CVE-2021-21303 is a medium-severity (CVSS 6.8) CWE-74 vulnerability in helm.sh/helm/v3. A fix is available for helm.sh/helm/v3 — see the affected versions and patch details below.
Improper Neutralization of Special Elements in Output in helm.sh/helm/v3
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-21303 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
helm.sh/helm/v3Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Since Helm 2 was released, a well-documented aspect of Helm is that the Helm chart's version number MUST follow the SemVer2 specification. In the past, Helm would not permit charts with malformed versions. At some point, a patch was merged that changed this - On a version parse error, the version number was simply passed along as-is. This provided a vector for malicious data to be injected into Helm and potentially used in various ways.
Core maintainers were able to send deceptive information to a terminal screen running the helm command, as well as obscure or alter information on the screen. In some cases, we could send codes that terminals used to execute higher-order logic, like clearing a terminal screen.
Further, during evaluation, the Helm maintainers discovered a few other fields that were not properly sanitized when read out of repository index files. This fix remedies all such cases, and once again enforces SemVer2 policies on version fields.
All users of the Helm 3 should upgrade.
Those who use Helm as a library should verify that they either sanitize this data on their own, or use the proper Helm API calls to sanitize the data.
Patches
This issue has been resolved in Helm 3.5.2.
While this fix does not constitute a breaking change, as all field formatting is now enforced as documented, it is possible that charts that were mistakenly allowed (but invalid) may no longer be available in search indexes. Specifically, malformed SemVer versions are no longer supported. This has always been the documented case, but it is true that malformed versions were allowed.
Note that this is the first security release since Helm 2's final deprecation. Helm 2 was not audited for vulnerability to this issue, and should be assumed vulnerable.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | helm.sh/helm/v3 | ≥ 3.0.0&&< 3.5.2 | 3.5.2go get helm.sh/helm/v3@v3.5.2 |
Affected Products
helmhelmDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for helm.sh/helm/v3, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update helm.sh/helm/v3 to 3.5.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21303 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
Is CVE-2021-21303 in your dependencies?
Find it across Go, including transitive dependencies.