What is CVE-2021-3900?

CVE-2021-3900 is a medium-severity vulnerability in Firefly-Iii Firefly Iii, classified under Cross-Site Request Forgery (CSRF). CVSS score: 6.5/10. Published 2021-10-27.

How severe is CVE-2021-3900?

Medium severity. CVSS v3 base score is 6.5 out of 10.

Is CVE-2021-3900 known to be exploited?

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

CSRF in Firefly-Iii Firefly Iii

CVE-2021-3900

firefly-iii is vulnerable to Cross-Site Request Forgery (CSRF)

Vulnerability class: CSRF (Cross-Site Request Forgery)

Published 2021-10-27 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N.

EPSS exploit prediction

EPSS: 0.006 (44.2th percentile), scored 2026-10-07.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 44th percentile — 44.2% 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-352 — Cross-Site Request Forgery (CSRF)

Public proof-of-concept exploits

References

huntr.dev/bounties/909e55b6-ef02-4143-92e4-bc3e8397db76 (Exploit, Third Party Advisory)
github.com/firefly-iii/firefly-iii/commit/c2c8c42ef3194d1aeba8c48240fe2e9063f77… (Patch, Third Party Advisory)

Frequently asked questions

What is CVE-2021-3900?: CVE-2021-3900 is a medium-severity vulnerability in Firefly-Iii Firefly Iii, classified under Cross-Site Request Forgery (CSRF). CVSS score: 6.5/10. Published 2021-10-27.
How severe is CVE-2021-3900?: Medium severity. CVSS v3 base score is 6.5 out of 10.
Is CVE-2021-3900 known to be exploited?: 2 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.