CVE-2026-84501: apache software foundation apache zookeeper Improper Output Neutralization for Logs

Vulnerability summary

CVE-2026-84501 is a medium (CVSS 5.3) Improper Output Neutralization for Logs vulnerability in apache software foundation apache zookeeper. EPSS estimates a 0.74% chance of exploitation in the next 30 days, higher than 53.7% of scored vulnerabilities.

Timeline

1. 16-09-2026
   PublishedVulnerability first disclosed.
2. 16-09-2026
   Last modifiedVulnerability information updated.

Description

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.

• CWE-117Improper Output Neutralization for LogsThe product constructs a log message from external input, but it does not neutralize or incorrectly neutralizes special elements when the message is written to a log file.