CVE-2021-3851Network attack vector

Open redirect via unchecked invalid-form return URLs

Published Oct 19, 2021 · Updated Aug 3, 2024

Open redirect in Firefly III before 5.6.2 allows remote authenticated users to send victims to an attacker-controlled site. Invalid-form handling redirects to the prior URL or an exception's redirect target without verifying that its host matches the application host. Exploitation requires victim interaction with a crafted request, after which the browser leaves the trusted Firefly III origin.

CVSS severity5.4
Medium
EPSS probability0.57%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Open redirect in Firefly III before 5.6.2 allows remote authenticated users to send victims to an attacker-controlled site. Invalid-form handling redirects to the prior URL or an exception's redirect target without verifying that its host matches the application host. Exploitation requires victim interaction with a crafted request, after which the browser leaves the trusted Firefly III origin.

The record

CVE
CVE-2021-3851
Published
Oct 19, 2021
Updated
Aug 3, 2024
Vendor
Firefly III
Product
Firefly III
Classifications
CWE-601, T1204.001
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Oct 19, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <5.6.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Firefly III · Firefly IIIversion=unspecified <5.6.2

Published CVSS scores

5.0huntr.devCVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:L
5.4NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
4.9NIST NVDAV:N/AC:M/Au:S/C:P/I:P/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-601URL Redirection to Untrusted Site ('Open Redirect')
T1204.001Malicious Link

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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