CVE-2026-53791: rsync < 3.5.0 Daemon IP Spoofing via PROXY Protocol Header

criticalCVSS 9.1CVE-2026-53791

CSIRTS triage

vendor: rsync product: rsync Authentication bypassaffected: <3.5.0

What: rsync daemon can be spoofed to accept connections from arbitrary IP addresses via a crafted PROXY protocol header.
Who is affected: rsync daemon instances before version 3.5.0 are affected.
Urgency: Critical priority; IP spoofing bypasses access controls and enables unauthorized connections.
Action: Upgrade to rsync 3.5.0 or later immediately.

AI-assisted analysis generated from the source advisory — verify against the original.

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Details

Source: Microsoft Security Response Center (INTL · vendor-psirt · site)
Severity: critical — CVSS 9.1
Published: 2026-08-11
Exploitation: Not in CISA KEV at last sync

Original advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-53791

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-537910.47% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 39% of all EPSS-scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-53791	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.

Recent advisories for rsync <

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

high[NEW] [high] Rsync: Multiple vulnerabilitiescert-bund · 2026-08-21
high[UPDATE] [high] Rsync: Multiple vulnerabilitiescert-bund · 2026-08-19
medium[UPDATE] [medium] Rsync: Vulnerability allows bypassing security measurescert-bund · 2026-08-18
highCVE-2026-70464: rsync daemon 2.0.0 before 3.5.0 contains a denial of service vulnerability that allows unauthe…nvd · 2026-08-13
highCVE-2026-70463: rsync 3.1.0 before 3.5.0 contains an authorization bypass in auth users directive parsing. The…nvd · 2026-08-13
mediumCVE-2026-70462: rsync 3.1.0 before 3.5.0 contains a signed integer overflow vulnerability in the I/O timeout i…nvd · 2026-08-13