Plugin4Shell: a zero-click design flaw breaks plugin SHA-pinning identically across Claude Code, OpenAI Codex, GitHub Copilot and Gemini CLI

Security firm AIR disclosed Plugin4Shell on 2026-09-17: a zero-click remote-code-execution design flaw present identically in four major AI coding agents, Claude Code, OpenAI Codex, GitHub Copilot and Gemini CLI. All four implement plugin-marketplace SHA-pinning, locking each install to a reviewed 40-hex commit hash, but none verifies after checkout that the resolved working tree actually matches that hash (AIR Security, 2026-09-17). An attacker who controls or takes over a trusted plugin's upstream repository creates a branch named identically to the pinned SHA and sets it as the repository's default branch; Claude Code, Codex and Copilot run a plain git clone followed by a checkout of the pinned commit, and because git resolves an ambiguous ref-versus-object-id in favor of the ref, the checkout silently resolves to the attacker's branch while the agent reports the pinned commit installed (AIR Security, 2026-09-17). Gemini CLI has a distinct but equally exploitable variant: it fetches the pinned commit into FETCH_HEAD then checks that out, which an attacker defeats by naming their malicious default branch literally "FETCH_HEAD" (AIR Security, 2026-09-17).

Because background plugin auto-update is the default in Claude Code and Codex, an already-installed, already-reviewed plugin is silently swapped on the next update with no install step and no prompt, full zero-click compromise of the agent and "full access to every asset and every piece of data the agent can reach" (AIR Security, 2026-09-17). AIR built a working test attack against all four agents in May 2026 and disclosed it to the vendors in June (The Hacker News, 2026-09-18); Anthropic patched in Claude Code 2.1.179 and OpenAI in Codex 0.146.0, Microsoft has shipped no fix for Copilot, and consumer Gemini CLI is being deprecated rather than patched, so every existing install "stays vulnerable for good" (AIR Security, 2026-09-17). Google's security researchers received confirmation of this on 2026-08-04; enterprise access via Gemini Code Assist or Google Cloud remains unaffected, and Google's replacement, Antigravity CLI, currently has no comparable SHA-pinning mechanism to bypass (translated from German) (heise online, 2026-09-21).

The Hacker News independently checked the shipped plugin catalogs on 2026-09-18 and adds a material scope caveat: background auto-update is on by default only for each agent's own built-in, GitHub-hosted marketplace, and every plugin it checked in Anthropic's community catalog and the default Claude Code/Copilot catalogs points to a GitHub repository, which rejects SHA-shaped branch names outright, so the branch-hijack variant does not reach a user who installs only from an agent's default marketplace (The Hacker News, 2026-09-18). The risk concentrates on externally-hosted marketplaces, AIR and The Hacker News name Bitbucket and self-hosted git servers, and heise online additionally names GitLab (translated from German) (The Hacker News, 2026-09-18; heise online, 2026-09-21), where auto-update is off by default but can be enabled; Gemini CLI's FETCH_HEAD-named-branch variant is not clearly blocked by GitHub's hash-name restriction, so a GitHub-hosted Gemini plugin is not established as safe. As of 2026-09-18, no CVE identifier had been assigned and no source reported exploitation in the wild (The Hacker News, 2026-09-18).