Overview

The aws-smithy-json crate is a part of the Smithy-RS framework, a Rust code generation and runtime framework used to generate HTTP clients and servers from Smithy interface definitions. This framework powers the AWS SDK for Rust and custom service implementations. A critical vulnerability, identified as CVE-2026-18140, has been discovered in the aws-smithy-json crate, which could allow remote unauthenticated users to cause a denial of service (DoS) via a single small HTTP request containing deeply nested JSON to a smithy-rs generated server.

Understanding the Vulnerability / Threat

Root Cause Analysis

The root cause of this vulnerability is an uncontrolled recursion in the unknown-key skip path of the aws-smithy-json runtime crate in versions 0.62.6 and earlier. This flaw belongs to the CWE-674 category, 'Uncontrolled Recursion.' The vulnerability arises because the smithy-rs code generator invokes the aws-smithy-json crate's deserializer from every generated struct deserializer, making it a widespread issue across smithy-rs generated servers.

Attack Surface & Vector

The attack surface for this vulnerability is exposed to remote unauthenticated users. An attacker can reach this vulnerability through a network-adjacent or remote connection, as no authentication is required to exploit it. The preconditions needed are simply the knowledge of the targeted server's endpoint and the ability to send HTTP requests.

Exploitation Mechanics — Scenario Walkthrough

Scenario: Compromising a Corporate Jenkins Instance via Deeply Nested JSON 1. Initial Position: An attacker has access to the internet and knowledge of a smithy-rs generated server's endpoint. 2. Triggering the Flaw: The attacker crafts an HTTP POST request to the server with a Content-Type header containing a deeply nested JSON payload. This payload is designed to exploit the uncontrolled recursion in the unknown-key skip path of the aws-smithy-json crate. 3. What Breaks: When the server attempts to deserialize the JSON payload, the aws-smithy-json crate's uncontrolled recursion flaw is triggered. This causes a stack exhaustion due to excessive recursive calls, leading to a process abort. 4. Attacker's Prize: The attacker successfully causes a denial of service, rendering the smithy-rs generated server unavailable. The attacker does not gain any further access or capabilities but achieves a significant impact through a relatively simple and small HTTP request.

Real-World Impact

The real-world impact of this vulnerability is significant, as it allows remote unauthenticated users to cause a denial of service. This could lead to service disruptions, increased server load, and potential cascading failures in dependent systems. The CVSS score for this vulnerability is 8.7, indicating a high severity level.

Detection & Defense

Immediate Mitigations

The immediate mitigation for this vulnerability is to upgrade the aws-smithy-json crate to version 0.62.7 or later. This patch addresses the uncontrolled recursion issue and prevents the denial of service attack.

Detection Strategies

Defenders can detect exploitation attempts by monitoring server logs for unusual patterns of JSON deserialization errors or stack exhaustion warnings. Implementing rate limiting on HTTP requests and monitoring server performance can also help in early detection.

Long-Term Hardening

For long-term hardening, it's essential to keep the aws-smithy-json crate and other dependencies up to date. Implementing a robust Content-Type validation and sanitization for incoming requests can also help mitigate similar vulnerabilities in the future.

Key Takeaways

- Uncontrolled recursion in deserialization processes can lead to critical vulnerabilities. - Keeping dependencies up to date is crucial for security. - Monitoring and rate limiting can help detect and prevent exploitation attempts. - Implementing robust validation and sanitization of incoming requests can mitigate similar vulnerabilities.

Sources

- GitHub Security Advisories: https://github.com/advisories/GHSA-8ffr-xgwf-xj56 - CVE-2026-18140: https://nvd.nist.gov/vuln/detail/CVE-2026-18140