External risk intelligence

OCaml JOSE RSA Signature Validation Flaw

CVE advisorySeverity: CRITICAL (CVSS 9.1)

CVE-2026-89086

The vulnerability exists in a library for OCaml used for JOSE/JWT processing. While such libraries are frequently used in internet-facing web services and APIs to handle authentication or tokens, the library itself is a development dependency, not a standalone edge service or appliance. Its exposure depends entirely on how a developer integrates it into their specific application.

Halo Surface Signal: 3 out of 5 — possibly public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

This advisory addresses a critical vulnerability in the jose library for OCaml. The issue lies in how the library validates RSA signatures, potentially leading to improper acceptance of invalid signatures. This could have implications for systems relying on these signatures for security and trust, depending on how the library is integrated into applications.

Invalid signatures may be accepted.
Affects systems using OCaml JOSE/JWT processing.
Confirm relevance and exposure to applications.

Attack Path

How an attacker could exploit the issue

An attacker could leverage this vulnerability by sending specially crafted data to a service that uses the affected OCaml jose library for signature validation. The library incorrectly accepts signatures without proper verification against the public key, potentially allowing an attacker to impersonate legitimate users or manipulate data.

Vulnerability is exposed via network.
Malicious signature is accepted by the library.
Allows unauthorized access and data manipulation.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could allow an attacker to forge RSA signatures for tokens or other signed data processed by applications using the affected library. When supported by the advisory, this could lead to unauthorized access or manipulation of data by presenting a forged signature as legitimate.

Forged digital signatures on signed data.
Signature validation bypasses public key check.
Unauthorized data access or modification.

Operational Fix

Recommended remediation, mitigation, and detection steps

The jose package for OCaml is susceptible to a critical vulnerability where RSA signature validation is incomplete, potentially leading to the acceptance of forged signatures. Responsibility for addressing this likely falls to the application development or platform engineering teams who integrate this library. The first practical step is to identify all applications utilizing this library, determine their business criticality and exposure, and then plan remediation based on risk.

Application development teams own the issue.
Verify affected application integrations.
Plan remediation based on risk.

Supplementary metadata

CVSS vector

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Frequently asked questions

What is the OCaml jose package?

The OCaml jose library is a set of tools used by developers to handle JSON Object Signing and Encryption (JOSE) and JSON Web Tokens (JWT). It provides the building blocks for verifying and creating secure data exchanges, such as authentication tokens, within applications built in the OCaml programming language.

Why is CVE-2026-89086 considered a critical flaw?

This vulnerability, classified as CWE-347, involves an error in cryptographic signature verification. The library checks if the RSA signature format is correct but fails to perform the mandatory step of validating the signature against the actual public key. Essentially, the code assumes a signature is authentic if it is formatted properly, allowing forged or invalid signatures to be accepted as trusted.

How can an attacker trigger this vulnerability?

An attacker triggers the bug by sending a specially crafted, invalid signature to an application that relies on the affected library. The vulnerability is not triggered by merely hosting the library; it requires the application to process malicious input as part of a legitimate-looking signature. Standard, well-formed signatures that do not attempt to bypass public key checks are not the focus of this flaw.

Is my application at risk from this JOSE vulnerability?

Halo Surface Signal indicates that your risk depends on how your developers integrated this library. Because this is a code-level dependency rather than a standalone service, you are primarily at risk if your application uses the library to verify untrusted tokens or signed data coming from the network, such as in internet-facing APIs or authentication gateways.

What should I do if I use the OCaml jose library?

Your first step is to work with your engineering team to audit your codebase and identify every application that includes the jose package. Once identified, evaluate whether those applications process signed data from outside sources. Prioritize updating the library to version 0.11.0 or later, as this version contains the necessary fixes to properly validate signatures against public keys.

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.