CVE-2021-27520: A cross-site scripting (XSS) issue in FUDForum 3.1.0 allows remote attackers to inject JavaScript via index.ph

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MEDIUM 6.1DOWNGRADE

CVE-2021-27520 is a medium-severity flaw (CVSS 6.1) affecting fudforum. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A cross-site scripting (XSS) issue in FUDForum 3.1.0 allows remote attackers to inject JavaScript via index.php in the "author" parameter.

Why this priority

EPSS top 6%: very likely to be exploited
Public exploit reference

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	6.1 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	6.4% (top 6%)
CISA exploited list	No
Weakness	CWE-79
Published	2021-03-19
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: fudforum

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-03-19 19:15 UTCPublished CVE record published
2026-06-17 03:45 UTCUpdated Record last modified

fudforum fudforum DOWNGRADE

Affected: 3.1.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 3.1.0

1. Check installed version

apt-cache policy fudforum || rpm -q fudforum || fudforum --version

2. No fixed release: DOWNGRADE to a release before 3.1.0 (Debian / Ubuntu)

apt-cache madison fudforum sudo apt install fudforum=<VERSION_BEFORE_3.1.0> sudo apt-mark hold fudforum

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list fudforum sudo dnf downgrade fudforum-<VERSION_BEFORE_3.1.0> sudo dnf versionlock add fudforum

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

Exploit-DB: FUDForum 3.1.0 - 'author' Reflected XSS
Nuclei scan template: FUDForum 3.1.0 - Cross-Site Scripting

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-27520?

fudforum: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-27520 being exploited?

CVE-2021-27520 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-27520 affect?

CVE-2021-27520 affects fudforum. Severity is medium (CVSS 6.1). A cross-site scripting (XSS) issue in FUDForum 3.1.0 allows remote attackers to inject JavaScript via index.php in the "author" parameter.

How urgent is CVE-2021-27520?

Priority 50/100. EPSS top 6%: very likely to be exploited; Public exploit reference

Software pages

PHP vulnerabilities

Related CVEs

CVE-2024-30951 MEDIUM 6.1 DOWNGRADE FUDforum v3.1.3 was discovered to contain a reflected cross-site scripting (XSS) vulnerability via the chpos p
CVE-2024-30950 LOW 3.5 DOWNGRADE A stored cross-site scripting (XSS) vulnerability in FUDforum v3.1.3 allows attackers to execute arbitrary web
CVE-2022-30863 MEDIUM 4.8 DOWNGRADE FUDForum 3.1.2 is vulnerable to Cross Site Scripting (XSS) via page_title param in Page Manager in the Admin C
CVE-2022-30861 MEDIUM 4.8 DOWNGRADE FUDforum 3.1.2 is vulnerable to Stored XSS via Forum Name field in Forum Manager Feature.
CVE-2022-30860 HIGH 7.2 UPGRADE FUDforum 3.1.2 is vulnerable to Remote Code Execution through Upload File feature of File Administration Syste

References

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