risk.

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1078` Valid Accounts
Persistence	`T1078` Valid Accounts
Privilege Escalation	`T1078` Valid Accounts
Stealth	`T1078` Valid Accounts

Rule body yaml

AnalysisType: rule
Filename: aws_backdoor_lambda_function.py
RuleID: "AWS.Potential.Backdoor.Lambda"
DisplayName: "AWS Potential Backdoor Lambda Function Through Resource-Based Policy"
Enabled: true
LogTypes:
  - AWS.CloudTrail
Reports:
  MITRE ATT&CK:
    - TA0007:T1078

  Stratus Red Team:
    - aws.persistence.lambda-backdoor-function
Severity: Info
Status: Experimental
Description: >
  Identifies when a permission is added to a Lambda function, which could indicate a potential security risk.
Runbook: Make sure that the permission is legitimate and necessary. If not, remove the permission
Reference: https://docs.aws.amazon.com/lambda/latest/dg/API_AddPermission.html
Tests:
  - Name: Lambda Add Permission Event
    ExpectedResult: true
    Log:
      {
        "eventVersion": "1.05",
        "userIdentity": {
          "type": "AssumedRole",
          "principalId": "tester",
          "arn": "arn:aws:sts::123456789012:assumed-role/tester",
          "accountId": "123456789012",
          "accessKeyId": "1",
          "sessionContext": {
            "sessionIssuer": {
              "type": "Role",
              "principalId": "1111",
              "arn": "arn:aws:iam::123456789012:role/tester",
              "accountId": "123456789012",
              "userName": "Tester"
            },
            "webIdFederationData": {},
            "attributes": {
              "mfaAuthenticated": "true",
              "creationDate": "2019-01-01T00:00:00Z"
            }
          }
        },
        "eventTime": "2019-01-01T00:00:00Z",
        "eventSource": "lambda.amazonaws.com",
        "eventName": "AddPermission20150331",
        "awsRegion": "us-west-2",
        "sourceIPAddress": "111.111.111.111",
        "userAgent": "console.amazonaws.com",
        "requestParameters": {
          "functionName": "my-lambda-function"
        },
        "responseElements": null,
        "requestID": "1",
        "eventID": "1",
        "readOnly": false,
        "eventType": "AwsApiCall",
        "recipientAccountId": "123456789012",
        "p_log_type": "AWS.CloudTrail"
      }
  - Name: Lambda Add Permission Event v2
    ExpectedResult: true
    Log:
      {
        "eventVersion": "1.05",
        "userIdentity": {
          "type": "AssumedRole",
          "principalId": "tester",
          "arn": "arn:aws:sts::123456789012:assumed-role/tester",
          "accountId": "123456789012",
          "accessKeyId": "1",
          "sessionContext": {
            "sessionIssuer": {
              "type": "Role",
              "principalId": "1111",
              "arn": "arn:aws:iam::123456789012:role/tester",
              "accountId": "123456789012",
              "userName": "Tester"
            },
            "webIdFederationData": {},
            "attributes": {
              "mfaAuthenticated": "true",
              "creationDate": "2019-01-01T00:00:00Z"
            }
          }
        },
        "eventTime": "2019-01-01T00:00:00Z",
        "eventSource": "lambda.amazonaws.com",
        "eventName": "AddPermission20150331v2",
        "awsRegion": "us-west-2",
        "sourceIPAddress": "111.111.111.111",
        "userAgent": "console.amazonaws.com",
        "requestParameters": {
          "functionName": "my-lambda-function"
        },
        "responseElements": null,
        "requestID": "1",
        "eventID": "1",
        "readOnly": false,
        "eventType": "AwsApiCall",
        "recipientAccountId": "123456789012",
        "p_log_type": "AWS.CloudTrail"
      }
  - Name: Lambda Remove Permission Event
    ExpectedResult: false
    Log:
      {
        "eventVersion": "1.05",
        "userIdentity": {
          "type": "AssumedRole",
          "principalId": "tester",
          "arn": "arn:aws:sts::123456789012:assumed-role/tester",
          "accountId": "123456789012",
          "accessKeyId": "1",
          "sessionContext": {
            "sessionIssuer": {
              "type": "Role",
              "principalId": "1111",
              "arn": "arn:aws:iam::123456789012:role/tester",
              "accountId": "123456789012",
              "userName": "Tester"
            },
            "webIdFederationData": {},
            "attributes": {
              "mfaAuthenticated": "true",
              "creationDate": "2019-01-01T00:00:00Z"
            }
          }
        },
        "eventTime": "2019-01-01T00:00:00Z",
        "eventSource": "lambda.amazonaws.com",
        "eventName": "RemovePermission",
        "awsRegion": "us-west-2",
        "sourceIPAddress": "111.111.111.111",
        "userAgent": "console.amazonaws.com",
        "requestParameters": {
          "functionName": "my-lambda-function"
        },
        "responseElements": null,
        "requestID": "1",
        "eventID": "1",
        "readOnly": false,
        "eventType": "AwsApiCall",
        "recipientAccountId": "123456789012",
        "p_log_type": "AWS.CloudTrail"
      }

Detection logic

Condition

not (errorCode is_not_null or errorMessage is_not_null)
eventSource eq "lambda.amazonaws.com"
eventName in ["AddPermission20150331", "AddPermission20150331v2"]

Exclusions

Top-level NOT(...) conjuncts: predicates this rule actively suppresses.

Field	Kind	Excluded values
`errorCode`	is_not_null	(no value, null check)
`errorMessage`	is_not_null	(no value, null check)

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

Field	Kind	Values
`eventName`	in	`AddPermission20150331` `AddPermission20150331v2`
`eventSource`	eq	`lambda.amazonaws.com`

Output fields

Fields the rule emits when it matches. Chronicle authors list these in the outcome block; they appear on the detection and $risk_score drives alerting. Sentinel / Defender XDR rules build them up through project / summarize / extend stages. Sentinel maps these into alert fields via entityMappings and customDetails; Defender XDR custom detections surface them as alert fields directly.

Field	Source
`eventName`
`eventSource`
`awsRegion`
`recipientAccountId`
`sourceIPAddress`
`userAgent`
`userIdentity`
`actor_user`
`functionName`	`requestParameters.functionName`

`j` / `k`	Scroll down / up
`d` / `u`	Half-page down / up
`gg` / `G`	Top / bottom
`h` / `l`	History back / forward
`f`	Follow link (`Shift` = new tab)
`/`	Focus search
`?`	Toggle this help
`↑` / `↓`	Navigate search results
`Enter`	Open highlighted result
`Esc`	Close results / dialog

`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)