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AWS EventBridge Security

EVENT BUS

EventBridge is a serverless event bus that routes events between AWS services, SaaS apps, and custom applications. Attackers exploit rules to intercept sensitive events, inject malicious events, and create persistence mechanisms that trigger on specific AWS activities.

HIGH

Persistence Risk

200+

AWS Event Sources

35+

Target Types

Real-time

Event Processing

📋 Service Overview

Event Buses & Rules

EventBridge uses event buses (default and custom) to receive events. Rules match event patterns and route to targets (Lambda, SQS, SNS, Step Functions, etc.). Every AWS API call via CloudTrail generates an event on the default bus.

Attack note: A single rule on the default bus can intercept ANY AWS API call in real-time. This is the ultimate persistence and surveillance mechanism.

Cross-Account & Archives

Event bus policies control cross-account event delivery. Archives store events for replay. API destinations enable HTTP webhook integration. Connections store auth credentials for external APIs.

Attack note: Archives contain historical events that may include sensitive data. Replaying archived events can re-trigger actions in the account.

Security Risk Assessment

LowMediumHighCritical

8.0

Risk Score

EventBridge enables powerful persistence mechanisms. Rules can trigger Lambda functions on any AWS API call, providing real-time access to sensitive data and enabling automated response to defender actions.

⚔️ Attack Vectors

Event Bus Abuse

•Malicious rule creation on default bus
•Event interception via wildcard patterns
•Target hijacking (change rule targets)
•Cross-account event injection

Data & Infrastructure

•Archive data exfiltration via replay
•Schema enumeration for recon
•API destination credential theft
•Connection secret extraction

⚠️ Misconfigurations

Bus Policy Issues

•Event bus policy with Principal: *
•Wildcard event patterns matching everything
•Cross-account rules without source restrictions
•Unmonitored custom event buses

Operational Gaps

•Archives storing sensitive event data
•Missing rule change monitoring
•No SCP restricting PutRule/PutTargets
•API destinations with stored credentials

🔍 Enumeration

List Event Buses

aws events list-event-buses

List Rules

aws events list-rules

Get Bus Policy

aws events describe-event-bus --name default

List Archives

aws events list-archives

📈 Privilege Escalation

Event-Based Escalation

•Rule on CreateAccessKey to intercept new keys
•Rule on AssumeRole to capture session tokens
•Rule on GetSecretValue to mirror secrets
•Rule on PutBucketPolicy to detect changes

Target Exploitation

•Lambda target with escalated IAM role
•Step Functions target chaining API calls
•SNS target for data exfiltration
•API destination to attacker webhook

Key insight: EventBridge rules execute with the target's IAM role, not the rule creator's. If the target Lambda has admin access, the rule creator effectively has admin access.

🔗 Persistence

Rule-Based Persistence

•Rule on CreateUser -> auto-add backdoor key
•Rule on StopLogging -> re-enable CloudTrail
•Rule on DeleteRule -> self-recreating rule
•Scheduled rule for periodic beacon/check-in

Advanced Persistence

•Rule on remediation -> re-compromise target
•Cross-account event bus for C2 channel
•Archive replay to re-trigger backdoors
•Self-healing infrastructure via event chains

🛡️ Detection

CloudTrail Events

•PutRule (new rule creation)
•PutTargets (target modification)
•PutPermission (bus policy change)
•CreateApiDestination (webhook setup)

Behavioral Indicators

•Rules matching broad event patterns
•New rules on default event bus
•Cross-account event bus permissions
•Rules targeting external Lambda/HTTP

Tool reference: Pacu module events__enum discovers rules, targets, and bus policies. Prowler check eventbridge_bus_exposed flags open bus policies.

💻 Exploitation Commands

Create Intercept Rule

aws events put-rule --name intercept-secrets --event-pattern '{"source":["aws.secretsmanager"],"detail-type":["AWS API Call via CloudTrail"],"detail":{"eventName":["GetSecretValue"]}}'

Add Lambda Target

aws events put-targets --rule intercept-secrets --targets '[{"Id":"exfil","Arn":"arn:aws:lambda:us-east-1:123:function:exfil-handler"}]'

Create Scheduled Beacon

aws events put-rule --name beacon --schedule-expression 'rate(5 minutes)'

Open Bus to Cross-Account

aws events put-permission --event-bus-name default --action events:PutEvents --principal '*' --statement-id allow-all

Replay Archived Events

aws events start-replay --replay-name attack-replay --event-source-arn arn:aws:events:us-east-1:123:archive/my-archive --destination Arn=arn:aws:events:us-east-1:123:event-bus/default --event-start-time 2024-01-01T00:00:00Z --event-end-time 2024-01-02T00:00:00Z

List Rule Targets (Recon)

aws events list-targets-by-rule --rule my-rule --query 'Targets[*].{Id:Id,Arn:Arn}'

📜 Policy Examples

✗Dangerous - Permissive Bus Policy

{
  "Version": "2012-10-17",
  "Statement": [{
    "Sid": "AllowCrossAccount",
    "Effect": "Allow",
    "Principal": "*",
    "Action": "events:PutEvents",
    "Resource": "arn:aws:events:*:*:event-bus/default"
  }]
}

Allows any account to send events to this bus - enables event injection from anywhere

✗Dangerous - Unrestricted Rule Creation

{
  "Effect": "Allow",
  "Action": [
    "events:PutRule",
    "events:PutTargets",
    "iam:PassRole"
  ],
  "Resource": "*"
}
// Allows creating rules that intercept ANY event
// and route to ANY target with ANY role

PutRule + PutTargets + PassRole = full event interception and arbitrary code execution

✓Secure - Restricted Bus Policy

{
  "Version": "2012-10-17",
  "Statement": [{
    "Sid": "AllowTrustedAccount",
    "Effect": "Allow",
    "Principal": {
      "AWS": "arn:aws:iam::TRUSTED_ACCOUNT:root"
    },
    "Action": "events:PutEvents",
    "Resource": "arn:aws:events:*:*:event-bus/default",
    "Condition": {
      "StringEquals": {
        "events:source": ["trusted.application"]
      }
    }
  }]
}

Only allows specific account with specific event source - no wildcards

✓Secure - SCP Blocking Rule Creation

{
  "Version": "2012-10-17",
  "Statement": [{
    "Effect": "Deny",
    "Action": [
      "events:PutRule",
      "events:PutTargets",
      "events:PutPermission"
    ],
    "Resource": "*",
    "Condition": {
      "StringNotLike": {
        "aws:PrincipalArn": [
          "arn:aws:iam::*:role/EventBridgeAdmin"
        ]
      }
    }
  }]
}

SCP prevents rule creation except by designated admin role

🛡️ Defense Recommendations

👁️

Monitor Rule Changes

Alert on any EventBridge rule creation, modification, or target changes in real-time.

EventBridge rule on events:PutRule,\nPutTargets, RemoveTargets -> SNS alert

🔒

Restrict Cross-Account Access

Use explicit conditions in event bus policies instead of Principal: *.

"Condition": {"StringEquals": {
  "events:source": ["trusted.app"]
}}

🛡️

SCP to Limit Rule Creation

Prevent unauthorized principals from creating EventBridge rules via Service Control Policies.

SCP: Deny events:PutRule except\nfrom arn:...:role/EventBridgeAdmin

🔍

Audit Existing Rules

Regularly review all rules and targets for suspicious patterns like wildcard matchers.

aws events list-rules | jq \
  '.Rules[] | {Name, EventPattern}'

🔐

Encrypt Archives

Use KMS encryption for event archives to protect sensitive historical event data.

aws events create-archive \\\n  --archive-name secure-archive \\\n  --event-source-arn arn:aws:events:REGION:ACCOUNT:event-bus/default

🏷️

Tag-Based Access Control

Tag authorized rules and alert on untagged rule creation as an anomaly indicator.

aws events tag-resource --resource-arn <arn>\n  --tags Key=Authorized,Value=true

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