Fix length check when accepting SFTP connections

[QuintenQVD0]

Changes

• GHSA-ghrq-5wpp-hxx5

Summary by CodeRabbit

• Bug Fixes
  • Improved robustness of SFTP subsystem request handling by adding defensive validation to prevent potential errors during request processing.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: fce87868-5cfb-420e-9d96-e2cb2bf92cc7

📥 Commits

Reviewing files that changed from the base of the PR and between 565b0e3 and 57e7786.

📒 Files selected for processing (1)

• sftp/server.go

📜 Recent review details

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (3)

• GitHub Check: Build and Test (ubuntu-22.04, 1.25.7, linux, amd64)
• GitHub Check: Build and Test (ubuntu-22.04, 1.26.0, linux, amd64)
• GitHub Check: Analyze (go)

🔇 Additional comments (1)

sftp/server.go (1)

146-147: Security fix correctly prevents panic on short payloads.

The addition of len(req.Payload) >= 4 before slicing prevents a potential DoS vulnerability where an attacker could send a malformed SSH request with a payload shorter than 4 bytes, causing a panic. The short-circuit evaluation of && ensures the slice operation only occurs when the length check passes.

The implementation correctly assumes SSH subsystem request payloads follow the SSH string wire format (4-byte big-endian length prefix + string data), and the check properly validates this assumption before attempting to slice. The code safely handles all edge cases:

• Payloads shorter than 4 bytes are rejected without panic
• Payloads of exactly 4 bytes correctly reject empty string comparisons
• Valid payloads containing "sftp" are properly accepted

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📝 Walkthrough

Walkthrough

In sftp/server.go, the AcceptInbound function now checks that req.Payload is at least 4 bytes long before slicing it. The subsystem match condition is extracted into an ok boolean, which is passed directly to req.Reply(ok, nil).

Changes

SFTP Payload Guard

Layer / File(s)	Summary
Payload bounds check in AcceptInbound sftp/server.go	Adds len(req.Payload) >= 4 guard before slicing req.Payload[4:]; extracts the combined condition into an ok boolean used in req.Reply.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

A bunny once sliced without checking the size,
And panicked one day — what a dreadful surprise!
Now four little bytes must be present and there,
Before any slicing is done with great care.
🐇 Hop safely, dear payload, no index out of bounds! 🎉

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Fix length check when accepting SFTP connections' directly and accurately describes the main change in the PR, which defensively adds a length check when accepting SFTP subsystem requests.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch QuintenQVD0/SSH-DDOS

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