CVE-2021-23840

What it is

Calls to EVP_CipherUpdate, EVP_EncryptUpdate and EVP_DecryptUpdate may overflow the output length argument in some cases where the input length is close to the maximum permissable length for an integer on the platform. In such cases the return value from the function call will be 1 (indicating success), but the output length value will be negative. This could cause applications to behave incorrectly or crash. OpenSSL versions 1.1.1i and below are affected by this issue. Users of these versions should upgrade to OpenSSL 1.1.1j. OpenSSL versions 1.0.2x and below are affected by this issue. However OpenSSL 1.0.2 is out of support and no longer receiving public updates. Premium support customers of OpenSSL 1.0.2 should upgrade to 1.0.2y. Other users should upgrade to 1.1.1j. Fixed in OpenSSL 1.1.1j (Affected 1.1.1-1.1.1i). Fixed in OpenSSL 1.0.2y (Affected 1.0.2-1.0.2x).

At a glance

Exploitability

Can be exploited over the network without logging in. High risk of service disruption.

Exploit odds

50.7% chance of exploitation in the next 30 days (EPSS). Higher than most.

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2021-02-16 17:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-06-17 03:38 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 50.7% in the next 30 days (EPSS).
• Severity: 7.5 HIGH (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2021-02-16 17:15 UTC

Last modified

2026-06-17 03:38 UTC

Exploit probability

50.7% · EPSS, next 30 days

Weakness

CWE-190 · Integer overflow

Record status

Modified

Affected

• OpenSSL < 1.0.2y
• OpenSSL < 1.1.1j
• Debian Linux 10.0
• Tenable Log Correlation Engine < 6.0.8
• Tenable Nessus Network Monitor 5.11.0
• Tenable Nessus Network Monitor 5.11.1
• Tenable Nessus Network Monitor 5.12.0
• Tenable Nessus Network Monitor 5.12.1

Attack profile CVSS 3.1 · 7.5

Attack vector

Network

Complexity

Low

Privileges needed

None

User interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

High

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2021-23840.md and follow it. Check whether this project is affected by CVE-2021-23840. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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