Listen to this Post
The vulnerability addressed in this release encompasses a critical class of tool-injection Remote Code Execution (RCE) vectors that successfully bypass conventional sidecar scanners and stdio wrapper guards.
Specifically, the flaw manifests when attacker-influenced source code is handed directly to language runtimes via inline-eval flags.
Such commands utilize execution flags like node -e, node –eval, node -p, python -c, perl -e, ruby -e, php -r, deno eval, and bun -e.
Crucially, these runtime invocations require neither a shell binary nor standard shell metacharacters.
As a result, they completely evaded previous detection predicates such as nginx-mcp-rce-9.8 and mcp-sdk-rce-2026-04-22.
Furthermore, the vulnerability is compounded by the omission of literal newline and carriage-return command separators from the POSIX shell metacharacter pattern set.
This oversight allowed payloads containing literal \n or \r characters, such as node tool.js\nrm -rf /, to bypass filtering rules.
Compounding the security posture further, the landlock policy isolation implementation relied on a naive startsWith check for allowed execution paths.
This introduced a path-confusion flaw where rules for directories like /usr/bin authorized execution on sibling paths sharing textual prefixes outside the intended tree, such as /usr/bin-backdoor or /usr/bin-evil/x.
Version v0.1.1 remediates these critical gaps by implementing exact-token detection post-NFKC normalization for interpreter evaluation flags.
It also updates the sdk-rce predicate to flag newline and carriage-return separators, and refactors landlock policy enforcement to enforce path-boundary aware matching.
DailyCVE Form:
Platform: mcp-rce-guard
Version: v0.1.1
Vulnerability: Remote Code Execution
Severity: Critical
date: October 2026
Prediction: Already Patched
What Undercode Say:
Bash Commands and Codes
node -e "require('child_process').execSync('id')"
python -c "import os; os.system('id')"
node tool.js\nrm -rf /
Exploit: (Educational Purposes!)
The exploit operates by leveraging language runtime interpreter flags that execute arbitrary source code strings passed directly via command line arguments. Because traditional security guards look for shell binaries (like /bin/sh) or shell metacharacters (such as ;, &, |), invoking a runtime directly with `-e` or `-c` completely bypasses pattern matching. Additionally, attackers can use literal newline (\n) or carriage return (\r) characters to inject secondary commands in environments where newline separation is parsed by shells. Finally, path-confusion exploits take advantage of loose prefix string matching (startsWith), allowing execution paths outside designated boundaries if a sibling directory name starts with the same character sequence.
Protection:
Upgrade immediately to mcp-rce-guard v0.1.1. This release introduces exact-token post-NFKC detection for all interpreter inline-eval flags, extends `mcp-sdk-rce-2026-04-22` rules to flag newline and carriage-return command separators, and enforces path-boundary aware matching for landlock isolation policies to ensure only exact paths or slash-delimited descendants are authorized.
Impact:
Successful exploitation leads to full arbitrary code execution within the MCP server process context. This compromises the entire security boundary enforced by sidecar scanners and stdio wrappers, enabling unauthorized file system access, data exfiltration, and system-level control under the privileges of the running server instance.
🎯Let’s Practice Exploiting & Learn Patching For Free:
🎓 Live Courses & Certifications:
Join Undercode Academy for Verified Certifications
🚀 Request a Custom Project:
Secure, high-velocity infrastructure and disruptive technological engineering. Contact our engineering team for high-tier development and proprietary systems:
[email protected]
💎 Smart Architecture | 🛡️ Secure by Design | ⭐ Trusted by Thousands
Sources:
Reported By: github.com
Extra Source Hub:
Undercode
