CVE-2026-55581: sonirico mcp-shell allowlist bypass via bash -c flag

CVE-2026-55581 · Severity: high · CVSS 8.4 · Published 2026-08-25

mcp-shell is a Model Context Protocol tool for secure command execution in Docker containers, designed to restrict operators to an allowlist of pre-approved binaries. A flaw in the validation logic allows an attacker to send `/bin/bash -c <arbitrary-command>` to the shell_exec tool and execute any command present in the container—such as `curl`, `wget`, or `env`—outside the allowlist. This works on default deployments with no authentication required, giving an attacker full command execution as the `mcpuser` account.

Technologies: github.com/sonirico/mcp-shell (Go), Sonirico MCP Shell. Vendors: Go, Sonirico.

Quick answers

Is CVE-2026-55581 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-55581 being exploited in the wild.
How severe is CVE-2026-55581?
High severity, with a CVSS score of 8.4, published 25 August 2026.
Which products and versions are affected?
github.com/sonirico/mcp-shell (Go) < 0.6.0, SEMVER:introduced=0,fixed=0.6.0; Sonirico MCP Shell < 0.6.0.
Is there a fix for CVE-2026-55581?
Yes. github.com/sonirico/mcp-shell (Go): upgrade to 0.6.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-55581 attacked?
It is attacked over the network.
Am I affected by CVE-2026-55581?
Check whether you run github.com/sonirico/mcp-shell (Go) or Sonirico MCP Shell at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • github.com/sonirico/mcp-shell (Go): upgrade to 0.6.0 or later
    go get github.com/sonirico/mcp-shell@v0.6.0

Technical details

The vulnerability is an OS command injection (CWE-78) stemming from incomplete argument validation in the secure mode enforcer. The default Docker image ships `security.yaml` with `/bin/bash` in the `allowed_executables` list. The validator in `security.go` splits the input command on whitespace and checks only the first token (e.g., `/bin/bash`) against the allowlist; it does not inspect or block shell command-mode flags like `-c`. When `/bin/bash -c id` is submitted, the validator extracts `/bin/bash`, matches it against the allowlist, then passes the entire command to the executor without detecting the `-c` flag. The executor then invokes `exec.CommandContext(ctx, "/bin/bash", "-c", "id")`, which spawns bash with the `-c` flag, executing the arbitrary `id` command as `mcpuser` (UID 1000). No authentication is required, and the vulnerability is present in all default deployments.

Affected products

  • sonirico mcp-shell < 0.6.0

Identifiers

  • CVE-2026-55581
  • GHSA-3x77-wg38-92r3
  • GO-2026-6290

Timeline

  • 2026-06-14: disclosed
  • 2026-06-14: patched: Version 0.6.0 released with fix
  • 2026-08-25: advisory

References

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