AWS S3 Bucket Public Write

Severity: critical
Compliance: CIS S3.8; PCI 1.3.1, 7.1, 7.2.1
Tags: AWS, Data Protection, Identity & Access Management, Impact:Data Destruction
Reference: https://docs.aws.amazon.com/AmazonS3/latest/dev/using-iam-policies.html
Source: github.com/panther-labs/panther-analysis

Ensures S3 buckets are not publicly writeable, preventing critical security vulnerabilities. Public write access allows attackers to upload malicious content, delete legitimate data, or consume storage for massive AWS bills. Attackers can use writable buckets for C2 infrastructure, malware distribution, or supply chain attacks.

MITRE ATT&CK coverage

Tactic	Techniques
Impact	`T1485` Data Destruction

Rule body yaml

AnalysisType: policy
Filename: aws_s3_bucket_public_write.py
PolicyID: "AWS.S3.Bucket.PublicWrite"
DisplayName: "AWS S3 Bucket Public Write"
Enabled: true
ResourceTypes:
  - AWS.S3.Bucket
Tags:
  - AWS
  - Data Protection
  - Identity & Access Management
  - Impact:Data Destruction
Reports:
  CIS:
    - S3.8
  PCI:
    - 1.3.1
    - 7.1
    - 7.2.1
  MITRE ATT&CK:
    - TA0040:T1485
Severity: Critical
Description: >
  Ensures S3 buckets are not publicly writeable, preventing critical security vulnerabilities. Public write access allows attackers to upload malicious content, delete legitimate data, or consume storage for massive AWS bills. Attackers can use writable buckets for C2 infrastructure, malware distribution, or supply chain attacks.
Runbook: |
  1. Query CloudTrail for s3:PutObject, s3:DeleteObject, and s3:PutObjectAcl events on the bucket in the past 30 days to identify any unauthorized write operations from unfamiliar IP addresses or principals
  2. Remove public write permissions from the bucket's ACL and bucket policy, enable all four S3 Block Public Access settings, and quarantine or delete any malicious objects that were uploaded
  3. Enable S3 server access logging and CloudTrail S3 data events for ongoing monitoring, and review how the public write access was configured to prevent recurrence
Reference: https://docs.aws.amazon.com/AmazonS3/latest/dev/using-iam-policies.html
Tests:
  - Name: Bucket Grants READ To All Authenticated Users
    ExpectedResult: true
    Resource:
      {
        "CreationDate": "2019-01-01T00:00:00Z",
        "EncryptionRules":
          [
            {
              "ApplyServerSideEncryptionByDefault":
                { "KMSMasterKeyID": null, "SSEAlgorithm": "AES256" },
            },
          ],
        "Grants":
          [
            {
              "Grantee":
                {
                  "DisplayName": "user.name",
                  "EmailAddress": null,
                  "ID": "11112223334445556667778899aaabbbcccdddeeee",
                  "Type": "CanonicalUser",
                  "URI": "http://acs.amazonaws.com/groups/global/AuthenticatedUsers",
                },
              "Permission": "READ",
            },
            {
              "Grantee":
                {
                  "DisplayName": null,
                  "EmailAddress": null,
                  "ID": null,
                  "Type": "Group",
                  "URI": "http://acs.amazonaws.com/groups/s3/LogDelivery",
                },
              "Permission": "READ",
            },
          ],
        "LifecycleRules": null,
        "Location": "us-east-2",
        "LoggingPolicy": null,
        "MFADelete": null,
        "Name": "bucket-name",
        "Owner":
          {
            "DisplayName": "user.name",
            "ID": "11112223334445556667778899aaabbbcccdddeeee",
          },
        "Policy": null,
        "PublicAccessBlockConfiguration":
          {
            "BlockPublicAcls": false,
            "BlockPublicPolicy": false,
            "IgnorePublicAcls": false,
            "RestrictPublicBuckets": false,
          },
        "Versioning": null,
      }
  - Name: Bucket Grants WRITE To All Authenticated Users
    ExpectedResult: false
    Resource:
      {
        "CreationDate": "2019-01-01T00:00:00Z",
        "EncryptionRules":
          [
            {
              "ApplyServerSideEncryptionByDefault":
                { "KMSMasterKeyID": null, "SSEAlgorithm": "AES256" },
            },
          ],
        "Grants":
          [
            {
              "Grantee":
                {
                  "DisplayName": "user.name",
                  "EmailAddress": null,
                  "ID": "11112223334445556667778899aaabbbcccdddeeee",
                  "Type": "CanonicalUser",
                  "URI": "http://acs.amazonaws.com/groups/global/AuthenticatedUsers",
                },
              "Permission": "WRITE",
            },
            {
              "Grantee":
                {
                  "DisplayName": null,
                  "EmailAddress": null,
                  "ID": null,
                  "Type": "Group",
                  "URI": "http://acs.amazonaws.com/groups/s3/LogDelivery",
                },
              "Permission": "WRITE",
            },
          ],
        "LifecycleRules": null,
        "Location": "us-east-2",
        "LoggingPolicy": null,
        "MFADelete": null,
        "Name": "bucket-name",
        "Owner":
          {
            "DisplayName": "user.name",
            "ID": "11112223334445556667778899aaabbbcccdddeeee",
          },
        "Policy": null,
        "PublicAccessBlockConfiguration":
          {
            "BlockPublicAcls": false,
            "BlockPublicPolicy": false,
            "IgnorePublicAcls": false,
            "RestrictPublicBuckets": false,
          },
        "Versioning": null,
      }
  - Name: Bucket Grants WRITE To All Users
    ExpectedResult: false
    Resource:
      {
        "CreationDate": "2019-01-01T00:00:00Z",
        "EncryptionRules":
          [
            {
              "ApplyServerSideEncryptionByDefault":
                { "KMSMasterKeyID": null, "SSEAlgorithm": "AES256" },
            },
          ],
        "Grants":
          [
            {
              "Grantee":
                {
                  "DisplayName": "user.name",
                  "EmailAddress": null,
                  "ID": "11112223334445556667778899aaabbbcccdddeeee",
                  "Type": "CanonicalUser",
                  "URI": "http://acs.amazonaws.com/groups/global/AllUsers",
                },
              "Permission": "WRITE",
            },
            {
              "Grantee":
                {
                  "DisplayName": null,
                  "EmailAddress": null,
                  "ID": null,
                  "Type": "Group",
                  "URI": "http://acs.amazonaws.com/groups/s3/LogDelivery",
                },
              "Permission": "WRITE",
            },
          ],
        "LifecycleRules": null,
        "Location": "us-east-2",
        "LoggingPolicy": null,
        "MFADelete": null,
        "Name": "bucket-name",
        "Owner":
          {
            "DisplayName": "user.name",
            "ID": "11112223334445556667778899aaabbbcccdddeeee",
          },
        "Policy": null,
        "PublicAccessBlockConfiguration":
          {
            "BlockPublicAcls": false,
            "BlockPublicPolicy": false,
            "IgnorePublicAcls": false,
            "RestrictPublicBuckets": false,
          },
        "Versioning": null,
      }
  - Name: Null Configuration
    ExpectedResult: true
    Resource:
      {
        "Arn": "arn:aws:s3:::something-prodsec",
        "LoggingPolicy": null,
        "Policy": null,
        "MFADelete": null,
        "ObjectLockConfiguration": null,
        "LifecycleRules": null,
        "Grants": null,
        "Versioning": null,
        "EncryptionRules": null,
        "PublicAccessBlockConfiguration": null,
        "AccountId": "123456789",
        "Tags":
          {
            "something": "ttd",
            "user": "prodsec",
            "service": "-",
            "owner": "-",
            "team": "-",
          },
        "Region": "us-west-2",
        "ResourceType": "AWS.S3.Bucket",
        "Name": "something-prodsec",
        "ResourceId": "arn:aws:s3:::something-prodsec",
        "TimeCreated": "2020-06-27T00:11:11Z",
        "Owner":
          {
            "DisplayName": "aws-corp",
            "ID": "11112223334445556667778899aaabbbcccdddeeee",
          },
      }

Detection logic

Condition

 not not

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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 S3 Bucket Public Write

MITRE ATT&CK coverage

Rule body yaml

Detection logic

Condition

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