CVE-2019-7167 in Zcash
Summary
by MITRE
Zcash, before the Sapling network upgrade (2018-10-28), had a counterfeiting vulnerability. A key-generation process, during evaluation of polynomials related to a to-be-proven statement, produced certain bypass elements. Availability of these elements allowed a cheating prover to bypass a consistency check, and consequently transform the proof of one statement into an ostensibly valid proof of a different statement, thereby breaking the soundness of the proof system. This misled the original Sprout zk-SNARK verifier into accepting the correctness of a transaction.
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Analysis
by VulDB Data Team • 08/08/2023
The vulnerability described in CVE-2019-7167 represents a critical flaw in the Zcash protocol's cryptographic proof system prior to the Sapling network upgrade. This issue specifically affected the Sprout network layer which utilized zk-SNARKs for privacy-preserving transactions. The vulnerability emerged from a fundamental weakness in the polynomial evaluation process during the proof generation phase, where certain mathematical elements were inadvertently exposed or could be derived by malicious actors. This flaw was particularly dangerous because it undermined the core security guarantee of zero-knowledge proofs, which should ensure that no information about the underlying transaction data is revealed while maintaining the validity of the proof.
The technical implementation of this vulnerability stems from a flaw in the key generation mechanism that occurs during the evaluation of polynomials within the zk-SNARK framework. According to CWE-310, this represents a cryptographic weakness where the system fails to properly maintain the security properties of its underlying mathematical constructs. The bypass elements generated during this process allowed an attacker to manipulate the proof verification system by transforming a valid proof for one statement into what appears to be a valid proof for an entirely different statement. This manipulation exploited the lack of proper consistency checking mechanisms that should have prevented such transformations from being accepted by the verifier. The vulnerability specifically targeted the soundness property of the proof system, which is fundamental to ensuring that only true statements can be proven valid.
The operational impact of CVE-2019-7167 was severe and far-reaching for the Zcash ecosystem, as it allowed for the creation of fraudulent transactions that could bypass the network's verification mechanisms. Attackers could potentially generate valid-looking proofs for invalid transactions, enabling them to spend coins without proper authorization or to manipulate transaction data in ways that would be undetectable to the network's Sprout verifier. This vulnerability directly compromised the integrity of the Zcash blockchain and could have led to significant financial losses. The attack vector was particularly insidious because it operated at the cryptographic layer, making it difficult to detect through traditional transaction monitoring systems. The vulnerability was particularly concerning given that it affected the network before the Sapling upgrade, which was designed to address these and other security weaknesses in the Zcash protocol.
The mitigation strategy for this vulnerability required a complete network upgrade to the Sapling protocol, which introduced improved cryptographic parameters and eliminated the mathematical weaknesses that enabled the attack. This upgrade represented a fundamental shift in how Zcash handled zero-knowledge proofs and required all network participants to transition to the new protocol version. The solution aligns with ATT&CK technique T1548.003, which involves the use of valid accounts to gain access to systems, but in this case the access was gained through exploitation of a cryptographic weakness rather than traditional authentication bypasses. The network upgrade also included enhanced consistency checks and improved polynomial evaluation methods that prevented the generation of bypass elements. Organizations and users were required to upgrade their software implementations to ensure compatibility with the new security parameters, as the old Sprout protocol remained vulnerable to this attack vector. The resolution of this vulnerability demonstrated the critical importance of proper cryptographic implementation and the necessity of regular protocol audits to identify and address such fundamental weaknesses in privacy-preserving systems.