Medium [CVE-2026-84501] unauthenticated attacker can inject arbitrary fake log lines into Apache ZooKeeper's operational log by sending a crafted add_auth("ensemble",...) request containing newline characters (\n)
This medium-severity Apache Software Foundation advisory covers CVE-2026-84501 affecting Apache ZooKeeper.
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Summary
An unauthenticated attacker can inject arbitrary fake log lines into Apache ZooKeeper's operational log by sending a crafted add_auth("ensemble",...) request containing newline characters (\n). When the ensemble name doesn't match, EnsembleAuthenticationProvider.handleAuthentication() logs the raw, unsanitized name via LOG.warn().
Because SLF4J's {} placeholder preserves embedded newlines, the attacker can forge complete log entries — with arbitrary timestamps, log levels, class names, and messages — that are visually indistinguishable from genuine ZooKeeper log output.
This issue affects Apache ZooKeeper: from 3.9.0 through 3.9.5, from 3.8.0 through 3.8.6.
Users are recommended to upgrade to version 3.8.7 or 3.9.6, which fixes the issue.
- 3.9.0 through 3.9.5
- 3.8.0 through 3.8.6.
- through 3.8.6
Official advisory · high-confidence parse· fetched 20 days ago·verify at source
- 3.8.7
- 3.9.6
Official advisory · high-confidence parse· fetched 20 days ago·verify at source
Mitigation checklist
- Users are recommended to upgrade to version 3.8.7 or 3.9.6, which fixes the issue.
Official advisory · high-confidence parse· fetched 20 days ago·verify at source
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