CVSS2
Attack Vector
NETWORK
Attack Complexity
MEDIUM
Authentication
NONE
Confidentiality Impact
NONE
Integrity Impact
PARTIAL
Availability Impact
NONE
AV:N/AC:M/Au:N/C:N/I:P/A:N
CVSS3
Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
NONE
User Interaction
REQUIRED
Scope
CHANGED
Confidentiality Impact
LOW
Integrity Impact
LOW
Availability Impact
NONE
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
AI Score
Confidence
High
EPSS
Percentile
53.0%
The fallback authentication endpoint served via Synapse was vulnerable to cross-site scripting (XSS) attacks. The impact depends on the configuration of the domain that Synapse is deployed on, but may allow access to cookies and other browser data, CSRF vulnerabilities, and access to other resources served on the same domain or parent domains.
This is fixed by #8444, which is included in Synapse v1.21.0.
If the homeserver is not configured to use reCAPTCHA, consent (terms of service), or single sign-on then the affected endpoint can be blocked at a reverse proxy:
/_matrix/client/r0/auth/.*/fallback/web
/_matrix/client/unstable/auth/.*/fallback/web
github.com/advisories/GHSA-3x8c-fmpc-5rmq
github.com/matrix-org/synapse/pull/8444
github.com/matrix-org/synapse/releases
github.com/matrix-org/synapse/releases/tag/v1.21.2
github.com/matrix-org/synapse/security/advisories/GHSA-3x8c-fmpc-5rmq
github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2020-238.yaml
matrix.org/blog/2020/10/15/synapse-1-21-2-released-and-security-advisory
nvd.nist.gov/vuln/detail/CVE-2020-26891
CVSS2
Attack Vector
NETWORK
Attack Complexity
MEDIUM
Authentication
NONE
Confidentiality Impact
NONE
Integrity Impact
PARTIAL
Availability Impact
NONE
AV:N/AC:M/Au:N/C:N/I:P/A:N
CVSS3
Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
NONE
User Interaction
REQUIRED
Scope
CHANGED
Confidentiality Impact
LOW
Integrity Impact
LOW
Availability Impact
NONE
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
AI Score
Confidence
High
EPSS
Percentile
53.0%