CVE-2025–59476: How a Log Formatter Flaw Let Attackers Forge Jenkins Audit Trails

Jenkins sits at the heart of countless CI/CD pipelines, which makes anything touching its logging layer worth paying attention to. CVE-2025–59476 is a log message injection vulnerability in Jenkins Core that let unvalidated user input distort log output, creating a path to falsified audit trails and misled monitoring systems.

What Is CVE-2025–59476?

The flaw sits in the Jenkins Core artifact (org.jenkins-ci.main:jenkins-core), specifically in the log formatter responsible for rendering user-supplied content into logs and console output. Because the formatter didn’t properly sanitize special characters, attackers could inject newline sequences and text crafted to mimic legitimate log entries. Affected surfaces include the core log formatter, console output, the CLI log console, and plugin-generated log streams.

How the Injection Works

When Jenkins logs data containing raw user input, that input often gets concatenated directly into the message, something like logger.info(“User input received: “ + input). If the input includes a newline followed by text styled like a log line, for example a fake “SEVERE” entry, the resulting output looks like a genuine new log record even though it’s part of the original message. That’s enough to fool anyone reviewing logs, and it can undermine automated monitoring or SIEM tooling that parses log lines individually.

Why It’s a Bigger Deal Than It Sounds

Forged log entries aren’t just cosmetic. They can mask unauthorized commands or deployments, mislead security audits, corrupt automated compliance trails, and generally erode trust in what should be a reliable forensic record. For organizations running Jenkins across regulated CI/CD environments, that’s a real problem: if the logs can lie, incident response and audit processes built on top of them inherit that unreliability.

The Fix

The issue was resolved with a patch introducing a transformMessage() function that processes every message before it reaches an output stream. It detects line breaks (\n, \r, \r\n), replaces them with explicit markers like [LF], [CR], and [CRLF], and adds continuation indicators with preserved indentation. The result: injected content stays visibly distinct from genuine log lines, so a forged “SEVERE” entry can no longer pass as real.

Mitigation Checklist

• Update Jenkins Core and all plugins to the patched version
• Sanitize any user-controlled input before it reaches a log statement
• Avoid raw string concatenation in logging calls
• Move toward structured log formats (JSON, for example) that resist this class of spoofing
• Add log integrity verification into CI/CD monitoring

The Takeaway

CVE-2025–59476 is a reminder that logging code doesn’t get a pass on input validation just because it isn’t a “user-facing” feature. As AI-driven observability and threat detection tools increasingly rely on parsing CI/CD logs automatically, log integrity becomes just as important as the security of the pipeline itself. Security starts with trustworthy records; logs included.

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