Stripping Inbound Waf Headers Before Trusting Them

Domain expertise for the mandatory safety companion to any AWS WAF header forwarding: a block rule that rejects inbound requests already carrying an x-amzn-waf-* header, so an attacker cannot forge the signal the origin trusts. Covers why the gap exists (AWS WAF does not strip pre-existing x-amzn-waf-* headers before inserting its own) and the rule placement.

Does not cover the forwarding workflows themselves; this is their required companion. The confidence-signal, interpolation, and client-IP references all point here.

Execute commands using the AWS MCP server when connected (sandboxed execution, audit logging, observability). Fall back to the AWS CLI otherwise.

Table of Contents

• Overview
• Workflow
• Why the spoofing gap exists
• A mandatory companion, not optional hardening
• Troubleshooting
• Procedure
• Security Considerations
• Additional Resources

Workflow

To add the inbound-header-stripping rule end to end, follow the procedure exactly. See the Procedure section below.

The procedure covers:

• Adding a block rule matching any inbound x-amzn-waf-* header
• Placing it before the forwarding rules
• Confirming and surfacing the console link

Why the spoofing gap exists

AWS WAF custom request handling does not strip a pre-existing x-amzn-waf-* header before inserting its own. A request that arrives already carrying one passes that forged value to the origin, so an origin trusting x-amzn-waf-bot-category:verified, a spoofed confidence value, or a spoofed client IP is bypassable.

Constraints:

• You MUST add a block rule that rejects any inbound request already carrying an x-amzn-waf-* header
• You MUST place that rule at a lower priority number than the forwarding rules, so it runs first and only AWS-WAF-set values reach the origin

A mandatory companion, not optional hardening

The vulnerability is the absence of a rule, not a misconfiguration that throws an error, so it is easy to ship header forwarding without it.

Constraints:

• You MUST treat this rule as a mandatory companion whenever the skill recommends header forwarding, not an optional hardening step
• You SHOULD confirm the stripping rule is present before declaring any forwarding workflow complete

Troubleshooting

The origin trusts a forged signal

No stripping rule is present, or it runs after the forwarding rules. Add it before them (Why the spoofing gap exists).

Legitimate requests are blocked by the strip rule

A legitimate upstream is setting an x-amzn-waf-* header. That is unusual; confirm the upstream and narrow the match if a specific known header must pass, but default to blocking all inbound x-amzn-waf-*.

Procedure

Overview

This procedure adds a block rule for inbound x-amzn-waf-* headers before the forwarding rules, then surfaces the console link.

Parameters

• web_acl_name, web_acl_id, scope (required): Identify the web ACL.

Constraints for parameter acquisition:

• You MUST ask for all required parameters upfront in a single prompt
• You MUST confirm which forwarding rules exist so the strip rule is placed before them

Steps

1. Verify dependencies

Constraints:

• You MUST confirm credentials with aws sts get-caller-identity
• You MUST identify the priority numbers of the forwarding rules

2. Add the inbound strip rule

Constraints:

• You MUST add a block rule matching any request that carries an x-amzn-waf-* header (a header match on the x-amzn-waf- prefix), at a lower priority number than the forwarding rules

• You MUST fetch the current LockToken before update-web-acl and pass the full rule set

• You MUST preserve the web ACL’s existing DefaultAction from the get-web-acl response and pass it back as {default_action}; do not assume Allow={}, since that would silently open all unmatched traffic on a web ACL whose default action is Block. For example, a Block rule whose ByteMatchStatement matches the x-amzn-waf- prefix on header keys:

  Copy

  aws wafv2 get-web-acl --name {web_acl_name} --scope {scope} --id {web_acl_id} --region {region}
  aws wafv2 update-web-acl --name {web_acl_name} --scope {scope} --id {web_acl_id} \
    --lock-token {lock_token} --default-action {default_action} \
    --rules '[{"Name":"StripInboundWafHeaders","Priority":0,"Action":{"Block":{}},"Statement":{"ByteMatchStatement":{"SearchString":"x-amzn-waf-","PositionalConstraint":"STARTS_WITH","FieldToMatch":{"Headers":{"MatchPattern":{"All":{}},"MatchScope":"KEY","OversizeHandling":"MATCH"}},"TextTransformations":[{"Priority":0,"Type":"LOWERCASE"}]}},"VisibilityConfig":{"SampledRequestsEnabled":true,"CloudWatchMetricsEnabled":true,"MetricName":"StripInboundWafHeaders"}}]' \
    --visibility-config SampledRequestsEnabled=true,CloudWatchMetricsEnabled=true,MetricName={web_acl_name} \
    --region {region}

3. Confirm and surface the console link

Constraints:

• You MUST confirm the strip rule runs before the forwarding rules

• You MUST present the web ACL console link and tell the customer to confirm the rule order:

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  https://us-east-1.console.aws.amazon.com/wafv2/homev2/web-acls?region={region}

Example

Example input

Example output

Troubleshooting

The origin trusts a forged value

The strip rule is missing or runs after the forwarding rules. Place it before them (Step 2).

Security Considerations

This procedure modifies a security control, so misconfiguration directly weakens the application’s defenses.

• Least-privilege IAM. You MUST grant only the specific wafv2: actions a task needs (for example wafv2:GetWebACL and wafv2:UpdateWebACL) rather than wafv2:* or the AWSWAFFullAccess managed policy.
• Ephemeral credentials. You MUST use IAM roles with temporary credentials (such as an EC2 instance profile, SSO session, or aws sts assume-role) rather than long-lived IAM user access keys when running these WAF CLI commands.
• Monitor configuration changes. You SHOULD enable AWS CloudTrail on wafv2 management events and set CloudWatch alarms on critical web ACL configuration changes (such as DeleteWebACL and UpdateWebACL rule removals) and on the web ACL’s BlockedRequests and CountedRequests metrics, so rule changes and sudden spikes in blocked or counted traffic are detected.
• Header-spoofing risk. Any x-amzn-waf-* signal forwarded to the origin can be forged inbound. You MUST add the inbound-header-stripping rule whenever a signal or client IP is forwarded (see stripping-inbound-waf-headers-before-trusting-them); without it the origin trusts a spoofable value.

Additional Resources

• Customizing web requests and responses in AWS WAF (AWS WAF Developer Guide) (opens in a new tab)
• How to use AWS WAF Bot Control for targeted bots signals and mitigate evasive bots with adaptive user experience (AWS Networking & Content Delivery Blog) (opens in a new tab)

Source

• references/stripping-inbound-waf-headers-before-trusting-them.md(opens in a new tab)
• Raw file(opens in a new tab)
• aws/agent-toolkit-for-aws(opens in a new tab)