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Synapse does not apply enough checks to servers requesting auth events of events in a room

High severity GitHub Reviewed Published May 24, 2023 in matrix-org/synapse • Updated Sep 24, 2024

Package

pip matrix-synapse (pip)

Affected versions

< 1.69.0

Patched versions

1.69.0

Description

Impact

Synapse is an open-source Matrix homeserver written and maintained by the Matrix.org Foundation. The Matrix Federation API allows remote homeservers to request the authorisation events of events in a room. This is necessary so that a homeserver receiving some events can validate that those events are legitimate and permitted in their room.
However, in versions of Synapse up to and including 1.68.0, a Synapse homeserver answering a query for authorisation events does not sufficiently check that the requesting server should be able to access them.

Authorisation events include power level events (the list of user IDs and their power levels at the time) and relevant membership events (including the display name of the sender of that event), as well as events like m.room.create, m.room.third_party_invite and m.room.join_rules. Non-authorisation events are unaffected, so it isn't possible to e.g. extract message contents this way.

This issue is only exploitable when a malicious actor knows the ID of a target room and the ID of an event from that room. In most cases, this makes exploitation infeasible. This issue is of negligible consequence for public rooms given that any server can easily join the room in order to be allowed to view authorisation events. Further, deployments in a closed federation where all homeservers are trustworthy are not affected.

Patches

The issue was patched in Synapse 1.69.0. Homeserver administrators are advised to upgrade.

Workarounds

Synapse can be configured with a list of servers that it is allowed to federate with federation_domain_whitelist. If this list is in use and all the servers on the list are trusted not to exploit this issue, then this issue is of no consequence.

This workaround is not practical for homeservers participating in open federation as interaction with any server not on the list would have to happen indirectly through servers that are, leading to inconsistent delays in message delivery.

References

Fixed in matrix-org/synapse#13823.

For more information

If you have any questions or comments about this advisory, e-mail us at [email protected].

References

@dkasak dkasak published to matrix-org/synapse May 24, 2023
Published to the GitHub Advisory Database May 24, 2023
Reviewed May 24, 2023
Published by the National Vulnerability Database May 26, 2023
Last updated Sep 24, 2024

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity High
Attack Requirements None
Privileges Required Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:H/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

EPSS score

0.113%
(46th percentile)

Weaknesses

CVE ID

CVE-2022-39335

GHSA ID

GHSA-45cj-f97f-ggwv

Source code

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