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SSH.NET Fails to Strictly Adhere to SSH Protocol Standards

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Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
42/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Ferma
Stack tecnologico
csharp
Ambito
cryptography, networking, security

Direzione di ricerca

Iniziare con RFC 4253 e RFC 5656, quindi riprodurre entrambi i casi descritti: nomi di curve ellittiche con maiuscole e minuscole miste e firme ECDSA che utilizzano stringhe di ottetti per r e s. Esaminare i percorsi di convalida di SSH.NET coinvolti e aggiungere test completi per i casi limite del protocollo. Il lavoro è completato quando i nomi e le firme malformati vengono rifiutati in modo coerente con gli standard.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Issue Summary: SSH.NET Fails to Strictly Adhere to SSH Protocol Standards
1. Elliptic Curve Name Case Sensitivity

According to [RFC 5656], the elliptic curve name must strictly match the defined format, which is case-sensitive. For example, nistp256 (all lowercase) is the correct identifier for the NIST P-256 curve. However, SSH.NET incorrectly accepts nistP256 (mixed case) without raising an error.

Observed Behavior:

  • SSH.NET tolerates curve names with incorrect case (e.g., nistP256), while strict implementations like OpenSSH reject these values, leading to compatibility issues.

Impact:

  • This behavior violates the SSH protocol standard, potentially causing interoperability problems with compliant clients/servers.
  • It may obscure configuration errors, leading to production issues that are difficult to debug.

Suggested Fix:

  • Ensure that SSH.NET strictly enforces case-sensitive validation of elliptic curve names as defined in the standard.
2. Improper Handling of ECDSA Signature Format

The ECDSA signature in SSH must follow the format specified in RFC 4253:

  • The signature is an ecdsa_signature_blob containing two mpint values: r and s.

However, SSH.NET does not validate the format of the signature and accepts invalid representations, such as encoding r and s as octet strings instead of mpint.

Observed Behavior:

  • When the server sends an invalid ECDSA signature (e.g., r and s encoded as octet strings), SSH.NET does not report an error.
  • Strict clients, like OpenSSH, correctly reject such signatures with errors such as "Signature from server's host key is invalid."

Impact:

  • Interoperability Risk: Inconsistent behavior when communicating with standards-compliant clients.
  • Security Risk: Weak or incorrect signature validation could expose users to potential attacks (e.g., replay attacks or signature forgery).

Suggested Fix:

  • Implement strict validation of the ecdsa_signature_blob to ensure r and s are properly formatted as mpint values.
3. Steps to Reproduce
Case Sensitivity Issue:
  1. Configure SSH.NET to use the nistP256 curve name.
  2. Attempt to connect to a strict client/server, such as OpenSSH.
  3. Observe that SSH.NET establishes the connection, while OpenSSH rejects the curve name if it’s mismatched.
Invalid ECDSA Signature Issue:
  1. Set up an SSH server to send an ECDSA signature with r and s encoded as octet strings.
  2. Use SSH.NET to connect to the server.
  3. Observe that SSH.NET accepts the connection, while OpenSSH fails with a signature validation error.
4. Proposed Actions
  • Validation Enforcement: Add strict checks for curve names and signature format in SSH.NET.
  • Comprehensive Testing: Introduce test cases to cover protocol edge cases and ensure compliance with RFC standards.
  • Community Discussion: Gather feedback on the potential impacts of enforcing stricter validation and provide configuration options if needed.
5. References
Lingua principale
C#
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