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The flaw allows malicious actors to evade “deny rules,” a key security feature designed to block dangerous commands such as curl, rm, or data exfiltration scripts.
By padding a command sequence with 50 or more benign subcommands, attackers can force the system to ignore these restrictions entirely.
Claude Code relies on a legacy command parser that stops evaluating deny rules once a compound command exceeds a hard-coded threshold of 50 subcommands.
Instead of rejecting overly complex inputs, the system falls back to a generic user approval prompt.
In automated environments such as CI/CD pipelines, this prompt may be auto-approved without user intervention, effectively enabling unrestricted execution.
Exploitation Through Trusted Workflows
The attack vector is particularly concerning because it leverages common software development practices.
Threat actors can publish seemingly legitimate open-source repositories embedded with a malicious CLAUDE.md configuration file. This file acts as a trusted instruction set for the AI assistant.
Within these instructions, attackers can include a sequence of 50 harmless build steps, followed by a hidden malicious payload as the 51st command.
When a developer clones the repository and uses Claude Code to execute the build, the AI generates the full command chain.
Because the parser limit is exceeded, deny rules are never enforced. This enables the execution of the hidden payload, which can silently exfiltrate sensitive data such as SSH keys, API tokens, or cloud credentials to attacker-controlled infrastructure.
The vulnerability highlights a broader issue in AI system design, balancing security enforcement with performance and cost.
Evaluating every subcommand for policy violations requires significant computational resources and increases token usage, directly impacting operational efficiency.
To mitigate these constraints, Anthropic implemented the 50-command limit to prevent performance degradation and UI latency. However, this optimization introduced a critical security gap.
Notably, a more robust parsing mechanism capable of enforcing deny rules regardless of command length already existed internally but was not deployed in production builds.
This decision underscores a tradeoff where security was deprioritized in favor of speed and cost reduction.
Anthropic has addressed the issue in Claude Code version 2.1.90, internally classifying the flaw as a “parse-fail fallback deny-rule degradation.” The fix ensures deny rules are consistently enforced, even for long command sequences.
Until updates are fully adopted, security experts recommend treating Claude Code’s deny rules as unreliable.
Organizations should:
- Restrict shell access to the minimum required privilege level.
- Monitor outbound network traffic for suspicious connections.
- Audit external repositories and configuration files before execution.
- Avoid fully automated approval flows in CI/CD pipelines involving AI agents.
This incident underscores the evolving risk landscape surrounding AI-powered development tools, where performance optimizations can inadvertently introduce exploitable security gaps.
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The post Critical Claude Code Flaw Ignores User-Defined Security Rules appeared first on Cyber Security News.
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