Restund
CVE-2021-21382
CRITICAL
Vulnerability Assessment
| Weakness |
CWE
OWASP Top 10:2025
A01:2025 - Broken Access Control
ASVS 5.0
V8 - Authorization Resource sphere isolation
MITRE ATT&CK
1 technique across 1 tactic: Collection The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up. |
| Impact | CVSS score of 9.6 (CRITICAL). Attack characteristics: network attack vector, low complexity, low privileges required, no user interaction not required. Public proof-of-concept exploit code is available. |
| Exploit ScenarioDerived | A remote attacker could access resources from an unintended security context by sending crafted requests to the affected application. |
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Alert meAnalysisAI
Restund is an open source NAT traversal server. Rated critical severity (CVSS 9.6), this vulnerability is remotely exploitable, low attack complexity. Public exploit code available and no vendor patch available.
RemediationAI
No vendor patch is available at time of analysis. Monitor vendor advisories for updates. Implement proper access controls, validate resource access permissions, use security boundaries.
Affects your stack
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Checking this CVE against your own stacks and servers is part of the Basic plan and up.
Technical ContextAI · show details
This vulnerability is classified as Exposure of Resource to Wrong Sphere (CWE-668), which allows attackers to access resources from an unintended security context. Restund is an open source NAT traversal server. The restund TURN server can be instructed to open a relay to the loopback address range. This allows you to reach any other service running on localhost which you might consider private. In the configuration that we ship (https://github.com/wireapp/ansible-restund/blob/master/templates/restund.conf.j2#L40-L43) the status interface of restund is enabled and is listening on 127.0.0.1.The status interface allows users to issue administrative commands to restund like listing open relays or draining connections. It would be possible for an attacker to contact the status interface and issue administrative commands by setting XOR-PEER-ADDRESS to 127.0.0.1:{{restund_udp_status_port}} when opening a TURN channel. We now explicitly disallow relaying to loopback addresses, 'any' addresses, link local addresses, and the broadcast address. As a workaround disable the status module in your restund configuration. However there might still be other services running on 127.0.0.0/8 that you do not want to have exposed. The turn module can be disabled. Restund will still perform STUN and this might already be enough for initiating calls in your environments. TURN is only used as a last resort when other NAT traversal options do not work. One should also make sure that the TURN server is set up with firewall rules so that it cannot relay to other addresses that you don't want the TURN server to relay to. For example other services in the same VPC where the TURN server is running. Ideally TURN servers should be deployed in an isolated fashion where they can only reach what they need to reach to perform their task of assisting NAT-traversal. Affected products include: Wire Restund.
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Severity by source
AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N
Primary rating from NVD · only source for this CVE.
CVSS VectorNVD
DescriptionNVD
Restund is an open source NAT traversal server. The restund TURN server can be instructed to open a relay to the loopback address range. This allows you to reach any other service running on localhost which you might consider private. In the configuration that we ship (https://github.com/wireapp/ansible-restund/blob/master/templates/restund.conf.j2#L40-L43) the status interface of restund is enabled and is listening on 127.0.0.1.The status interface allows users to issue administrative commands to restund like listing open relays or draining connections. It would be possible for an attacker to contact the status interface and issue administrative commands by setting XOR-PEER-ADDRESS to 127.0.0.1:{{restund_udp_status_port}} when opening a TURN channel. We now explicitly disallow relaying to loopback addresses, 'any' addresses, link local addresses, and the broadcast address. As a workaround disable the status module in your restund configuration. However there might still be other services running on 127.0.0.0/8 that you do not want to have exposed. The turn module can be disabled. Restund will still perform STUN and this might already be enough for initiating calls in your environments. TURN is only used as a last resort when other NAT traversal options do not work. One should also make sure that the TURN server is set up with firewall rules so that it cannot relay to other addresses that you don't want the TURN server to relay to. For example other services in the same VPC where the TURN server is running. Ideally TURN servers should be deployed in an isolated fashion where they can only reach what they need to reach to perform their task of assisting NAT-traversal.
Threat Intelligence Overview
NVD
CVE.org
EPSS
GitHub
How this decision was reached
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What every action level means
SSVC AssessmentENISA
Priority Score
Attack Technique
Information disclosure occurs when an application unintentionally exposes sensitive data that aids attackers in reconnaissance or directly compromises security.
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