How severe is CVE-2021-3851?

CVE-2021-3851 has a CVSS score of 5.4/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-3851?

CVE-2021-3851 affects the following packages: grumpydictator/firefly-iii (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2021-3851?

Update grumpydictator/firefly-iii to 5.6.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3851 is resolved across your whole dependency graph.

How do I detect CVE-2021-3851 in my Packagist dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for grumpydictator/firefly-iii, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-3851 if there is no patch (or I can't update yet)?

Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

Is CVE-2021-3851 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-3851, including 1 in-the-wild exploitation 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-2021-3851?

CVE-2021-3851 has an EPSS (Exploit Prediction Scoring System) score of 0.6%, placing it in the 46th 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. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-3851?

CVE-2021-3851 is classified as Open Redirect (CWE-601). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-3851 published, and has it been updated?

CVE-2021-3851 was published on October 19, 2021 and was last updated on July 9, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐘

🐘 Packagist

Not in CISA KEV

MEDIUM severity

CVE-2021-3851 — firefly-iii

MEDIUMFix: firefly-iii/firefly-iii@8662dfa

CVE-2021-3851 is a medium-severity (CVSS 5.4) Open Redirect vulnerability in grumpydictator/firefly-iii. 1 public exploit reference exists, so weaponization risk is real. A fix is available for grumpydictator/firefly-iii — see the affected versions and patch details below.

Open Redirect in firefly-iii

Also known asGHSA-5fvx-5p2r-4mvp

Published

Oct 19, 2021

Updated

Jul 9, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

1 known

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.6%probability of exploitation in next 30 days

Lower Risk+0.40%

Lower risk than most CVEs46th percentile — riskier than 46% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-3851 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

🐘grumpydictator/firefly-iii

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

Description

firefly-iii is vulnerable to URL Redirection to Untrusted Site

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`grumpydictator/firefly-iii`	all versions	5.6.2`composer require grumpydictator/firefly-iii:^5.6.2`

Affected Products

1 product · 1 configurations

Application

firefly iiifirefly-iii

< 5.6.2

range

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.

firefly-iii is vulnerable to URL Redirection to Untrusted Site

huntr.devNVDOct 2021

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for grumpydictator/firefly-iii, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update grumpydictator/firefly-iii to 5.6.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3851 is resolved across your whole dependency graph.
Workarounds
Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

Frequently Asked Questions

firefly-iii is vulnerable to URL Redirection to Untrusted Site

O3 Security · Impact-Aware SCA

Is CVE-2021-3851 in your dependencies?

Find it across Packagist, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

5.4/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

Low

User Interaction

Required

Scope

Changed

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

CWE Classification

CWE-601Open Redirect

Weaponization

No known public exploit tooling found.

OpenSSF Scorecard

Package health score

firefly-iii

4.6/10

Code-Review0/10

Maintained10/10

Branch-Protection3/10

Assessed Oct 5, 2026

Timeline

Disclosed

Oct 19, 2021

First exploit

Oct 21, 2021

2 days after disclosure — rapid exploitation

Last updated

Jul 9, 2026

Identifiers

CVE-2021-3851

GHSA-5fvx-5p2r-4mvp

References

Advisorynvd.nist.gov Fixgithub.com Fixhuntr.dev Webgithub.com

Related vulnerabilities

This CVE is medium severity; 8 of 8 other CVEs affecting grumpydictator/firefly-iii are rated as high or higher (top: critical).

CVE-2026-50886GHSA-6jq6-x4cx-qvcmGHSA-5q8v-j673-m5v4CVE-2024-37893GHSA-29w6-c52g-m8jcCVE-2024-22075CVE-2023-1788CVE-2023-1789

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedJul 9, 2026

CVE-2021-3851 — firefly-iii

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Frequently Asked Questions

Is CVE-2021-3851 in your dependencies?