# CVE-2021-21382: Unsafe loopback forwarding interface in Restund

> Attackers can exploit unsafe loopback forwarding in Restund to access internal services and issue administrative commands via the status interface.

- **Severity:** High 8.6 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 11 Jun 2021
- **Updated:** 3 Aug 2024

## What is CVE-2021-21382?

CVE-2021-21382 is a high-severity server-side request forgery vulnerability in wireapp restund, published on 11 Jun 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-862 (Missing Authorization), CWE-668 (Exposure of Resource to Wrong Sphere)

## Which versions are affected by CVE-2021-21382?

It affects 1 product.


Products: Wire Restund.

## How do I fix CVE-2021-21382?

No fix has been published for CVE-2021-21382 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-21382 being exploited?

There is no known exploitation of CVE-2021-21382 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-21382 and fix it if it is affected.

What it is: CVE-2021-21382 is a high-severity server-side request forgery vulnerability in wireapp restund, published on 11 Jun 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-21382 in the wild and no public exploit code.

Affected product: wireapp restund. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-21382/ (Markdown: https://insights.sbomify.com/CVE-2021-21382/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-21382
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-21382
- OSV: https://osv.dev/vulnerability/CVE-2021-21382

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-21382/
Summaries are generated and may contain mistakes; the official description above is the record.