# CVE-2026-43617: Rsync authorization bypass in hostname-based ACLs

Source: https://junglewise.ai/threats/cve-2026-43617-rsync-authorization-bypass-in-hostname-based-acls  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-05-20. Updated: 2026-05-20  
License: CC BY 4.0 with a link back

CVE-2026-43617 · Severity: medium · CVSS 4.8 · Published 2026-05-20

Rsync is a widely used utility for synchronizing files between systems. A security flaw in the rsync server (daemon) allows unauthorized users to bypass access restrictions that are based on hostnames. This could allow an attacker to connect to a file share they were intended to be blocked from, potentially leading to unauthorized data access or modification.

Technologies: [RsyncProject Rsync](https://junglewise.ai/threats/technologies/rsync). Vendors: [RsyncProject](https://junglewise.ai/threats/vendors/rsyncproject).

## Quick answers

**Is CVE-2026-43617 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-43617 being exploited in the wild.

**How severe is CVE-2026-43617?** Medium severity, with a CVSS score of 4.8, published 20 May 2026.

**Which products and versions are affected?** RsyncProject Rsync < 3.4.3.

**Is there a fix for CVE-2026-43617?** A patch was published on 2026-05-20: Fixed in version 3.4.3 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-43617 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-43617?** Check whether you run RsyncProject Rsync at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

An authorization bypass exists in rsync versions prior to 3.4.3 when the rsync daemon is configured to use a chroot. The vulnerability occurs because reverse DNS lookups for connecting clients are performed after the process has entered the chroot environment. If the chroot lacks necessary system configuration files (such as /etc/resolv.conf or NSS modules), the lookup fails and defaults the hostname to 'UNKNOWN'. An attacker who controls the PTR record for their source IP can exploit this failure to bypass 'hosts deny' rules that target specific hostnames. This allows connections from hosts that administrators intended to block, though IP-based ACLs remain effective.

## Affected products

- RsyncProject rsync < 3.4.3

## Timeline

- 2026-05-19: disclosed
- 2026-05-20: patched: Fixed in version 3.4.3
- 2026-05-20: advisory

## References

- [https://github.com/RsyncProject/rsync/releases/tag/v3.4.3](https://github.com/RsyncProject/rsync/releases/tag/v3.4.3)
- [https://github.com/RsyncProject/rsync/security/advisories/GHSA-rjfm-3w2m-jf4f](https://github.com/RsyncProject/rsync/security/advisories/GHSA-rjfm-3w2m-jf4f)
- [https://www.vulncheck.com/advisories/rsync-authorization-bypass-via-hostname-resolution](https://www.vulncheck.com/advisories/rsync-authorization-bypass-via-hostname-resolution)

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