CVE-2026-81687 in openssl_encrypt
Summary
by MITRE • 08/27/2026
openssl_encrypt versions before 1.4.9 fail to enforce a time ceiling on key derivation function iteration counts specified in file metadata. Attackers can craft files with extremely high KDF iteration counts to consume CPU resources for unbounded periods before password verification occurs.
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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 input validation and resource management within the cryptographic key derivation process. This flaw specifically affects applications that utilize this library to decrypt files or data protected by passwords, where the encryption parameters are stored alongside the ciphertext in the file metadata. The core technical deficiency lies in the absence of an upper bound check on the iteration count specified for the Key Derivation Function (KDF). In secure cryptographic implementations, KDFs such as PBKDF2, scrypt, or Argon2 employ a high number of iterations to deliberately slow down password guessing attacks, thereby increasing the computational cost required by attackers. However, this mechanism relies on the assumption that input parameters are within reasonable bounds. When openssl_encrypt fails to enforce a time ceiling or maximum iteration limit, it allows an attacker to specify arbitrarily large values for these counts directly in the file's metadata header.
From an operational perspective, this vulnerability enables a highly effective Denial of Service (DoS) attack vector. An adversary can craft malicious files containing extremely high KDF iteration counts and distribute them through various channels such as email attachments, web uploads, or shared storage systems. When a victim application attempts to decrypt these crafted files using openssl_encrypt, the system is forced to execute the key derivation process with the specified excessive iterations before it even begins password verification. This results in significant CPU consumption that can persist for unbounded periods, effectively hanging the processing thread and exhausting server resources. Unlike traditional DoS attacks that rely on network bandwidth saturation or rapid connection requests, this attack targets computational efficiency, making it particularly difficult to mitigate through standard rate-limiting or firewall rules because the malicious payload appears as a legitimate encrypted file until decryption is attempted.
This flaw aligns with CWE-400: Uncontrolled Resource Consumption, specifically highlighting how insufficient control over resource limits can lead to system degradation. Furthermore, in the context of the MITRE ATT&CK framework, this vulnerability facilitates techniques associated with Impact categories, particularly Denial of Service (T1499) and potentially Resource Hijacking if the computational resources are leveraged for other malicious purposes such as cryptocurrency mining during the prolonged execution time. The lack of validation on cryptographic parameters is a common oversight in older or less rigorously audited libraries, where developers may assume that input data comes from trusted sources or fails to anticipate adversarial manipulation of metadata fields intended only for internal configuration use.
To mitigate this vulnerability, organizations must immediately upgrade openssl_encrypt to version 1.4.9 or later, which implements strict validation logic to cap the maximum allowable iteration count based on acceptable performance thresholds and security requirements. For systems that cannot be upgraded instantly due to legacy constraints, a temporary workaround involves implementing an application-level pre-processing step that parses file metadata before passing it to the encryption library. This intermediary layer should validate all cryptographic parameters, rejecting any files where KDF iteration counts exceed a predefined safe maximum, such as 10,000 or 100,000 iterations depending on the specific algorithm and hardware capabilities. Additionally, deploying Web Application Firewalls (WAFs) with rules to detect unusually large metadata payloads in upload endpoints can provide an additional layer of defense against the distribution of these crafted files. Regular security audits focusing on cryptographic implementation details are essential to prevent similar misconfigurations where performance optimization is prioritized over robust input validation.