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« AWS S3 Object Versioning Suspended AWS S3 Unauthenticated Bucket Access by Rare Source »
Elastic Docs ›Elastic Security Solution [8.18] ›Detections and alerts ›Prebuilt rule reference

AWS S3 Static Site JavaScript File Uploaded

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AWS S3 Static Site JavaScript File Uploaded

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This rule detects when a JavaScript file is uploaded or accessed in an S3 static site directory (static/js/) by an IAM user or assumed role. This can indicate suspicious modification of web content hosted on S3, such as injecting malicious scripts into a static website frontend.

Rule type: esql

Rule indices: None

Severity: medium

Risk score: 47

Runs every: 5m

Searches indices from: now-9m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

  • Domain: Cloud
  • Data Source: AWS
  • Data Source: Amazon Web Services
  • Data Source: AWS S3
  • Tactic: Impact
  • Use Case: Web Application Compromise
  • Use Case: Cloud Threat Detection
  • Resources: Investigation Guide

Version: 1

Rule authors:

  • Elastic

Rule license: Elastic License v2

Investigation guide

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Triage and Analysis

Investigating AWS S3 Static Site JavaScript File Uploaded

An S3 PutObject action that targets a path like static/js/ and uploads a .js file is a potential signal for web content modification. If done by an unexpected IAM user or outside of CI/CD workflows, it may indicate a compromise.

Possible Investigation Steps

  • Identify the Source User: Check aws.cloudtrail.user_identity.arn, access key ID, and session type (IAMUser, AssumedRole, etc).
  • Review File Content: Use the S3 GetObject or CloudTrail requestParameters to inspect the uploaded file for signs of obfuscation or injection.
  • Correlate to Other Events: Review events from the same IAM user before and after the upload (e.g., ListBuckets, GetCallerIdentity, IAM activity).
  • Look for Multiple Uploads: Attackers may attempt to upload several files or modify multiple directories.

False Positive Analysis

  • This behavior may be expected during app deployments. Look at:
  • The user_agent.original to detect legitimate CI tools (like Terraform or GitHub Actions).
  • Timing patterns—does this match a regular release window?
  • The origin IP and device identity.

Response and Remediation

  • Revert Malicious Code: Replace the uploaded JS file with a clean version and invalidate CloudFront cache if applicable.
  • Revoke Access: If compromise is confirmed, revoke the IAM credentials and disable the user.
  • Audit IAM Policies: Ensure that only deployment users can modify static site buckets.
  • Enable Bucket Versioning: This can allow for quick rollback and historical review.

Rule query

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from logs-aws.cloudtrail* metadata _id, _version, _index
| where

    // filter on CloudTrail logs for S3 PutObject actions
    event.dataset == "aws.cloudtrail"
    and event.provider == "s3.amazonaws.com"
    and event.action in ("GetObject","PutObject")

    // filter for IAM users, not federated identities
    and aws.cloudtrail.user_identity.type in ("IAMUser", "AssumedRole")

    // filter for S3 static site bucket paths from webpack or similar
    and aws.cloudtrail.request_parameters LIKE "*static/js/*.js*"

    // exclude common IaC tools and automation scripts
    and not (
        user_agent.original LIKE "*Terraform*"
        or user_agent.original LIKE "*Ansible*"
        or user_agent.original LIKE "*Pulumni*"
    )

// extract bucket and object details from request parameters
| dissect aws.cloudtrail.request_parameters "%{{?bucket.name.key}=%{bucket.name}, %{?host.key}=%{bucket.host}, %{?bucket.object.location.key}=%{bucket.object.location}}"

// filter for specific bucket and object structure
| dissect bucket.object.location "%{}static/js/%{bucket.object}"

// filter for JavaScript files
| where ENDS_WITH(bucket.object, ".js")
| keep
    aws.cloudtrail.user_identity.arn,
    aws.cloudtrail.user_identity.access_key_id,
    aws.cloudtrail.user_identity.type,
    aws.cloudtrail.request_parameters,
    bucket.name,
    bucket.object,
    user_agent.original,
    source.ip,
    event.action,
    @timestamp

Framework: MITRE ATT&CKTM

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