Paper 2023/1325

The Grant Negotiation and Authorization Protocol: Attacking, Fixing, and Verifying an Emerging Standard

Florian Helmschmidt, University of Stuttgart
Pedram Hosseyni, University of Stuttgart
Ralf Kuesters, University of Stuttgart
Klaas Pruiksma, University of Stuttgart
Clara Waldmann, University of Stuttgart
Tim Würtele, University of Stuttgart

The Grant Negotiation and Authorization Protocol (GNAP) is an emerging authorization and authentication protocol which aims to consolidate and unify several use-cases of OAuth 2.0 and many of its common extensions while providing a higher degree of security. OAuth 2.0 is an essential cornerstone of the security of authorization and authentication for the Web, IoT, and beyond, and is used, among others, by many global players, like Google, Facebook, and Microsoft. Because of historically grown limitations and issues of OAuth 2.0 and its various extensions, prominent members of the OAuth community decided to create GNAP, a new and completely resigned authorization and authentication protocol. Given GNAP's advantages over OAuth 2.0 and its support within the OAuth community, GNAP is expected to become at least as important as OAuth 2.0. In this paper, we present the first formal security analysis of GNAP. We build a detailed formal model of GNAP, based on the Web Infrastructure Model (WIM) of Fett, Küsters, and Schmitz. Based on this model, we provide formal statements of the key security properties of GNAP, namely, authorization, authentication, and session integrity for both authorization and authentication. In the process of trying to prove these properties, we have discovered several attacks on GNAP. We present these attacks as well as modifications to the protocol that prevent them. These modifications have been incorporated into the GNAP specification after discussion with the GNAP working group. We give the first formal security guarantees for GNAP, by proving that GNAP, with our modifications applied, satisfies the mentioned security properties. GNAP was still an early draft when we started our analysis, but is now on track to be adopted as an IETF standard. Hence, our analysis is just in time to help ensure the security of this important emerging standard.

Available format(s)
Cryptographic protocols
Publication info
Published elsewhere. Major revision. ESORICS 2023
authorization protocolsformal security analysisweb security
Contact author(s)
flori @ nhelmschmidt de
pedram hosseyni @ sec uni-stuttgart de
ralf kuesters @ sec uni-stuttgart de
klaas pruiksma @ sec uni-stuttgart de
clara waldmann @ sec uni-stuttgart de
tim wuertele @ sec uni-stuttgart de
2023-09-08: approved
2023-09-05: received
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      author = {Florian Helmschmidt and Pedram Hosseyni and Ralf Kuesters and Klaas Pruiksma and Clara Waldmann and Tim Würtele},
      title = {The Grant Negotiation and Authorization Protocol: Attacking, Fixing, and Verifying an Emerging Standard},
      howpublished = {Cryptology ePrint Archive, Paper 2023/1325},
      year = {2023},
      note = {\url{}},
      url = {}
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