What is CVE-2021-3851?

CVE-2021-3851 is a medium-severity vulnerability in Firefly-Iii Firefly Iii, classified under URL Redirection to Untrusted Site (Open Redirect). CVSS score: 5.4/10. Published 2021-10-19.

How severe is CVE-2021-3851?

Medium severity. CVSS v3 base score is 5.4 out of 10.

Open Redirect in Firefly-Iii Firefly Iii

CVE-2021-3851

firefly-iii is vulnerable to URL Redirection to Untrusted Site

Vulnerability class: Open Redirect

Published 2021-10-19 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.006 (45.1th percentile), scored 2026-10-01.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 45th percentile — 45.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.006 · newest: 0.006 · change: 0.000

Affected products

Firefly-Iii Firefly Iii
Firefly-Iii Firefly-Iii/firefly-Iii — versions unspecified

Weakness classification (CWE)

CWE-601 — URL Redirection to Untrusted Site (Open Redirect)

References

huntr.dev/bounties/549a1040-9b5e-420b-9b80-20700dd9d592 (Exploit, Patch, Third Party Advisory)
github.com/firefly-iii/firefly-iii/commit/8662dfa4c0f71efef61c31dc015c6f723db83… (Patch, Third Party Advisory)

Frequently asked questions

What is CVE-2021-3851?: CVE-2021-3851 is a medium-severity vulnerability in Firefly-Iii Firefly Iii, classified under URL Redirection to Untrusted Site (Open Redirect). CVSS score: 5.4/10. Published 2021-10-19.
How severe is CVE-2021-3851?: Medium severity. CVSS v3 base score is 5.4 out of 10.