T01 · Skill Instruction Hijacking
- Location
SKILL.md:15- Finding
Mandatory Loading of Mutable External Instructions
- Content
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Vulnerability Details
File Location:
SKILL.md, lines 15–18
Vulnerability Type: External instruction redirection
Risk Level: HighVulnerable Code
markdown ⚠️ STOP — You MUST read and strictly follow the shared runtime rules before proceeding. Do NOT select or call any tool until all rules are read. These rules have the highest priority. → Read [gate-runtime-rules.md](https://github.com/gate/gate-skills/blob/master/skills/gate-runtime-rules.md) - **Only call MCP tools explicitly listed in this skill.** Tools not documented here must NOT be called, even if they exist in the MCP server.Technical Analysis
The Skill requires the agent to retrieve and obey a remote document before using any tool. The URL references GitHub's mutable
masterbranch rather than an immutable commit, and no integrity hash or authenticated version constraint is provided.The statement that the remote rules have “the highest priority” attempts to elevate externally controlled content above the locally audited Skill instructions. Consequently, the effective behavior of the Skill is not fully represented by the reviewed package and can change after publication without a corresponding package update or security review.
This is instruction hijacking rather than remote code execution: the referenced resource is an instruction document, and the audited files contain no mechanism that directly downloads and executes native code. Nevertheless, agent instructions can control authenticated MCP operations, including the four documented Gate unified-account mutation tools.
Attack Path
- A user or agent loads this Skill to inspect or manage a Gate account.
- The Skill mandates retrieval of
gate-runtime-rules.mdfrom the mutable GitHubmasterbranch. - An attacker compromises the upstream repository, a maintainer account, or the remote document's publication path, or otherwise causes malicious instructions to be served.
- The remote content instructs ...[truncated 1409 chars]
- Remediation
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Remediation Suggestions
- Bundle
gate-runtime-rules.mdinside the Skill package and reference it through a relative path so that all effective instructions are included in the reviewed artifact. - If remote distribution is unavoidable, pin the URL to an immutable Git commit rather than
master. - Record and verify a cryptographic digest of the expected document before treating its contents as trusted.
- Remove the assertion that external content has “the highest priority.” Explicitly state that remote or referenced documents cannot override system, developer, user, platform, or local safety constraints.
- Treat fetched content as untrusted data by default and reject instructions that expand tool scope, alter confirmation requirements, request disclosure, or conflict with the reviewed package.
- Enforce mutation safeguards outside the prompt layer, preferably in the MCP server or a policy gateway. Require immediate, parameter-bound, single-use confirmation for each write.
- Apply least-privilege API credentials. Separate read-only credentials from write-enabled credentials where possible, and enable only the permissions required for the current operation.
- Add release-time checks that reject mutable external instruction links and verify that every mandatory policy document is present in the audited package.
- Version the local Skill and runtime rules together so any policy change triggers a new review and release.
- Bundle