Summary
Denial of service attack via incorrect parameters in Matrix Synapse
Workarounds
Homeserver administrators could limit access to the federation API to trusted servers (for example via federation_domain_whitelist).
Impact
A malicious or poorly-implemented homeserver can inject malformed events into a room by specifying a different room id in the path of a /send_join, /send_leave, /invite or /exchange_third_party_invite request.
This can lead to a denial of service in which future events will not be correctly sent to other servers over federation.
This affects any server which accepts federation requests from untrusted servers.
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
CVE-2020-26257 has a CVSS score of 6.5 (High). The vector is network-reachable, 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 (1.23.1); 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.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
Issue is resolved by https://github.com/matrix-org/synapse/pull/8776.
Frequently Asked Questions
- What is CVE-2020-26257? CVE-2020-26257 is a high-severity cross-site scripting (XSS) vulnerability in matrix-synapse (pip), affecting versions < 1.23.1. It is fixed in 1.23.1. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- How severe is CVE-2020-26257? CVE-2020-26257 has a CVSS score of 6.5 (High). 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-2020-26257? matrix-synapse (pip) versions < 1.23.1 is affected.
- Is there a fix for CVE-2020-26257? Yes. CVE-2020-26257 is fixed in 1.23.1. Upgrade to this version or later.
- Is CVE-2020-26257 exploitable, and should I be worried? Whether CVE-2020-26257 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-2020-26257 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-2020-26257? Upgrade
matrix-synapseto 1.23.1 or later.