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CVE-2020-26891

Cross-site scripting (XSS) vulnerability in the fallback authentication endpoint

6.1 MediumFix available

AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth//fallback/web or /_matrix/client/unstable/auth//fallback/web Synapse endpoints.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
Required

What an attacker can affect

Confidentiality
Low
Integrity
Low
Availability
None

Exploit likelihood

1.92%Up 0.5 points since Apr 14, 2021
90%50%10%1%

What each source reports

NVD Score used 6.1

AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth//fallback/web or /_matrix/client/unstable/auth//fallback/web Synapse endpoints.

Record
CVE-2020-26891
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
CVSS 2.0
4.3 MediumAV:N/AC:M/Au:N/C:N/I:P/A:N
Weakness
CWE-79
Updated
2026-11-02
CVE List No score

AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth//fallback/web or /_matrix/client/unstable/auth//fallback/web Synapse endpoints.

Record
CVE-2020-26891
Updated
2026-11-02
EUVDEUVD-2020-0108 No score

AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth//fallback/web or /_matrix/client/unstable/auth//fallback/web Synapse endpoints.

Record
EUVD-2020-0108
Updated
2026-11-03
GitHub AdvisoriesGHSA-3x8c-fmpc-5rmq 5.1

Cross-site scripting (XSS) vulnerability in the fallback authentication endpoint

Impact

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.

Patches

This is fixed by #8444, which is included in Synapse v1.21.0.

Workarounds

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
Record
GHSA-3x8c-fmpc-5rmq
CVSS 4.0
5.1 MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Weakness
CWE-79
Updated
2026-11-02
OSVALPINE-CVE-2020-26891 6.1

AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth//fallback/web or /_matrix/client/unstable/auth//fallback/web Synapse endpoints.

Record
ALPINE-CVE-2020-26891
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Updated
2026-11-03
OSV 6.1

AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth//fallback/web or /_matrix/client/unstable/auth//fallback/web Synapse endpoints.

Record
CVE-2020-26891
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Updated
2026-11-02
Show 5 moreShow less
OSVDEBIAN-CVE-2020-26891 6.1

AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth//fallback/web or /_matrix/client/unstable/auth//fallback/web Synapse endpoints.

Record
DEBIAN-CVE-2020-26891
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Updated
2026-11-02
OSVGHSA-3x8c-fmpc-5rmq 5.1

Cross-site scripting (XSS) vulnerability in the fallback authentication endpoint

Impact

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.

Patches

This is fixed by #8444, which is included in Synapse v1.21.0.

Workarounds

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
Record
GHSA-3x8c-fmpc-5rmq
CVSS 4.0
5.1 MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Weakness
CWE-79
Updated
2026-11-02
OSVUBUNTU-CVE-2020-26891 6.1

AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth//fallback/web or /_matrix/client/unstable/auth//fallback/web Synapse endpoints.

Record
UBUNTU-CVE-2020-26891
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Updated
2026-11-02
OSVopenSUSE-SU-2024:11041-1 No score

matrix-synapse-1.43.0-1.1 on GA media

These are all security issues fixed in the matrix-synapse-1.43.0-1.1 package on the GA media of openSUSE Tumbleweed.

Record
openSUSE-SU-2024:11041-1
Updated
2026-11-02
OSVPYSEC-2020-238 No score

AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth//fallback/web or /_matrix/client/unstable/auth//fallback/web Synapse endpoints.

Record
PYSEC-2020-238
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
matrix-synapse
PyPI
 before 1.21.0
1.21.0
synapse
Alpine:v3.13
 before 1.21.1-r0
1.21.1-r0
synapse
Alpine:v3.14
 before 1.21.1-r0
1.21.1-r0
synapse
Alpine:v3.15
 before 1.21.1-r0
1.21.1-r0
synapse
Alpine:v3.16
 before 1.21.1-r0
1.21.1-r0
synapse
Alpine:v3.17
 before 1.21.1-r0
1.21.1-r0
synapse
Alpine:v3.18
 before 1.21.1-r0
1.21.1-r0
synapse
Alpine:v3.19
 before 1.21.1-r0
1.21.1-r0
synapse
Alpine:v3.20
 before 1.21.1-r0
1.21.1-r0
synapse
Alpine:v3.21
 before 1.21.1-r0
1.21.1-r0
Show 8 moreShow less
Affected packages
PackageAffectedFixed
synapse
Alpine:v3.22
 before 1.21.1-r0
1.21.1-r0
synapse
Alpine:v3.23
 before 1.21.1-r0
1.21.1-r0
synapse
Alpine:v3.24
 before 1.21.1-r0
1.21.1-r0
matrix-synapse
PyPI
 before 1.21.0, plus listed versions
0.33.50.33.5.10.33.60.33.6rc10.33.7
Show 45 moreShow less
0.33.7rc10.33.7rc20.33.80.33.8rc20.33.90.34.00.34.0.10.34.0rc10.34.0rc20.34.1.10.99.00.99.0rc10.99.0rc20.99.0rc30.99.0rc40.99.10.99.1.10.99.1rc10.99.1rc20.99.20.99.2rc10.99.30.99.3.10.99.3.20.99.3rc10.99.40.99.4rc10.99.50.99.5.10.99.5.20.99.5rc11.0.01.0.0rc11.0.0rc21.0.0rc31.1.01.1.0rc11.1.0rc21.10.01.10.0rc11.10.0rc21.10.0rc31.10.0rc51.10.11.11.0
1.21.0
matrix-synapse
Ubuntu:Pro:18.04:LTS
 All versions, plus listed versions
0.19.2+dfsg-60.24.0+dfsg-10.24.0+dfsg-1ubuntu0.1~esm10.24.0+dfsg-1ubuntu0.1~esm4
 None known
matrix-synapse
Ubuntu:Pro:20.04:LTS
 All versions, plus listed versions
1.3.0-11.4.0-11.5.0-11.5.1-11.6.0-1
Show 12 moreShow less
1.6.1-11.7.0-21.7.1-11.7.2-11.7.3-11.8.0-11.9.0-11.9.1-11.10.0-11.10.0-21.11.0-11.11.0-1ubuntu0.1~esm2
 None known
matrix-synapse
Ubuntu:Pro:22.04:LTS
 All versions, plus listed versions
1.39.0-11.47.0-21.47.1-11.48.0-11.49.0-1
Show 7 moreShow less
1.49.2-11.50.1-11.50.2-11.51.0-11.52.0-11.53.0-11.53.0-1ubuntu0.1~esm2
 None known
matrix-synapse
openSUSE:Tumbleweed
From an advisory covering 11 CVEs
 before 1.43.0-1.1
1.43.0-1.1

History

History
RecordedChangedSources
affected, facts
affected, aliases, referencesOSV
affected
aliasesEUVD
first indexedCVE List, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2020-26891 is a medium vulnerability (CVSS 6.1) in Matrix Synapse, published October 16, 2020. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 1.92%, the 80th percentile. Fixed in matrix-synapse 1.21.0.

References

Show 18 moreShow less
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Left out here: 79 versions. The JSON record has them.

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