CVE-2021-28121: Virtual Robots.txt before 1.10 does not block HTML tags in the robots.txt field.

Q: How do I fix CVE-2021-28121?

virtual robots.txt: upgrade to 1.10. Exact commands for each OS are on this page.

Q: Is CVE-2021-28121 being exploited?

CVE-2021-28121 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-28121 affect?

CVE-2021-28121 affects virtual robots.txt. Severity is critical (CVSS 9.8). Virtual Robots.txt before 1.10 does not block HTML tags in the robots.txt field.

Q: How urgent is CVE-2021-28121?

Priority 58/100. Critical severity; Remotely exploitable, no login, no user action

This month

CRITICAL 9.8REMOTEUPGRADE

CVE-2021-28121 is a critical-severity flaw (CVSS 9.8) affecting virtual robots.txt. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Virtual Robots.txt before 1.10 does not block HTML tags in the robots.txt field.

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.7% (top 23%)
CISA exploited list	No
Published	2021-08-12
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: virtual robots.txt project

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

Am I affected?

Timeline

2021-08-12 22:15 UTCPublished CVE record published
2026-06-17 03:45 UTCUpdated Record last modified

virtual robots.txt project virtual robots.txt UPGRADE

Affected: < 1.10

Fixed in: 1.10

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy virtual-robots.txt || rpm -q virtual-robots.txt || virtual-robots.txt --version

2. UPGRADE to 1.10 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade virtual-robots.txt

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade virtual-robots.txt

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

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-2021-28121?

virtual robots.txt: upgrade to 1.10. Exact commands for each OS are on this page.

Is CVE-2021-28121 being exploited?

CVE-2021-28121 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-2021-28121 affect?

CVE-2021-28121 affects virtual robots.txt. Severity is critical (CVSS 9.8). Virtual Robots.txt before 1.10 does not block HTML tags in the robots.txt field.

How urgent is CVE-2021-28121?

Priority 58/100. Critical severity; Remotely exploitable, no login, no user action

References

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