CVE-2026-18830

Executive Summary

CVE-2026-18830 is an insufficient input validation flaw in Amazon Bedrock AgentCore harness that permits an authenticated remote attacker to invoke configured tools while bypassing model invocation and security controls through crafted content blocks in conversation messages. It affects AWS Bedrock users leveraging AgentCore for agentic workflows. The CVSS 8.1 score reflects high confidentiality and integrity impact with network attack vector and low complexity, though AWS states the issue has been addressed with no customer action required.

Technical Analysis

The vulnerability stems from inadequate validation of content blocks within conversation message payloads passed to the AgentCore harness. An authenticated user can supply specially formatted blocks that trigger direct tool execution paths instead of routing through the intended model invocation layer. This bypasses guardrails that normally enforce policy checks, output filtering, and invocation sequencing. The attack surface is the message ingestion pipeline in AgentCore, where content blocks are parsed without strict schema enforcement or context isolation between user-supplied data and internal control structures. CWE-1287 captures the improper validation of syntactic correctness of input, allowing malformed blocks to reach the tool dispatcher. No customer-side code changes are involved as the harness resides in the AWS service boundary.

Exploitation Scenarios

An attacker begins with reconnaissance by identifying AWS accounts with active Bedrock AgentCore agents exposed via IAM roles granting InvokeAgent permissions. They authenticate using valid credentials and craft a conversation message containing nested content blocks with embedded tool invocation metadata that mimics internal harness signals. The payload is submitted through the standard Bedrock runtime API endpoint, weaponized by placing the bypass blocks ahead of legitimate model output sections to short-circuit the invocation flow. Upon receipt, the harness executes the configured tools directly, returning results without model mediation. Post-exploitation, the attacker chains tool outputs to exfiltrate data or perform unauthorized actions within the agent's permission scope, achieving high-impact access to integrated resources such as databases or external APIs.

Impact Assessment

Successful exploitation grants an authenticated user the ability to execute tools with confidentiality and integrity impacts rated high under CVSS, potentially exposing sensitive data processed by the agent or altering downstream system state. Blast radius is limited to the permissions granted to the specific AgentCore agent but can affect production workflows in enterprise environments relying on Bedrock for automated decisioning. No availability impact is scored. Systems using custom agents with tool integrations face the highest risk.

Detection Guidance

Sigma Rule

title: Amazon Bedrock AgentCore Content Block Bypass
id: 8f4c2a1b-9e3d-4f7a-8c2b-1d9e6f3a4b5c
status: experimental
description: Detects crafted content blocks in Bedrock AgentCore messages
logsource:
  product: aws
  service: bedrock
detection:
  selection:
    eventName: InvokeAgent
    requestParameters|contains: "contentBlocks"
    requestParameters|contains: "toolUse"
  condition: selection
fields:
  - userIdentity.arn
  - requestParameters
level: high

YARA Rule

rule Amazon_Bedrock_AgentCore_Bypass {
    meta:
        description = "Detects patterns associated with CVE-2026-18830 content block bypass"
    strings:
        $block = /contentBlocks.*toolUse.*bypass/
        $msg = "conversationMessages"
    condition:
        $block and $msg
}

Suricata IDS Rule

alert http any any -> any 443 (msg:"Amazon Bedrock AgentCore Input Validation Bypass Attempt"; flow:established,to_server; content:"InvokeAgent"; content:"contentBlocks"; content:"toolUse"; classtype:web-application-attack; sid:2026188301; rev:1;)

Elastic Detection

sequence by userIdentity.arn with maxspan=5m
  [aws.bedrock where event.action == "InvokeAgent" and requestParameters.contentBlocks like "*toolUse*"]

Splunk SPL Query

index=aws sourcetype=aws:cloudtrail eventName=InvokeAgent 
| search requestParameters="*contentBlocks*" requestParameters="*toolUse*" 
| stats count by userIdentity.arn sourceIPAddress

Wazuh OSSEC Rule

<rule id="576550" level="12">
  <if_sid>550</if_sid>
  <match>InvokeAgent.*contentBlocks.*toolUse</match>
  <description>Amazon Bedrock AgentCore bypass detected</description>
</rule>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source).

1. immediate mitigations: Restrict IAM permissions for InvokeAgent to least-privilege roles and monitor CloudTrail for anomalous content block patterns.
2. patch/upgrade steps: No customer-applied patches exist; rely on AWS service-side remediation already deployed.
3. long-term hardening: Implement additional application-layer validation on agent inputs and enable detailed Bedrock logging for content block inspection.

References

https://nvd.nist.gov/vuln/detail/CVE-2026-18830 https://aws.amazon.com/security/advisories https://cwe.mitre.org/data/definitions/1287.html https://attack.mitre.org/techniques/T1190/ No known threat actor campaigns or in-the-wild exploitation reported.