CVE-2026-81721 in openssl_encrypt
Summary
by MITRE • 08/27/2026
openssl_encrypt before 1.4.9 fails to validate KDF cost parameters in encrypted file metadata and keystore headers, allowing attackers to trigger unbounded memory allocation. Attackers can craft malicious encrypted files declaring arbitrarily large Argon2, scrypt, or balloon KDF parameters to exhaust system memory and crash the process without authentication.
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Analysis
by VulDB Data Team • 08/27/2026
The vulnerability identified in OpenSSL versions prior to 1.4.9 represents a critical failure in input validation within its cryptographic key derivation functions, specifically affecting encrypted file metadata and keystore headers. This flaw stems from an insufficient check on cost parameters associated with memory-hard KDF algorithms such as Argon2, scrypt, and balloon. When the library processes these structures to derive encryption keys or verify integrity, it accepts parameter values declared in the header without verifying that they fall within reasonable bounds for system resources. Consequently, an attacker can craft a malicious encrypted file containing arbitrarily large parameters for memory allocation requirements during the key derivation phase. This lack of boundary checking allows the application to attempt allocating massive amounts of RAM based on the untrusted input provided in the metadata headers.
From a technical perspective, this issue is classified under CWE-789: Uncontrolled Memory Allocation and CWE-20: Improper Input Validation. The operational impact is severe as it enables a denial-of-service attack without requiring any form of authentication or prior access privileges. By submitting a specially crafted file with extreme cost parameters, an attacker can force the OpenSSL process to consume excessive system memory until the operating system's out-of-memory killer terminates the application or the service becomes unresponsive due to resource exhaustion. This type of attack is particularly dangerous in environments where encrypted files are processed automatically by services such as email gateways, document management systems, or backup utilities that may not validate file integrity before processing them for decryption.
The exploitation vector aligns with MITRE ATT&CK technique T1496: Resource Hijacking, specifically the sub-technique of memory exhaustion to disrupt service availability. Unlike attacks that require code execution or privilege escalation, this vulnerability allows a remote attacker to degrade system stability simply by delivering a malicious payload. The absence of authentication requirements makes it feasible for attackers to target public-facing services or shared storage locations where files can be uploaded or placed without strict access controls. This creates a significant risk for organizations relying on OpenSSL for data protection at rest, as the integrity of the decryption process is compromised by the inability to trust metadata parameters.
Mitigation strategies must prioritize immediate patching to version 1.4.9 or later, where these validation checks have been implemented to enforce maximum limits on KDF cost parameters. In addition to upgrading, administrators should implement input sanitization at the application layer if possible, ensuring that any file processing logic rejects headers with suspiciously large values before passing them to the cryptographic library. Furthermore, deploying resource monitoring and rate-limiting mechanisms can help detect and mitigate attempts to exhaust system memory in real-time. Security teams should also review configurations for services handling encrypted data to ensure they are not vulnerable to similar unbounded allocation issues across other supported algorithms until all systems are updated.