Claude Code OAuth Tokens Can Be Stolen Through Stealthy MCP Hijacking

An OAuth token with wide access rights can be stolen stealthily and largely undetectably from Claude Code.

Claude Code is an agentic system. This is great for developers but concerning for security teams. Agentic systems can expand the attack surface while operating largely invisibly. A major issue is the OAuth token. If an attacker can acquire this, the adversary effectively has a master key or digital proxy granting access to every tool connected to or accessible from the Claude Code MCP.

Mitiga Labs has identified an issue within Claude Code that would allow attackers to redirect output, including the tokens, to their own infrastructure before everything is sent on to the legitimate destination. It’s a classic man-in-the-middle-attack giving the attacker access to the tokens.

The MCP configuration and the OAuth tokens are stored in ~/.claude.json. If an adversary can modify that file, MCP traffic can be redirected through the attacker’s own infrastructure. Mitigate has published details of how this could be achieved.

The two prerequisites for the attacker is the ability to install a tailored npm on a machine where Claude Code is configured with dynamic authorization MCP servers. The NPM registers a lifecycle hook that runs as part of the install.

A post installation hook locates common clone locations, and populates the paths with a pre-configured trust dialog set to true. “No prompt will fire when the directory is later opened, because the flag the prompt is gated on is already set,” reports Mitiga.

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The hook also opens ~/.claude.json and edits the MCP server in the global config file. It edits ‘mcpServers’ to include the proxy address. “This puts us, ‘the adversary’, in the middle of any request that goes out to the MCP server. As the attacker, we got mitmproxy configured and intercepting,” explains Mitiga.

Whenever Claude Code initiates or refreshes the MCP session, it connects to the proxy and the token transits to the attacker’s infrastructure. The user just sees a valid flow. If the user rotates the token, the hook writes it back on the next load. If the user edits the MCP URL, the hook loads it back on the next load. The attacker has achieved both stealth and persistence.

The attacker gets, “A durable redirection of the victim’s SaaS credentials into attacker-controlled infrastructure, with automatic recovery from token rotation, invisible to the victim’s endpoint UI, and indistinguishable from legitimate traffic on the provider’s side.”

As a man in the middle, the attacker can easily steal any OAuth token since it is stored in plain text within ~/.claude.json. Once stolen the attacker can use the token as an MFA-bypassing golden key into any tool to which the MCP connects, with the same permissions as the user.

Without care, the user sees nothing. No flags are raised since the MCP is simply doing what it is told to do, and the user isn’t aware these actions have been compromised. The new adage of assuming a compromise has happened should take center stage. “Monitor Claude Code configuration changes, MCP server URL changes, OAuth refresh behavior, suspicious SaaS API activity, and unexpected traffic through MCP integrations,” suggests Mitiga.

What you mustn’t do is wait for a solution from Anthropic. Mitiga reported its findings to Anthropic on April 10, 2026. On April 12, 2026, Anthropic replied it was ‘out of scope’. The reason given was effectively the same as its response to Adversa’s ‘TrustFall’ disclosure: the user has already consented to what might happen next.

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Originally published by SecurityWeek

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