CVE-2021-29433 Denial of service (via resource exhaustion) due to improper input validation

What is CVE-2021-29433?

CVE-2021-29433 is a medium-severity denial of service vulnerability in matrix-org sydent, published on 15 Apr 2021. Any signed-in user can exploit it over the network, with no action from the victim.

Official description

Sydent is a reference Matrix identity server. In Sydent versions 2.2.0 and prior, sissing 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. A patch for the vulnerability is in version 2.3.0. No workarounds are known to exist.

Weakness: CWE-400, CWE-20

Which versions are affected by CVE-2021-29433?

3 open source packages ship an affected version, including matrix-sydent. It affects 1 product.

Products include Matrix Sydent.

How do I fix CVE-2021-29433?

Upgrade matrix-sydent to 2.3.0 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-2021-29433 being exploited?

There is no known exploitation of CVE-2021-29433 in the wild and no public exploit code.

Other vulnerabilities in matrix-org sydent

• CVE-2023-38686 Sydent does not verify email server certificates Critical 9.3
• CVE-2021-29431 SSRF in Sydent due to missing validation of hostnames High 7.7
• CVE-2021-29430 Denial of service attack via memory exhaustion High 7.5
• CVE-2019-11842 matrix-sydent and matrix-synapse Use Cryptographically Weak PRNG High 7.5
• CVE-2019-11340 Matrix Sydent mishandles emails Medium 5.9

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-pw4v-gr34-2553, PYSEC-2021-24.