CVE-2021-3901Public exploitNetwork attack vector

Service interruption via unprotected bill rescan

Published Oct 27, 2021 · Updated Aug 3, 2024

Cross-site request forgery in Firefly III 5.6.2 and earlier allows remote attackers to trigger a bill rescan through an authenticated user. The bills.rescan route accepts a state-changing GET request, and its bill-page link supplies neither a POST boundary nor a CSRF token. Exploitation requires an authenticated user to submit the crafted cross-origin request and interrupts availability by forcing bill-matching work.

CVSS severity3.5
Low
EPSS probability0.55%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Cross-site request forgery in Firefly III 5.6.2 and earlier allows remote attackers to trigger a bill rescan through an authenticated user. The bills.rescan route accepts a state-changing GET request, and its bill-page link supplies neither a POST boundary nor a CSRF token. Exploitation requires an authenticated user to submit the crafted cross-origin request and interrupts availability by forcing bill-matching work.

The record

CVE
CVE-2021-3901
Published
Oct 27, 2021
Updated
Aug 3, 2024
Vendor
Firefly III
Product
Firefly III
Classifications
CWE-352, T1204.001
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Oct 27, 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

Compare your product and version with the public record. A matching version still requires validation against your environment.

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

8.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
6.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:P
3.5huntr.devCVSS:3.0/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:N/A:L

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 exploitPublished

Weakness, pattern, technique

CWE-352Cross-Site Request Forgery (CSRF)
T1204.001Malicious Link

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo