CVE-2021-23840: OpenSSL EVP cipher update integer overflow causes negative output length

Automated analysis

Generated by VULONE's analysis model from the NVD record, CISA KEV and EPSS data on 20 September 2026. Confidence: high.

high priorityCVSS 7.5 with network reachability and high EPSS, but no KEV listing or documented exploitation and impact limited to availability.

What it is

Calls to EVP_CipherUpdate, EVP_EncryptUpdate and EVP_DecryptUpdate can overflow the output length argument when the input length approaches the platform's maximum integer value. The function returns success (1) while the output length becomes negative, so applications may behave incorrectly or crash. OpenSSL 1.1.1i and earlier and 1.0.2x and earlier are affected; fixes are in 1.1.1j and 1.0.2y.

Impact

An attacker who can drive very large input lengths into affected EVP calls can trigger incorrect application behavior or a crash, giving a denial of service. The CVSS vector shows no confidentiality or integrity impact, only availability.

Attack surface

Reachable over the network per the CVSS vector (AV:N) with no privileges or user interaction required, but only where an application passes near-maximum input lengths into the affected EVP update functions. The record does not identify specific exposed services or protocols.

Exploitation

Not listed in CISA KEV and no ransomware usage is documented. EPSS is high (about 0.51 probability, 98.9th percentile), and references are vendor and third-party advisories plus patch commits, with no public exploit tag.

What to do

• Upgrade OpenSSL to 1.1.1j or later; 1.0.2 users must move to 1.0.2y (premium support) or 1.1.1j.
• Inventory and update downstream products that bundle OpenSSL, including Debian, Oracle, Tenable, McAfee, Fujitsu and Node.js components listed in the record.
• Apply vendor advisories and patches referenced for affected products rather than relying on OpenSSL upgrades alone.
• Where immediate patching is not possible, review applications that pass very large buffers to EVP_CipherUpdate, EVP_EncryptUpdate or EVP_DecryptUpdate and add input length bounds checks.

Detection

• Monitor application and service logs for crashes or abnormal terminations in processes using OpenSSL EVP encryption or decryption routines.
• Track OpenSSL library versions across hosts and containers to find systems still on 1.1.1i or earlier or 1.0.2x or earlier.
• Watch for unusually large input sizes or memory allocation failures in services that perform bulk encryption or decryption.

This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.