CVE-2026-81700 in openssl_encrypt
Summary
by MITRE • 08/27/2026
openssl_encrypt versions before 1.4.9 contain a signature verification vulnerability in gpg_runner.verify_detached that accepts revoked and expired keys by only checking VALIDSIG status without inspecting REVKEYSIG, EXPKEYSIG, or gpg exit codes. Attackers holding compromised-then-revoked signing keys or expired project keys can bypass signature verification to execute malicious plugins in the host process.
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Analysis
by VulDB Data Team • 08/27/2026
The vulnerability identified in openssl_encrypt versions prior to 1.4.9 represents a critical failure in cryptographic key validation logic within the gpg_runner.verify_detached function. This component is responsible for verifying detached digital signatures, which are essential for ensuring the integrity and authenticity of data or plugins before they are executed by the host process. The core technical flaw lies in an incomplete implementation of OpenPGP signature verification standards. Specifically, the code relies exclusively on the presence of a VALIDSIG status line from GnuPG to determine that a signature is valid. This approach ignores critical security indicators such as REVKEYSIG and EXPKEYSIG, which signal that the signing key has been revoked or expired respectively. Furthermore, the implementation fails to inspect the exit codes returned by the gpg command-line tool, which often provide additional context regarding why a verification might have failed despite producing a VALIDSIG status under certain edge cases or misconfigurations.
From an industry standard perspective, this vulnerability aligns with CWE-347: Improper Verification of Cryptographic Signature. The flaw stems from trusting the output of a cryptographic operation without fully validating the state and trustworthiness of the public key used to generate that signature. In robust security architectures, verifying a signature is not merely about confirming the mathematical correctness of the hash but also ensuring the identity associated with the signing key remains valid within its lifecycle. By neglecting to check for revocation or expiration status, the software violates fundamental principles of trust management defined in cryptographic best practices and standards such as RFC 4880 (OpenPGP Message Format). This oversight creates a significant gap where technically correct signatures are accepted even when they originate from compromised or no-longer-trusted entities.
The operational impact of this vulnerability is severe, particularly for systems relying on openssl_encrypt to load and execute plugins or extensions in a host process. An attacker who possesses a signing key that was previously valid but has since been revoked due to compromise, or an expired project key, can forge signatures that the application will accept as legitimate. This allows the execution of malicious code under the guise of authorized software. For instance, if a plugin is signed with a compromised developer's key that they have subsequently revoked for security reasons, the vulnerable version of openssl_encrypt would still validate this signature and proceed to execute the associated payload. This effectively bypasses all integrity checks designed to prevent unauthorized or tampered code execution, leading to potential remote code execution, data exfiltration, or complete system compromise depending on the privileges of the host process.
This behavior also maps closely to MITRE ATT&CK technique T1059: Command and Scripting Interpreter, specifically in contexts where attackers use signed scripts or plugins to evade detection mechanisms that rely solely on signature validity rather than source reputation. The ability to bypass verification using revoked keys is a sophisticated evasion tactic often seen in advanced persistent threats seeking to maintain persistence while avoiding alerts triggered by unsigned or improperly signed binaries. Security teams must recognize that the presence of a valid cryptographic signature does not equate to trustworthiness if the key's lifecycle status is ignored.
To mitigate this vulnerability, immediate updates to openssl_encrypt version 1.4.9 or later are required, as these versions correct the verification logic by properly inspecting REVKEYSIG and EXPKEYSIG statuses along with gpg exit codes. In environments where updating is not immediately feasible, administrators should implement strict allow-lists for known good signing keys rather than relying solely on signature validity checks. Additionally, integrating a key management system that actively monitors certificate revocation lists (CRL) or OpenPGP web of trust status can provide an additional layer of defense. It is also advisable to audit existing plugins and scripts to ensure they are signed by currently active and uncompromised keys, thereby reducing the attack surface available to adversaries attempting to exploit this logic flaw.