CVE-2021-29431 SSRF in Sydent due to missing validation of hostnames

What is CVE-2021-29431?

CVE-2021-29431 is a high-severity server-side request forgery 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. Sydent can be induced to send HTTP GET requests to internal systems, due to lack of parameter validation or IP address blacklisting. It is not possible to exfiltrate data or control request headers, but it might be possible to use the attack to perform an internal port enumeration. This issue has been addressed in in 9e57334, 8936925, 3d531ed, 0f00412. A potential workaround would be to use a firewall to ensure that Sydent cannot reach internal HTTP resources.

Weakness: CWE-918, CWE-20

Which versions are affected by CVE-2021-29431?

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

Products include Matrix Sydent.

How do I fix CVE-2021-29431?

Upgrade matrix-sydent to 2.3.0 or later.

Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.

Is CVE-2021-29431 being exploited?

There is no known exploitation of CVE-2021-29431 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-2021-29430 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
• CVE-2021-29432 Malicious users could control the content of invitation emails Medium 5.3

Sources

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

Also known as GHSA-9jhm-8m8c-c3f4, PYSEC-2021-22.