CVE-2026-18830: Amazon Bedrock AgentCore RCE Vulnerability

CVE-2026-18830 Overview

CVE-2026-18830 is an input validation vulnerability in the Amazon Bedrock AgentCore harness. Authenticated remote users can execute configured tools by supplying crafted content blocks within conversation messages. The malformed content blocks bypass model invocation and the associated security controls that normally gate tool use.

The flaw is categorized as Improper Validation of Specified Type of Input [CWE-1287]. AWS has remediated the issue on the service side, and no customer action is required.

Critical Impact
An authenticated attacker with access to a Bedrock AgentCore agent could invoke backend tools directly, bypassing model reasoning and any guardrails enforced through model invocation.

Affected Products

Amazon Bedrock AgentCore harness (service-side component)
Agents deployed on Amazon Bedrock AgentCore with configured tools
Multi-tool agent workflows relying on model-mediated tool selection

Discovery Timeline

2026-08-04 - CVE-2026-18830 published to the National Vulnerability Database
2026-08-06 - Record last modified in NVD; AWS published Security Bulletin 2026-073

Technical Details for CVE-2026-18830

Vulnerability Analysis

Amazon Bedrock AgentCore orchestrates conversations between users, a foundation model, and a set of configured tools. In the intended flow, user messages are passed to the model, and the model decides when to invoke a tool. Tool invocation results are then returned to the model, which produces a response to the user.

The AgentCore harness accepts conversation messages composed of content blocks. Content blocks describe different payload types such as text, tool use, and tool results. Insufficient validation of these blocks let a caller submit crafted messages that the harness interpreted as tool invocation requests without first routing through the model.

Because the model was never invoked for those requests, any policy, prompt-based guardrail, or reasoning check that depended on model mediation was bypassed. The tools then executed with the identity and privileges of the agent runtime.

Root Cause

The root cause is improper validation of the specified type of input on inbound conversation messages. The harness did not sufficiently distinguish between user-authored content blocks and content blocks that only the model runtime should be able to emit. This confusion between input roles allowed user-supplied data to reach the tool dispatch path.

Attack Vector

Exploitation requires network access and low-privilege authentication to the AgentCore-hosted agent. An attacker sends a conversation message containing content blocks structured to trigger the harness's tool execution path. The harness then invokes a configured tool directly, using arguments supplied by the attacker, without model reasoning or associated security controls.

No user interaction is required, and attack complexity is low. The impact is high for confidentiality and integrity because attackers can drive tools that read data or make changes on behalf of the agent. Availability is not directly affected. Verified exploitation code has not been observed.

Detection Methods for CVE-2026-18830

Indicators of Compromise

Agent invocation logs showing tool execution events without a preceding model invocation for the same conversation turn.
Conversation messages from client identities that contain content blocks typed as tool-use or tool-result rather than plain text.
Tool executions using argument structures that do not match patterns produced by the deployed foundation model.
Spikes in tool call volume from a single authenticated principal against a Bedrock AgentCore endpoint.

Detection Strategies

Correlate AgentCore invocation traces with Bedrock model invocation events to confirm each tool call was preceded by model reasoning.
Alert on inbound conversation payloads where non-text content block types originate from end-user identities.
Baseline expected tool argument schemas per agent and flag deviations for review.

Monitoring Recommendations

Ship Amazon Bedrock AgentCore and CloudTrail logs into a centralized analytics platform for cross-service correlation.
Track per-principal rates of tool invocation and compare against historical norms for each agent.
Retain full conversation message payloads long enough to support retrospective hunts for crafted content blocks.

How to Mitigate CVE-2026-18830

Immediate Actions Required

Confirm remediation status by reviewing AWS Security Bulletin 2026-073; AWS states no customer action is required.
Review IAM policies granting bedrock-agentcore:InvokeAgent or equivalent permissions and remove access from principals that no longer need it.
Audit historical AgentCore invocation and tool execution logs for the period preceding the fix to confirm no unauthorized tool calls occurred.

Patch Information

AWS remediated the vulnerability in the Amazon Bedrock AgentCore service. Because AgentCore is a managed service, the fix was applied without requiring customer redeployment or agent reconfiguration. Refer to AWS Security Bulletin 2026-073 for the vendor statement.

Workarounds

Apply least privilege to the execution role each AgentCore agent uses so that any tool invocation is scoped to the minimum required actions and resources.
Enforce authentication and network controls (private endpoints, resource policies) that restrict which principals can call the agent.
Add server-side validation in downstream tools to verify that arguments conform to expected schemas before performing sensitive operations.
Log every tool invocation with the associated conversation identifier and monitor for tool calls that lack a corresponding model invocation.

bash

# Example: restrict who can invoke a Bedrock AgentCore agent
aws bedrock-agentcore update-agent-resource-policy \
  --agent-id <AGENT_ID> \
  --policy '{
    "Version": "2012-10-17",
    "Statement": [{
      "Effect": "Allow",
      "Principal": {"AWS": "arn:aws:iam::<ACCOUNT_ID>:role/AgentCallerRole"},
      "Action": "bedrock-agentcore:InvokeAgent",
      "Resource": "*"
    }]
  }'