How severe is CVE-2021-29431?

CVE-2021-29431 has a CVSS 3.x base score of 7.7, rated high severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability none.

Is CVE-2021-29431 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (64th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-29431?

CVE-2021-29431 affects Matrix Sydent. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-29431?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2021-29431 published?

CVE-2021-29431 was published on 2021-04-15 and last updated on 2026-06-17.

CVE-2021-29431

CVE-2021-29431 is a high-severity vulnerability in Matrix Sydent with a CVSS 3.x base score of 7.7. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-20.

Key facts

Severity: High (CVSS 3.x base score 7.7)
CVSS v2: 4.0
EPSS exploit prediction: 1% (64th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-20
Affected product: Matrix Sydent
Published: 2021-04-15
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2021-29431?: 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.
How severe is CVE-2021-29431?: CVE-2021-29431 has a CVSS 3.x base score of 7.7, rated high severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability none.
Is CVE-2021-29431 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (64th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-29431?: CVE-2021-29431 affects Matrix Sydent. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-29431?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2021-29431 published?: CVE-2021-29431 was published on 2021-04-15 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:matrix:sydent:*:*:*:*:*:*:*:*

More vulnerabilities in Matrix Sydent

CVE-2023-38686 — Critical (CVSS 9.3): Sydent is an identity server for the Matrix communications protocol. Prior to version 2.5.6, if configured to send…
CVE-2021-29430 — High (CVSS 7.5): Sydent is a reference Matrix identity server. Sydent does not limit the size of requests it receives from HTTP clients.…
CVE-2019-11842 — High (CVSS 7.5): An issue was discovered in Matrix Sydent before 1.0.3 and Synapse before 0.99.3.1. Random number generation is…
CVE-2019-11340 — Medium (CVSS 5.9): util/emailutils.py in Matrix Sydent before 1.0.2 mishandles registration restrictions that are based on e-mail domain,…
CVE-2021-29432 — Medium (CVSS 5.3): Sydent is a reference matrix identity server. A malicious user could abuse Sydent to send out arbitrary emails from the…
CVE-2021-29433 — Medium (CVSS 4.3): Sydent is a reference Matrix identity server. In Sydent versions 2.2.0 and prior, sissing input validation of some…

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