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Fencer Vulns
high 8.6

CVE-2021-21382 json

Unsafe loopback forwarding interface in Restund

published 2021-06-11 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-8688

cvss
8.6
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
1.5%
EPSS 1.5% chance of exploitation in 30 days, percentile 73%
public exploits
0
none known

Summary

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.

Patch / fix

  • wire restund 0.4.15

Exploit prediction

1.47% chance of exploitation in the next 30 days, higher than 73% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/wireapp/restund/pull/7 referenced PoC, tagged "exploit" by a source, unverified
  • github.com https://github.com/wireapp/restund/security/advisories/GHSA-96j5-w9jq-pv2x referenced PoC, tagged "exploit" by a source, unverified
  • github.com https://github.com/wireapp/ansible-restund/blob/master/templates/restund.conf.j2#L40-L43 referenced PoC, tagged "exploit" by a source, unverified
  • talosintelligence.com https://talosintelligence.com/vulnerability_reports/TALOS-2018-0732 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 8.6 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N
NVD 3.1 9.6 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N
NVD 2.0 5.5 AV:N/AC:L/Au:S/C:P/I:P/A:N

Affected products

  • wireapp restund

Classification

Weakness
  • CWE-668 Exposure of Resource to Wrong Sphere
  • CWE-862 Missing Authorization

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.