How severe is CVE-2021-40346?

CVE-2021-40346 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-40346?

Affected package information for CVE-2021-40346 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2021-40346?

No fixed version has been published for CVE-2021-40346 yet. Monitor the advisory for updates, apply mitigations recommended by the vendor, and use O3 Security to track when a fix becomes available in your stack.

Is CVE-2021-40346 actively exploited in the wild?

Yes. There are 9 known exploit references for CVE-2021-40346, including 5 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-40346?

CVE-2021-40346 has an EPSS (Exploit Prediction Scoring System) score of 92.4%, placing it in the 100th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. A score above 50% indicates significantly elevated exploitation risk — immediate patching is strongly recommended.

When was CVE-2021-40346 published, and has it been updated?

CVE-2021-40346 was published on September 8, 2021 and was last updated on April 11, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CVE-2021-40346

HIGH

Also known asBIT-haproxy-2021-40346

Published

Sep 8, 2021

Updated

Apr 11, 2026

Affected

0 pkgs

Patched

None yet

Exploits

9 known

EPSS Exploitation Probability

via FIRST.org ↗

92.4%probability of exploitation in next 30 days

Very High Risk100th percentile0.00%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Description

An integer overflow exists in HAProxy 2.0 through 2.5 in htx_add_header that can be exploited to perform an HTTP request smuggling attack, allowing an attacker to bypass all configured http-request HAProxy ACLs and possibly other ACLs.

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.