Summary
User content sandbox can be confused into opening arbitrary documents
Workarounds
There are no known workarounds.
Impact
The user content sandbox can be abused to trick users into opening unexpected documents after several user interactions. The content can be opened with a blob origin from the Matrix client, so it is possible for a malicious document to access user messages and secrets.
CVE-2021-21320 has a CVSS score of 2.6 (Low). The vector is network-reachable, low privileges required, and user interaction required. 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 (3.15.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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This has been fixed by https://github.com/matrix-org/matrix-react-sdk/pull/5657, which is included in 3.15.0.
Frequently Asked Questions
- What is CVE-2021-21320? CVE-2021-21320 is a low-severity security vulnerability in matrix-react-sdk (npm), affecting versions < 3.15.0. It is fixed in 3.15.0.
- How severe is CVE-2021-21320? CVE-2021-21320 has a CVSS score of 2.6 (Low). 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.
- Which versions of matrix-react-sdk are affected by CVE-2021-21320? matrix-react-sdk (npm) versions < 3.15.0 is affected.
- Is there a fix for CVE-2021-21320? Yes. CVE-2021-21320 is fixed in 3.15.0. Upgrade to this version or later.
- Is CVE-2021-21320 exploitable, and should I be worried? Whether CVE-2021-21320 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
- What actually determines whether CVE-2021-21320 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.
- How do I fix CVE-2021-21320? Upgrade
matrix-react-sdkto 3.15.0 or later.