Summary
Open redirects on some federation and push requests
Workarounds
Requests to internal IP addresses could be blocked at the system or network level.
Impact
Requests to user provided domains were not restricted to external IP addresses when calculating the key validity for third-party invite events and sending push notifications. This could cause Synapse to make requests to internal infrastructure. The type of request was not controlled by the user, although limited modification of request bodies was possible.
For the most thorough protection server administrators should remove the deprecated federation_ip_range_blacklist from their settings after upgrading to Synapse v1.25.0 which will result in Synapse using the improved default IP address restrictions. See the new ip_range_blacklist and ip_range_whitelist settings if more specific control is necessary.
Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites. Typical impact: phishing and credential harvesting via a trusted domain.
CVE-2021-21273 has a CVSS score of 3.1 (Medium). The vector is network-reachable, no 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 (1.25.0); 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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Issue is resolved by #8821. Further improvements to protect homeservers by default were made in #8870 and #8954.
Frequently Asked Questions
- What is CVE-2021-21273? CVE-2021-21273 is a medium-severity open redirect vulnerability in matrix-synapse (pip), affecting versions < 1.25.0. It is fixed in 1.25.0. Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites.
- How severe is CVE-2021-21273? CVE-2021-21273 has a CVSS score of 3.1 (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.
- Which versions of matrix-synapse are affected by CVE-2021-21273? matrix-synapse (pip) versions < 1.25.0 is affected.
- Is there a fix for CVE-2021-21273? Yes. CVE-2021-21273 is fixed in 1.25.0. Upgrade to this version or later.
- Is CVE-2021-21273 exploitable, and should I be worried? Whether CVE-2021-21273 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-2021-21273 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-2021-21273? Upgrade
matrix-synapseto 1.25.0 or later.