Why Logback 1.2.7 Is Vulnerable and How to Safely Upgrade
This article explains the Logback CVE‑2021‑42550 vulnerability affecting versions before 1.2.9, outlines the three conditions required for exploitation, compares its severity to Log4j, and provides concrete steps—including upgrading to 1.2.9 and setting the configuration file read‑only—to protect Java applications.
Background
Following the high‑profile Log4j vulnerability, many developers assumed switching to Logback would avoid similar risks, only to discover a new Logback flaw that also permits remote code execution.
Vulnerability Details
Logback versions lower than 1.2.9 are affected by CVE‑2021‑42550 . An attacker who can modify logback.xml, runs a vulnerable Logback version, and either restarts the application or sets scan=true can inject malicious LDAP configuration and execute arbitrary code.
Permission to edit logback.xml Logback version < 1.2.9
Application restart or scan=true before the attack
The official announcement rates the vulnerability as MEDIUM , explicitly stating it is less severe than the Log4j issue (CVE‑2021‑44228).
Mitigation
Upgrade Logback to version 1.2.9 or later and configure the Logback file as read‑only. For Spring Boot projects, versions prior to 2.5.8 and 2.6.2 still depend on vulnerable Logback. Update the logback.version property in pom.xml as follows:
<properties>
<java.version>1.8</java.version>
<logback.version>1.2.9</logback.version>
</properties>Conclusion
Although the Logback issue is rated only medium severity, it underscores the necessity of continuously monitoring framework release notes for security patches. Regularly checking official changelogs and reviewing code differences between versions helps developers quickly identify and remediate vulnerabilities.
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