CVE-2021-21394 Denial of service (via resource exhaustion) due to improper input validation on third-party identifier endpoints
Attackers can cause resource exhaustion and denial of service in Synapse homeservers via improper input validation on third-party identifier endpoints.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2021-21394?
CVE-2021-21394 is a medium-severity denial of service vulnerability in Fedoraproject Fedora, published on 12 Apr 2021. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.
Weakness: CWE-20
Which versions are affected by CVE-2021-21394?
3 open source packages ship an affected version, including matrix-synapse. It affects 2 products.
| Package | Affected | Fixed in |
|---|---|---|
matrix-synapse PyPI | All versions | 1.28.0 |
matrix-synapse PyPI | All versions | 1.28.0 |
matrix-synapse PyPI | All versions | 1.28.0 |
Products include Fedoraproject Fedora, Matrix Synapse.
How do I fix CVE-2021-21394?
Upgrade matrix-synapse to 1.28.0 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2021-21394 being exploited?
There is no known exploitation of CVE-2021-21394 in the wild and no public exploit code.
Other vulnerabilities in Fedoraproject Fedora
Sources
Also known as GHSA-w9fg-xffh-p362, PYSEC-2021-27, UBUNTU-CVE-2021-21394.