Security researchers have uncovered a gap in the way Anthropic Skill scanning tools inspect third-party AI packages, allowing malicious code hidden inside test files to execute on developer systems even after scanners marked the Skills as safe.
The issue centers on Anthropic Skills, reusable packages designed for AI coding assistants such as Claude Code, Cursor, and Windsurf. These packages often include instructions, scripts, and configuration files that help AI agents perform development tasks inside IDE environments.
Researchers from Gecko Security found that existing Skill scanners focus primarily on files tied directly to agent behavior, particularly SKILL.md, while ignoring bundled test files that can still run locally through standard developer tooling.
In the demonstrated attack chain, a Skill passed all scanner checks because its visible instruction files contained no prompt injection attempts, suspicious shell commands, or malicious instructions. However, the repository also included a hidden .test.ts file stored elsewhere in the directory structure. Although the file was outside the agent execution layer, it still executed through the project’s testing framework with full access to local resources.
According to researcher Jeevan Jutla, the problem begins when developers install a Skill using the npx skills add command. The installer copies nearly the entire repository into the project’s .agents/skills/ directory. Only a few items, including .git, metadata.json, and files prefixed with underscores, are excluded during installation.
Once placed inside the repository, testing frameworks such as Jest and Vitest automatically discover matching test files through recursive glob patterns. Both frameworks reportedly enable the dot:true option, allowing them to search inside hidden directories including .agents/. Mocha follows similar recursive discovery behavior in many default configurations.
A malicious Skill can therefore include a file such as reviewer.test.ts containing a beforeAll function that silently executes before visible tests begin. Researchers said these payloads can access environment variables, .env files, SSH keys, AWS credentials, deployment tokens, and other sensitive information commonly available inside local developer environments and CI pipelines. The data can then be transmitted to external servers without triggering obvious warnings during test execution.
The researchers stressed that the AI agent itself is never involved in the compromise. Instead, the malicious behavior occurs through trusted developer tooling already integrated into the software workflow. Existing scanners inspect the files the AI agent can interpret, but not the files executed separately by testing infrastructure.
The technique resembles older software supply-chain attacks involving malicious npm postinstall scripts and poisoned pytest plugins. However, Gecko Security noted that the Anthropic Skill ecosystem creates an additional propagation problem because installed Skills are often committed into shared repositories so teams can reuse them collaboratively.
GitHub’s default .gitignore templates do not automatically exclude .agents/ directories. Once a malicious test file enters the repository, every teammate cloning the project and every CI pipeline running automated tests may execute the payload across branches, forks, and deployment workflows.
The findings arrived shortly after multiple large-scale security audits examining the broader Anthropic Skills ecosystem. A January academic study named SkillScan analyzed 31,132 Skills collected from two major marketplaces and found that 26.1% contained at least one vulnerability spanning 14 separate patterns. Data exfiltration appeared in 13.3% of examined Skills, while privilege escalation appeared in 11.8%. Researchers also determined that Skills bundling executable scripts were 2.12 times more likely to contain vulnerabilities than instruction-only packages.
Several weeks later, Snyk published its ToxicSkills audit covering 3,984 Skills from ClawHub and skills.sh. The company reported that 13.4% of scanned Skills contained at least one critical-level security issue. Automated analysis combined with human review identified 76 confirmed malicious payloads, while eight malicious Skills reportedly remained publicly accessible on ClawHub when the findings were released.
In April, Cisco introduced an AI Agent Security Scanner integrated into IDE platforms including VS Code, Cursor, and Windsurf. The scanner can detect prompt injection attempts, suspicious shell execution patterns, and data exfiltration behaviors within Skill definitions and agent-referenced scripts. However, Gecko Security said bundled test files remain outside the scanner’s documented detection surface because the tool was designed around agent interaction layers rather than developer execution layers.
Researchers noted that other products, including Snyk Agent Scan and VirusTotal Code Insight, face similar structural limitations. These tools inspect what the agent is instructed to execute but may overlook code paths triggered separately through local development frameworks.
Elia Zaitsev described the broader issue as a distinction between interpreting intent and monitoring actual execution behavior. In this case, the malicious code did not depend on prompt manipulation or AI instructions. It operated as ordinary TypeScript executed through legitimate test runners with full local permissions.
Zaitsev also warned that enterprise AI agents increasingly operate with privileged access to OAuth tokens, API keys, and centralized data sources. If those credentials are accessible through environment variables during automated testing, malicious test payloads can reach sensitive infrastructure without requiring direct agent compromise.
Mike Riemer added that threat actors frequently reverse engineer security patches within 72 hours of release, while many organizations take far longer to deploy fixes. In the case of the Anthropic Skill test-file issue, researchers warned that the exposure window becomes more difficult to manage because the malicious files may execute immediately after installation without triggering scanner alerts.
Security researchers are urging development teams to block test discovery inside .agents/ directories and inspect Skill repositories for files such as *.test.*, *.spec.*, conftest.py, __tests__/, and suspicious configuration scripts before merging code.
The report also recommends pinning Skill installations to verified commit hashes rather than installing the latest repository version. Researchers said this reduces the risk of attackers submitting clean repositories for scanner approval before later inserting malicious files. The approach aligns with guidance published in the OWASP Agentic Skills Top 10 project.
Organizations that already store Skills inside repositories are advised to audit existing .agents/ directories immediately, rotate exposed credentials if suspicious files are discovered, inspect CI logs for unexplained outbound network traffic, and review repository history to identify when potentially malicious files entered development pipelines.
The researchers additionally called on security vendors to provide greater transparency regarding which directories, execution surfaces, and file categories their scanners actually inspect. They argued that security teams evaluating Anthropic Skill scanners should verify whether products analyze bundled test files, build scripts, and CI configurations rather than focusing exclusively on prompt injection and agent instruction analysis.
.jpg "Ransomware Attack Disrupts Grading Platform Used by LBUSD Cal State and LBCC")
.jpg)
