External risk intelligence

Openssl_encrypt Authentication Bypass via AES-CTR Fallback

CVE advisorySeverity: CRITICAL (CVSS 9.3)

CVE-2026-74901

The vulnerability exists in a software library (openssl_encrypt) used for cryptographic operations. While libraries can be embedded in internet-facing applications, they are also frequently used in internal tools, scripts, or non-networked backend processes. There is no inherent design requiring this library to be exposed directly to the public internet in common deployment patterns.

Authentication Bypass

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

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A vulnerability has been identified in a cryptographic library that could allow unauthorized modification of data without detection, potentially impacting the integrity of sensitive information. The issue arises from a fallback mechanism in AES-GCM decryption that, when triggered by decryption failures, reverts to an unauthenticated mode, allowing attackers to alter data in transit. The main concern is confirming relevance and exposure, as this library can be used in various contexts, both internal and external.

Data integrity risk without detection.
Affects cryptographic operations, a core security function.
Confirm relevance and exposure of this library.

Attack Path

How an attacker could exploit the issue

An attacker could bypass authentication by intercepting and altering encrypted data, causing the `openssl_encrypt` library to fall back to an less secure mode. This could allow an attacker to modify ciphertext in transit, enabling undetected bit-flipping attacks.

No authentication required to access.
Triggered by AES-GCM decryption failure.
Bypasses integrity checks for data modification.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could allow an attacker to bypass authentication checks when decrypting data using the `openssl_encrypt` library. When decryption fails, the system may fall back to an unauthenticated mode, enabling an attacker to alter encrypted data in transit, leading to undetected modifications.

Encrypted data integrity.
Attackers modify ciphertext in transit.
Undetected data alteration.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability in openssl_encrypt affects applications utilizing its cryptographic functions. Owners of applications that use this library should first identify all deployments, determine their external reachability and business criticality, and then confirm the accountable team for remediation, likely application or platform owners. Planning for remediation should be risk-based, considering factors like exposure and impact.

Confirm application ownership and reachability.
Verify if the affected library is in use.
Plan remediation based on identified risk.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

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Frequently asked questions

What is openssl_encrypt?

Openssl_encrypt is a cryptographic software library designed to help developers implement encryption and decryption functions within their applications. It serves as a foundational building block for securing sensitive data. Because it handles core security tasks, it is commonly integrated into a wide range of software, including web applications, automated scripts, and backend service components that need to protect data during storage or transit.

How does CVE-2026-74901 create a security weakness?

This vulnerability is classified as Improper Verification of Cryptographic Signature (CWE-347). It occurs when the library fails to properly authenticate encrypted data. If the library encounters a decryption error, it incorrectly falls back to a less secure mode instead of stopping. This flaw lets an attacker manipulate encrypted data in transit, bypassing integrity checks so the system accepts altered information as if it were legitimate.

When does this vulnerability trigger?

The vulnerability is triggered specifically when the library attempts to decrypt data using AES-GCM and encounters a failure. This failure causes the library to revert to the unauthenticated AES-CTR mode. Simply using the library for encryption or decryption under normal, successful conditions does not trigger the fallback; the flaw only becomes active when a decryption error occurs and the system processes that error by switching to the weaker, unauthenticated mechanism.

Why should I care if my service is not internet-facing?

Halo Surface Signal indicates this library is versatile, appearing in both internet-facing applications and internal, non-networked backend processes. While external exposure increases the risk of remote tampering, internal systems are also vulnerable if an attacker gains any foothold within your network. You should care if you manage any software utilizing this library, as the threat relies on the ability to manipulate data in transit, regardless of whether that transit is public or private.

What is the first step to address CVE-2026-74901?

Begin by auditing your software inventory to determine where the openssl_encrypt library is currently utilized. Once you have a clear list of affected applications, categorize them by business criticality and assess their connectivity. Because this library is a component, you will likely need to coordinate with the specific application or platform teams responsible for each deployment to verify the version in use and coordinate a secure update.

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.