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OpenCode patch closes Windows MCP sign-in code-execution hole
The Hacker News reports Anomaly patched OpenCode so a hostile MCP OAuth URL could no longer run PowerShell on Windows and WSL during sign-in.
- New headline Was: “OpenCode Flaw Lets Malicious MCP Servers Execute Code During Sign-In on Windows and WSL”
- Our desk filed its story
Anomaly patched its OpenCode AI coding agent so a Model Context Protocol server could no longer run code on a Windows or WSL machine during MCP sign-in, The Hacker News reported Thursday, citing Pillar Security1.
OpenCode uses MCP servers to add outside tools1. During authentication, the server supplies the OAuth authorization URL that the client opens in a browser1. On Windows and WSL, OpenCode passed that address into a helper that built a PowerShell command without escaping it, so a crafted URL could execute as the signed-in developer1.
The helper ran only when a developer started sign-in, such as with opencode mcp auth, not merely by adding the server or opening a session1. An attacker needed control of a remote MCP server the developer had added; that server could be hostile from the start, later compromised, or tied to a domain that changed hands1.
macOS and Linux were not affected because OpenCode opened the browser there without sending the address through a shell1. The Windows path used an older release of the open package, whose command-injection issue on Windows was reported in 2023 and fixed in open 111. Checking that the URL used HTTP or HTTPS would not have blocked the attack, because the payload used an ordinary HTTPS address1.
Anomaly shipped the change in version 1.18.33 on Sept. 281. The Hacker News said the pull request was framed as a browser-launch refactor and that neither it nor the release notes described a security flaw1. Builds 1.0.137 through 1.18.32 remain at risk of being missed by scanners because Anomaly did not request a CVE or publish an advisory1.
Pillar reported the issue privately on Aug. 29 and disclosed it on Oct. 19, rating it critical with a proposed score of 9.8 out of 101. According to Pillar, Anomaly said a malicious MCP server falls outside its threat model, and the project's security policy treats user-configured MCP servers as outside its trust boundary1.
Pillar reported no signs the flaw had been used in attacks and published no indicators of past exploitation1. The Hacker News said the issue is separate from a recently disclosed breach of OpenCode's hosted Console that may have exposed some stored API keys1.
