Bias of Polynomial Coefficients in Secret Sharing
Published Sources: OSV
What is affected, what to do
Affected versions of this crate allowed for a bias when generating random polynomials for Shamir Secret Sharing, where instead of being within the range `[0, 255]` they were instead in the range `[1, 255]`. A description from Cure53, who originally found the issue, is available: > The correct method to select a random polynomial would be to select all coefficients (including the most significant coefficient) uniformly in the range 0..255 (inclusive).
The available sources name no fixed version. Check the vendor guidance in the references below.
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Metrics
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Priority
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CVSS v3
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EPSS score
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- Severity
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Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 6.87 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- no public PoC known
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- —
- Published
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
Affected versions of this crate allowed for a bias when generating random polynomials for Shamir Secret Sharing, where instead of being within the range `[0, 255]` they were instead in the range `[1, 255]`. A description from Cure53, who originally found the issue, is available: > The correct method to select a random polynomial would be to select all coefficients (including the most significant coefficient) uniformly in the range 0..255 (inclusive). Otherwise, knowledge that a coefficient in a polynomial cannot be 0 permits the exclusion of single byte values for the shared secret given one share less than required. [...] Exploiting this weakness necessitates sharing the same secret multiple times. In this scenario, an attacker could exclude an exponential number of values for each of the shared bytes until sufficiently few values remain for brute forcing. Cure53 estimates that under ideal circumstances (e.g., a 2-out-of-N scheme) a shared secret can be reconstructed if the same secret has been distributed 500-1500 times. Secrets that have been shared a low amount of times (ideally, once) would not be impacted. However, secrets that are repeatedly shared may be vulnerable, especially if the shares are still available, and should be rotated. The vulnerability does not impact reconstitution of secrets: secrets that have already been split can be recombined without issue. The flaw can be corrected by changing the lower bound of the polynomial coefficient range in the `sharks::math::random_polynomial` function to `0`. The `blahaj` crate has been made available with a fixed version of the code, after attempts to reach the maintainer of the `sharks` crate were unsuccessful.