提出 #765933: GitHub Dropbear 2025.89 Improper Verification of Cryptographic Signature

The specific issue is located in curve25519.c:
Line 484 directly uses s+32 for computation, with no validation for S >= L at any point.

According to the RFC 8032 §5.1.7 specification, an out-of-bounds S should be rejected. Without this check, we can generate an equivalent signature by constructing S' = S + L. My local testing confirms that Dropbear's verification process accepts both the original signature and its S+L variant. Furthermore, the paper "The Provable Security of Ed25519: Theory and Practice" by Jacqueline Brendel, Cas Cremers, Dennis Jackson, and Mang Zhao (https://ieeexplore.ieee.org/stampPDF/getPDF.jsp?arnumber=9519456) emphasizes that strictly rejecting such non-canonical signatures is necessary to maintain Strong Existential Unforgeability under Chosen Message Attacks (SUF-CMA).

Historically, there have been multiple signature malleability vulnerabilities in other projects (e.g., CVE-2020-36843, CVE-2024-45193). This inconsistency with the specification breaks signature uniqueness and could potentially impact higher-level security policies or auditing logic that rely on it.

• ソース: ⚠️ https://github.com/mkj/dropbear/issues/406
• ユーザー: pythok (UID 95793)
• 送信: 2026年02月23日 17:02 (1 月 ago)
• モデレーション: 2026年03月07日 10:07 (12 days later)
• ステータス: 承諾済み
• VulDBエントリ: 349652 [mkj Dropbear 迄 2025.89 S Range Check src/curve25519.c unpackneg 弱い認証]
• ポイント: 20