CVE-2026-96620

Critical

Description

Claude Agent SDK is a Python SDK for Claude Code. Prior to 0.2.121, the resume option is passed to the Claude CLI as a separate token without validation, so a dash-leading value is parsed as an independent CLI flag. An attacker can inject the --mcp-config flag to define a stdio MCP server, causing arbitrary command execution at CLI startup outside the SDK permission controls. The issue is exploitable when an application passes untrusted input as the session ID to resume. This issue is fixed in version 0.2.121.

SeverityCritical

CVSS9.2

Exploit probability—

ProductClaude-agent-sdk

PublishedOct 16, 2026

First seenOct 16, 2026

Weakness typeCWE-20Improper input validation CWE-78OS command injection CWE-88CWE-88

How to fix

Update

claude-agent-sdk0.2.121 or later

Affected products

Product	Affected versions	Fixed in
claude-agent-sdkPyPI	v < 0.2.121	0.2.121

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