ssh: distinguish 'key type rejected by server' from invalid passphrase (issue #1356)

[Diogo-Damasceno]

Description

Fixes #1356. When an sshd server rejects a public key because its algorithm is not in PubkeyAcceptedAlgorithms (e.g. server allows only ssh-ed25519 while the key is RSA), NetExec currently prints the misleading Could not decrypt private key, invalid password. That sends the user down the wrong path (blaming the passphrase instead of the server config).

Root cause (verified against Paramiko 5.0.0 auth_handler.py): in this situation Paramiko raises with one of:

• Auth rejected: pubkey algorithm '...' unsupported or disabled
• Unable to agree on a pubkey algorithm for signing a '...' key! (AuthenticationException)
• An RSA key was specified, but no RSA pubkey algorithms are configured!

Change: added _is_key_type_rejected() in nxc/protocols/ssh.py and call it from plaintext_login() in both the AuthenticationException and SSHException branches, emitting Key type rejected by server (pubkey algorithm not allowed). Wrong-passphrase (Invalid key) and encrypted-key (Private key file is encrypted) cases keep their existing behavior.

Dependencies: none new; only touches SSH login error handling.

AI usage disclosure: this PR was produced with the assistance of an AI agent (Hermes Agent, model tencent/hy3). The change and the new tests were written and executed by the agent; the human contributor performed the final self-review and verification (see Checklist). Per the project AI policy, this references an accepted issue (#1356).

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Deprecation of feature or functionality
• This change requires a documentation update
• This requires a third party update (such as Impacket, Dploot, lsassy, etc)
• This PR was created with the assistance of AI (Hermes Agent / tencent-hy3; see Description for extent)

Setup guide for the review

• Trigger the bug: set on the target SSH server PubkeyAcceptedAlgorithms ssh-ed25519 in sshd_config, then run netexec ssh <host> -u <user> --key-file <rsa_or_ecdsa_key> -p <wrong_or_right_pass>.
  • Before: NetExec prints Could not decrypt private key, invalid password (misleading).
  • After: NetExec prints Key type rejected by server (pubkey algorithm not allowed).
• Regression check: with a key whose algorithm is allowed, a wrong passphrase still prints Could not decrypt private key, invalid password (unchanged).
• Local test: pytest tests/test_ssh_key_type_rejection.py (4 tests, using the real Paramiko 5.x rejection messages). Note: I could not run the full NetExec suite locally because aardwolf does not build on Python 3.14 (see Install fails with Python 3.14 #1241); the change is isolated to SSH login error handling and is covered by the new unit tests.

Screenshots (if appropriate):

N/A (CLI log-level change; see "Setup guide" for before/after output).

Checklist:

• I have ran Ruff against my changes (poetry run ruff check .)
• I have added or updated the tests/e2e_commands.txt file if necessary (SSH key-rejection is not a new module/feature; covered by unit test)
• If reliant on changes of third party dependencies, such as Impacket, dploot, lsassy, etc, I have linked the relevant PRs in those projects
• I have linked relevant sources that describes the added technique (Paramiko 5.0.0 paramiko/auth_handler.py rejection messages; see Description)
• I have performed a self-review of my own code (not an AI review) — done by the human contributor
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (PR here: https://github.com/Pennyw0rth/NetExec-Wiki)

[github-actions]

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• Type of change

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[Diogo-Damasceno]

Pushed a follow-up commit addressing the lint job:

• Dropped an unused paramiko import in the test file (Ruff flagged imported but unused).
• Ran ruff format across nxc/protocols/ssh.py and the test so they match the project style (only line-wrapping changed — no logic touched, comments kept).
• Re-ran the unit tests locally: 4 passed on Python 3.14.7 with paramiko 5.0.0.

The PR description already includes all the template sections (Description, Type of change, Setup guide for the review, Checklist). The pr-template-check bot fired on the first push before the body was filled — it's all there now.

Root cause note for reviewers: this is purely an error-message correctness fix. Paramiko 5.x raises distinct messages when the server rejects a key algorithm (e.g. PubkeyAcceptedAlgorithms restricting to ed25519 while the key is RSA); those were previously collapsed into the misleading Could not decrypt private key, invalid password, sending users down the wrong path. Happy to adjust wording or add an e2e entry if you'd prefer.