How severe is CVE-2021-21265?

CVE-2021-21265 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-21265?

CVE-2021-21265 affects the following packages: october/backend (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2021-21265?

Update october/backend to 1.1.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21265 is resolved across your whole dependency graph.

How do I detect CVE-2021-21265 in my Packagist dependencies?

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

How do I mitigate CVE-2021-21265 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2021-21265 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21265 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-21265?

CVE-2021-21265 has an EPSS (Exploit Prediction Scoring System) score of 1.7%, placing it in the 78th 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-2021-21265?

CVE-2021-21265 is classified as CWE-644 (CWE-644). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21265 published, and has it been updated?

CVE-2021-21265 was published on March 10, 2021 and was last updated on October 24, 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

HIGH severity

CVE-2021-21265 — backend

HIGHFix: octobercms/library@f29865a

CVE-2021-21265 is a high-severity (CVSS 7.5) CWE-644 vulnerability in october/backend. A fix is available for october/backend — see the affected versions and patch details below.

October CMS vulnerable to Potential Host Header Poisoning on misconfigured servers

Also known asGHSA-xhfx-hgmf-v6vp

Published

Mar 10, 2021

Updated

Oct 24, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 5, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.7%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs78th percentile — riskier than 78% of all scored CVEsHighest risk

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

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected

🐘october/backend

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

Impact

When running on servers that are configured to accept a wildcard as a hostname (i.e. the server routes any request, regardless of the HOST header to an October CMS instance) the potential exists for Host Header Poisoning attacks to succeed. See the following resources for more information on Host Header Poisoning:

Patches

A feature has been added in v1.1.2 to allow a set of trusted hosts to be specified in the application.

Workarounds

Apply https://github.com/octobercms/library/commit/f86fcbcd066d6f8b939e8fe897409d152b11c3c6 & https://github.com/octobercms/october/commit/f638d3f78cfe91d7f6658820f9d5e424306a3db0 to your installation manually if unable to upgrade to v1.1.2.
Check that the configuration setting cms.linkPolicy is set to force.

Alternative Workaround

Check to make sure that your web server does not accept any hostname when serving your web application.

Add an entry called testing.tld to your computer's host file and direct it to your server's IP address
Open the address testing.tld in your web browser
Make sure an October CMS website is not available at this address

If an October CMS website is returned, configure your webserver to only allow known hostnames. If you require assistance with this, please contact your server administrator.

References

Reported by Abdullah Hussam

For More Information

If you have any questions or comments about this advisory:

Email us at [email protected]

Threat Assessment

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`october/backend`	all versions	1.1.2`composer require october/backend:^1.1.2`

Affected Products

1 product · 1 configurations

Application

octoberoctobercms

< 1.1.2

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for october/backend, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update october/backend to 1.1.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21265 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### Impact When running on servers that are configured to accept a wildcard as a hostname (i.e. the server routes any request, regardless of the HOST header to an October CMS instance) the potential exists for Host Header Poisoning attacks to succeed. See the following resources for more information on Host Header Poisoning: - https://portswigger.net/web-security/host-header - https://dzone.com/articles/what-is-a-host-header-attack ### Patches A feature has been added in v1.1.2 to allow a set of trusted hosts to be specified in the application. ### Workarounds - Apply https://github.com/oc