External risk intelligence

openssl_encrypt PQc Decapsulation Fallback Deterministic Shared Secret Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.3)

CVE-2026-74900

The vulnerability exists in a library (openssl_encrypt/pqc.py) used for cryptographic operations. While it is network-accessible, the exposure depends on whether the host application implementing this library exposes the specific post-quantum cryptography (PQC) decapsulation functions to internet-facing traffic. It is not inherently a public-facing service by design.

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 details a critical vulnerability in the `openssl_encrypt` library concerning its post-quantum cryptography (PQC) implementation. A failure in the decapsulation process can lead to a silent fallback, allowing an attacker with minimal private key information to derive a shared secret, potentially decrypting all encrypted data. The main concern is confirming relevance and exposure due to the library's specific usage context.

Vulnerability allows deterministic secret key generation.
Leadership should remember PQC fallback risks data compromise.
Confirm if this crypto library is in use.

Attack Path

How an attacker could exploit the issue

An attacker with access to 16 bytes of a private key can exploit a flaw in the KEM decapsulation process. This flaw causes the system to silently fall back to a simulation mode, enabling the attacker to compute the shared secret. Once the shared secret is known, the attacker can decrypt all encrypted data.

Requires 16 bytes of private key.
Triggered by KEM decapsulation failure.
Risk: All ciphertext can be decrypted.

Live Threat

Current exploitation, exposure, and threat context

When supported by the advisory, KEM decapsulation failures could silently fall back to simulation mode, allowing an attacker with 16 bytes of the private key and public encapsulated data to compute a deterministic shared secret. This could enable decryption of all ciphertext when the fallback is triggered without error.

Private key data could be at risk.
Attackers could compute shared secrets.
All ciphertext may be decrypted.

Operational Fix

Recommended remediation, mitigation, and detection steps

This critical vulnerability in openssl_encrypt's pqc.py requires immediate attention from teams responsible for cryptographic libraries and applications that utilize them. The immediate priority is to identify all instances of the affected technology, assess their exposure and business criticality, and locate the accountable owners. Planning remediation should be risk-based, considering the potential for silent decryption.

Identify application owners for affected libraries.
Verify reachability and criticality of instances.
Plan remediation based on identified risk.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is openssl_encrypt?

openssl_encrypt is a software library designed to handle cryptographic operations. It is often integrated into applications to manage secure data processing, including newer post-quantum cryptography (PQC) standards. Developers use this library to implement encryption and key exchange mechanisms, ensuring that information remains private and authenticated during digital communications.

What is the security weakness in CVE-2026-74900?

This vulnerability is classified as CWE-391: Unchecked Error Condition. Within the library's PQC implementation, a failure during key encapsulation mechanism (KEM) decapsulation—which should trigger an error—instead silently defaults to a simulation mode. This fallback generates a weak, predictable shared secret that relies on insufficient private key data, allowing an attacker to bypass encryption protections.

How is this vulnerability triggered?

The flaw is triggered specifically when a KEM decapsulation process fails. If the process does not encounter a failure, the fallback mode is not invoked, and the vulnerability does not manifest. To exploit this, an attacker needs 16 bytes of the private key along with the public encapsulated key data to compute the shared secret.

Is my system at risk for CVE-2026-74900?

According to Halo Surface Signal, risk depends on how your application uses the library. While the library itself is network-accessible, you are only at risk if your specific software implementation exposes these PQC decapsulation functions to internet-facing traffic. It is not automatically exposed just because the library is present in your environment.

How should I respond to this advisory?

Begin by inventorying your applications to identify which ones utilize the openssl_encrypt library. Once mapped, confirm if those applications use the PQC components and verify if they are reachable by external traffic. Focus your remediation efforts on the most critical services that rely on this library for encryption, and prioritize applying the necessary software updates to eliminate the flawed code.

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.