What is CVE-2021-3466?

CVE-2021-3466 is a critical-severity vulnerability in GNU Libmicrohttpd, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 9.8/10. Published 2021-03-25.

How severe is CVE-2021-3466?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Is CVE-2021-3466 known to be exploited?

1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Buffer overflow in GNU Libmicrohttpd

CVE-2021-3466

A flaw was found in libmicrohttpd. A missing bounds check in the post_process_urlencoded function leads to a buffer overflow, allowing a remote attacker to write arbitrary data in an application that uses libmicrohttpd. The highest threat from this vulnerability is to data confidentiality and integrity as well as system availability. Only version 0.9.70 is vulnerable.

Vulnerability class: Buffer Overflow

Published 2021-03-25 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.087 (95.0th percentile), scored 2026-10-01.

Low probability of exploitation in the next 30 days, but still tracked. 95th percentile — 95.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.087 · newest: 0.087 · change: 0.000

Affected products

GNU Libmicrohttpd — versions 0.9.70
Fedoraproject Fedora — versions 32, 33, 34
Red Hat Enterprise Linux — versions 6.0, 7.0, 8.0
Libmicrohttpd — versions libmicrohttpd 0.9.70

Weakness classification (CWE)

CWE-120 — Buffer Copy without Checking Size of Input (Classic Buffer Overflow)

Public proof-of-concept exploits

fkie-cad/nvd-json-data-feeds

References

bugzilla.redhat.com/show_bug.cgi (Patch, Third Party Advisory, Issue Tracking)
lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/… (vendor-advisory)
lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/… (vendor-advisory)
lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/… (vendor-advisory)
security.gentoo.org/glsa/202311-08 (vendor-advisory)

Frequently asked questions

What is CVE-2021-3466?: CVE-2021-3466 is a critical-severity vulnerability in GNU Libmicrohttpd, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 9.8/10. Published 2021-03-25.
How severe is CVE-2021-3466?: Critical severity. CVSS v3 base score is 9.8 out of 10.
Is CVE-2021-3466 known to be exploited?: 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.