External risk intelligence

Nutshell Token Creation Vulnerability Allows Disk Fill Exploit

CVE advisorySeverity: CRITICAL (CVSS 9.1)

CVE-2025-65548

Nutshell is a mint implementation for the Cashu protocol, which is designed to function as an internet-facing service for issuing and managing digital tokens. Since it operates as a web-accessible service or API to facilitate token transactions, its endpoints are commonly reachable over the public internet in standard deployment patterns.

Cashu Nutshell

before 0.18.0

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

This advisory concerns a vulnerability in the Nutshell software, a component of the Cashu token system, that could allow an attacker to overwhelm the system's database and storage. While the potential impact is a denial-of-service, the primary concern is to confirm if your organization utilizes this specific technology.

Attackers can fill storage with arbitrary data.
Understand if this Cashu component is in use.
Assess potential service disruption if exposed.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by sending specially crafted cashu tokens to a vulnerable Nutshell mint. Because the mint does not check the size of the preimage data when spending these tokens, an attacker can provide excessively large preimages. This would cause the mint to store this arbitrary data, potentially filling up its database and disk space.

Accessible via the network.
Sending tokens with large preimages.
Denial of service, database/disk exhaustion.

Live Threat

Current exploitation, exposure, and threat context

When supported by the advisory, this vulnerability could allow an attacker to exploit how NUT-14 tokens are spent by versions of nutshell before 0.18.0. This could lead to the mint's database and disk being filled with arbitrary data, potentially impacting service availability.

Mint database and disk storage.
Spending tokens with oversized preimages.
Service denial due to disk exhaustion.

Operational Fix

Recommended remediation, mitigation, and detection steps

The Nutshell project, specifically versions prior to 0.18.0, is susceptible to a denial-of-service vulnerability due to unvalidated preimage sizes. This could allow an attacker to fill the mint's database and disk with arbitrary data, impacting its availability. Initial steps should focus on identifying all instances of affected Nutshell deployments, assessing their exposure and business criticality, locating the accountable owner, and then planning remediation or mitigation based on the assessed risk.

Identify affected deployments and owners.
Verify public reachability and business impact.
Plan risk-based remediation or mitigation.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

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Frequently asked questions

What is Nutshell and how is it used?

Nutshell is a software implementation of a mint for the Cashu protocol. Cashu is a Chaumian e-cash system, and Nutshell serves as the backend service that manages digital tokens, handles minting operations, and processes transactions. It essentially functions as the financial engine where tokens are created and redeemed.

What is the vulnerability in CVE-2025-65548?

This vulnerability is an Improper Validation of Specified Quantity of Input (CWE-1284). In affected versions of Nutshell, the software fails to check the size of preimage data when processing spent tokens. This allows an attacker to submit tokens containing excessively large data, which the mint then writes to its local storage.

How can an attacker trigger this issue?

An attacker triggers this by interacting with a mint running a vulnerable version of Nutshell and submitting a crafted cashu token that includes an oversized preimage. Simply performing valid minting operations without large, malicious preimages does not trigger this storage exhaustion; the bug specifically relies on the missing size check during token redemption.

How does Halo Surface Signal rate this threat?

Halo Surface Signal flags this as 'Likely' because Nutshell is designed to operate as an internet-facing service to facilitate token exchanges. Because these minting endpoints are frequently exposed to the public internet to remain accessible to users, instances are often reachable by remote attackers, increasing the risk of unauthorized disk-filling attempts.

Do I need to update my Nutshell deployment?

If you are running any version of Nutshell earlier than 0.18.0, you should prioritize an update to resolve this flaw. Begin by identifying all active instances of the software in your infrastructure, determine if they are exposed to the network, and coordinate with your technical team to patch the software to the latest secure version to prevent potential service disruption.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.