CVE-2026-15603

morgan vulnerable to Log Forging via unescaped Unicode line separators

Workarounds

None. The affected values include the request URL and request headers, which are core to access logging, so there is no practical format-based mitigation. Upgrade to 1.12.0.

Morgan writes attacker-controlled request data to the access log through its tokens. The 1.11.0 fix neutralizes C0 control characters, DEL, and backslash, but it does not escape the Unicode line separators U+0085 (NEL), U+2028 (LINE SEPARATOR), or U+2029 (PARAGRAPH SEPARATOR). These code points are reachable through several tokens, including the request URL (:url), request headers such as :user-agent and :referrer, arbitrary :req[header] values, and the Basic auth username (:remote-user). NEL is valid obs-text in HTTP header values, so an attacker can place it in an ordinary request header.

When such a value is logged, the raw code points survive into the emitted access-log record, and downstream ECMAScript-aware or Unicode-aware log processing splits the single physical record into multiple logical records. This allows an attacker to forge log entries. The built-in combined, common, default, and short formats are affected, as well as any custom format. This is an incomplete fix of CVE-2026-5078.

CVE-2026-15603 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.12.0); upgrading removes the vulnerable code path.

Kodem intelligence

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Users should upgrade to version 1.12.0.

1. What is CVE-2026-15603? CVE-2026-15603 is a medium-severity security vulnerability in morgan (npm), affecting versions < 1.12.0. It is fixed in 1.12.0.
2. How severe is CVE-2026-15603? CVE-2026-15603 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of morgan are affected by CVE-2026-15603? morgan (npm) versions < 1.12.0 is affected.
4. Is there a fix for CVE-2026-15603? Yes. CVE-2026-15603 is fixed in 1.12.0. Upgrade to this version or later.
5. Is CVE-2026-15603 exploitable, and should I be worried? Whether CVE-2026-15603 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-15603 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-15603? Upgrade morgan to 1.12.0 or later.