High [CVE-2026-70455] Denial of Service via Zstandard compression thread exhaustion
This high-severity Red Hat Linux advisory covers CVE-2026-70455 affecting Red Hat Enterprise Linux 10, Red Hat package: rsync.
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Summary
rsync 3.4.2 before 3.5.0 contains a denial of service vulnerability that allows a remote sender to exhaust system resources by specifying the --zt short alias for --compress-threads, which bypasses the refuse options directive's string matching on long option names.
Attackers can specify --zt=N with a large value to spawn an unbounded number of Zstandard worker threads on the receiver, exhausting available thread and memory resources. By sending requests using the --zt Zstandard compression alias, an attacker can bypass security directives and spawn an excessive number of worker threads.
RHEL 9 and older versions are unaffected. Red Hat severity: Important — CVSS 7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).
Weakness: CWE-770. Affected Red Hat products: Red Hat Enterprise Linux 10.
Red Hat fixing advisory: RHSA-2026:67463.
Affected products named by the advisory: Red Hat package: rsync.
- < 3.4.2
- < 3.5.0
Official advisory · high-confidence parse· fetched 9 days ago·verify at source
- 3.5.0
- rsync-0:3.5.0-3.el10_2
- RHSA-2026:67463
Official advisory · high-confidence parse· fetched 9 days ago·verify at source
Mitigation checklist
- If the rsync daemon is unused, immediately disable rsyncd.service to eliminate the attack surface. For active deployments, restrict network access (TCP 873) strictly to trusted hosts using firewall rules. Do not rely on refuse options = compress-threads for protection, as the --zt alias bypasses this configuration. Network isolation is the most effective defense.
Official advisory · high-confidence parse· fetched 9 days ago·verify at source
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