CVE-2020-15093 in tough Library
Summary
by MITRE
The tough library (Rust/crates.io) prior to version 0.7.1 does not properly verify the threshold of cryptographic signatures. It allows an attacker to duplicate a valid signature in order to circumvent TUF requiring a minimum threshold of unique signatures before the metadata is considered valid. A fix is available in version 0.7.1. CVE-2020-6174 is assigned to the same vulnerability in the TUF reference implementation.
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Analysis
by VulDB Data Team • 10/29/2020
The vulnerability described in CVE-2020-15093 affects the tough library, a rust implementation of the Trust Update Framework (TUF) used for securing software distribution systems. This library serves as a critical component in verifying the authenticity and integrity of software updates by implementing cryptographic signature verification mechanisms. The flaw resides in the library's insufficient validation of signature thresholds, creating a fundamental weakness in the TUF security model that undermines the integrity protection mechanisms designed to prevent unauthorized software modifications. The issue specifically impacts versions prior to 0.7.1, making any system relying on this library vulnerable to signature duplication attacks that bypass the intended security controls.
The technical flaw manifests in the library's failure to properly enforce the minimum threshold requirement for cryptographic signatures within the TUF framework. In a properly functioning TUF implementation, metadata files should require a minimum number of unique signatures from trusted parties before being accepted as valid. However, the vulnerable tough library allows attackers to create duplicate signatures that satisfy the threshold requirement without actually providing additional legitimate verification. This vulnerability directly maps to CWE-326, which addresses insufficient cryptographic strength, and more specifically to CWE-347, which deals with improper verification of cryptographic signatures. The flaw enables an attacker to manipulate the signature validation process by simply duplicating existing valid signatures, thereby circumventing the intended security controls that require multiple independent verifications.
The operational impact of this vulnerability is significant within software supply chain security contexts where TUF is employed to protect against malicious updates and tampering. Attackers can exploit this weakness to bypass the threshold-based signature verification that is essential for preventing single points of failure in the security model. When systems rely on TUF to ensure that software updates have been validated by multiple parties, this vulnerability allows adversaries to compromise the integrity protection by simply replicating valid signatures. The consequence extends beyond individual systems to potentially affect entire software distribution networks, as the vulnerability undermines the fundamental trust model that TUF aims to establish. This weakness is particularly dangerous in environments where software updates are automatically applied, as it could enable attackers to inject malicious code without detection, since the duplicated signatures would appear legitimate to the verification process.
The mitigation for this vulnerability requires immediate deployment of version 0.7.1 of the tough library, which includes proper signature threshold validation. Organizations should conduct comprehensive inventory assessments to identify all systems using affected versions of the library and implement remediation procedures. The fix addresses the core issue by ensuring that signature thresholds are properly enforced and that duplicate signatures cannot be used to circumvent the verification process. Security teams should also consider implementing additional monitoring for unusual signature patterns and conduct regular security assessments of their software supply chain components. This vulnerability highlights the importance of proper cryptographic implementation in security frameworks and demonstrates how seemingly minor flaws in signature validation can have substantial impacts on overall system security. The issue is related to ATT&CK technique T1553.006, which covers credential dumping and signature validation bypass, emphasizing the need for robust cryptographic verification mechanisms in security frameworks.