CVE-2026-15603 is a medium-severity (CVSS 5.3) CWE-117 vulnerability in morgan. A fix is available for morgan — see the affected versions and patch details below.
morgan vulnerable to Log Forging via unescaped Unicode line separators
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-15603.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-15603 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 394,803 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
morgannpmDescription
Impact
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.
Patches
Users should upgrade to version 1.12.0.
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | morgan | all versions | 1.12.0npm install morgan@1.12.0 |
Affected Products
morganmorgan_projectDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for morgan, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update morgan to 1.12.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-15603 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
Log forging flaw in `morgan` allows unauthenticated remote attackers to inject Unicode line separators into log tokens. This can lead to log record misinterpretation by downstream systems, potentially impacting security monitoring and incident response in Red Hat products that use `morgan` as an HTTP request logger…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.Source: Red Hat security advisory for CVE-2026-15603 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-15603 in your dependencies?
Find it across npm, including transitive dependencies.