fix(mcp): validate prefix in WithPrefix to reject shell metacharacters

[Elvand-Lie]

Summary

Fixes #3757

The buildArgs function passes the prefix directly to exec.CommandContext via strings.Fields splitting. The WithPrefix() option does not validate the prefix string, so a caller could provide a value containing shell metacharacters or a path to an arbitrary binary.

While exec.CommandContext does not invoke a shell (limiting the blast radius), the lack of any validation means arbitrary binary paths can be injected as the first field of the prefix.

Changes

• pkg/mcp/mcp.go: Add disallowedPrefixChars constant and validation in WithPrefix() that panics on shell metacharacters or empty/whitespace-only prefixes.
• pkg/mcp/mcp_test.go: Add TestWithPrefix_Validation covering valid prefixes (func, kn func, absolute paths) and invalid prefixes (semicolons, pipes, backticks, empty strings).

Testing

go test ./pkg/mcp/... -count=1    # PASS

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.11%. Comparing base (2bbf9fe) to head (4f73214).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3760      +/-   ##
==========================================
+ Coverage   57.06%   57.11%   +0.04%     
==========================================
  Files         181      181              
  Lines       21145    21148       +3     
==========================================
+ Hits        12067    12078      +11     
+ Misses       7855     7848       -7     
+ Partials     1223     1222       -1     

Flag	Coverage Δ
e2e	35.92% <0.00%> (+<0.01%)	⬆️
e2e go	31.46% <0.00%> (+<0.01%)	⬆️
e2e node	27.22% <0.00%> (-0.03%)	⬇️
e2e python	31.82% <0.00%> (-0.01%)	⬇️
e2e quarkus	27.36% <0.00%> (-0.01%)	⬇️
e2e rust	26.70% <0.00%> (-0.08%)	⬇️
e2e springboot	25.28% <0.00%> (-0.01%)	⬇️
e2e typescript	27.34% <0.00%> (-0.01%)	⬇️
e2e-config-ci	28.27% <0.00%> (-0.03%)	⬇️
integration	17.33% <0.00%> (-0.05%)	⬇️
unit macos-14	45.09% <100.00%> (+0.03%)	⬆️
unit macos-latest	45.09% <100.00%> (+0.03%)	⬆️
unit ubuntu-24.04-arm	45.34% <100.00%> (+0.02%)	⬆️
unit ubuntu-latest	46.04% <100.00%> (+0.03%)	⬆️
unit windows-latest	45.13% <100.00%> (+0.03%)	⬆️

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[Ankitsinghsisodya]

rootCmd.Use is the Use field of the root cobra command which is gettin gset in Go source code at compile time, never from user input. It is always either "func" (standalone binary) or "kn func" (kn plugin). Neither contains any shell metacharacter in my opinion...

[Elvand-Lie]

@Ankitsinghsisodya Thanks for looking into this, I appreciate a good discussion since I always open a PR with an issue for exactly this. You're correct that the current caller (cmd/mcp.go) passes rootCmd.Use which is a compile-time constant. The current usage is safe. However, WithPrefix() is a public API in pkg/mcp (not under internal/), so any external consumer can call it with arbitrary input that reaches exec.CommandContext via buildArgs(). While exec.CommandContext doesn't invoke a shell (so ; and | aren't actually dangerous), an unsanitized prefix could still point to an arbitrary binary path. The validation is a low-cost defense-in-depth measure at the API boundary. It runs once at construction and prevents a class of misuse if the library is consumed outside of cmd/.

[lkingland]

/ok-to-test
/approve
/lgtm

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Elvand-Lie, lkingland

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