CVE-2021-21303

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.

CVE-2021-21303 has a CVSS score of 6.5 (Medium). The vector is requires local access, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.5.2); upgrading removes the vulnerable code path.

Kodem intelligence

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.

1. What is CVE-2021-21303? CVE-2021-21303 is a medium-severity security vulnerability in helm.sh/helm/v3 (go), affecting versions >= 3.0.0, < 3.5.2. It is fixed in 3.5.2.
2. How severe is CVE-2021-21303? CVE-2021-21303 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of helm.sh/helm/v3 are affected by CVE-2021-21303? helm.sh/helm/v3 (go) versions >= 3.0.0, < 3.5.2 is affected.
4. Is there a fix for CVE-2021-21303? Yes. CVE-2021-21303 is fixed in 3.5.2. Upgrade to this version or later.
5. Is CVE-2021-21303 exploitable, and should I be worried? Whether CVE-2021-21303 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-21303 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-21303? Upgrade helm.sh/helm/v3 to 3.5.2 or later.