How severe is CVE-2021-28833?

CVE-2021-28833 has a CVSS score of 6.1/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-28833?

CVE-2021-28833 affects the following packages: qiita-markdown (RubyGems). Ecosystems affected: RubyGems.

How do I fix CVE-2021-28833?

Update qiita-markdown to 0.34.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-28833 is resolved across your whole dependency graph.

How do I detect CVE-2021-28833 in my RubyGems dependencies?

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

How do I mitigate CVE-2021-28833 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2021-28833 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-28833 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-2021-28833?

CVE-2021-28833 has an EPSS (Exploit Prediction Scoring System) score of 0.7%, placing it in the 53th 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-2021-28833?

CVE-2021-28833 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2021-28833 published, and has it been updated?

CVE-2021-28833 was published on June 21, 2021 and was last updated on July 9, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

💎

💎 RubyGems

Not in CISA KEV

MEDIUM severity

CVE-2021-28833 — qiita-markdown

MEDIUMFix: increments/qiita-markdown@b5d4e60

CVE-2021-28833 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in qiita-markdown. A fix is available for qiita-markdown — see the affected versions and patch details below.

qiita-markdown Cross-site Scripting vulnerability

Also known asGHSA-9p29-94hp-8rvc

Published

Jun 21, 2021

Updated

Jul 9, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.7%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs53th percentile — riskier than 53% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2021-28833 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

💎qiita-markdown

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.

Description

Increments Qiita::Markdown before 0.34.0 allows XSS via a crafted gist link, a different vulnerability than CVE-2021-28796.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
💎RubyGems	`qiita-markdown`	all versions	0.34.0`bundle update qiita-markdown --conservative`

Affected Products

1 product · 1 configurations

Application

qiita\increments

< 0.34.0

1 version

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for qiita-markdown, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update qiita-markdown to 0.34.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-28833 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

Increments Qiita::Markdown before 0.34.0 allows XSS via a crafted gist link, a different vulnerability than CVE-2021-28796.

O3 Security · Impact-Aware SCA

Is CVE-2021-28833 in your dependencies?

Find it across RubyGems, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

6.1/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

Required

Scope

Changed

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

CWE Classification

CWE-79Cross-site Scripting (XSS)

Weaponization

No known public exploit tooling found.

OpenSSF Scorecard

Package health score

qiita-markdown

7.0/10

Maintained10/10

Code-Review10/10

Branch-Protection5/10

Assessed Oct 5, 2026

Timeline

Disclosed

Jun 21, 2021

Last updated

Jul 9, 2026

Identifiers

CVE-2021-28833

GHSA-9p29-94hp-8rvc

References

Advisorygithub.com Advisoryvuln.ryotak.me Advisorynvd.nist.gov Webgithub.com Webgithub.com Packagegithub.com

Related vulnerabilities

Also affects qiita-markdown

This CVE is medium severity, in line with the other 1 CVE affecting qiita-markdown.

CVE-2021-28796

Same weakness: CWE-79

This CVE is medium severity; 7 of 7 other CVEs with CWE-79 are rated as high or higher (top: high).

CVE-2026-91127CVE-2026-77606CVE-2026-84992CVE-2026-77608CVE-2026-77607GHSA-9rcc-pmj8-ffhrCVE-2026-77616Browse all CWE-79

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedJul 9, 2026