CVE-2026-15603: Morgan Morgan Vulnerable To Log Forging Vulnerability

MODERATE
Advisory Identifier: GHSA-jxfw-x594-9x9m
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CVSS 5.3
CWE-117
Published
Advisory Summary

CVE-2026-15603 is a MODERATE (CVSS 5.3) morgan vulnerable to log forging vulnerability (CWE-117: Improper Output Neutralization for Logs) affecting Morgan libraries in the npm ecosystem. The issue involves morgan vulnerable to log forging via unescaped unicode line separators. Remediation is available by upgrading affected packages to version 1.12.0.

CVE-2026-15603 at a Glance

CVE Identifier CVE-2026-15603
GHSA Identifier GHSA-jxfw-x594-9x9m
Severity MODERATE
CVSS v3.1 Base Score 5.3
CWE Classification CWE-117: Improper Output Neutralization for Logs
Affected Product Morgan
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 5.3

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity Low (I:L)
Availability None (A:N)

How to Fix CVE-2026-15603

To remediate this vulnerability, upgrade all affected Morgan dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

morgan fixed: 1.12.0 
npm install morgan@1.12.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

npm ls morgan

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-15603, inspect your project dependencies using npm:

npm ls morgan

Analyze the terminal output to locate any version of Morgan in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.12.0), your application is affected.

Note: The npm ls morgan command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-117: Improper Output Neutralization for Logs, where the 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. In Morgan, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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.

CVE-2026-15603 Affected Morgan Versions

1 component(s)
Fixed Versions:
1.12.0
Product / Package Ecosystem Affected Versions Fixed Version
morgan
npm/morgan
npm
< 1.12.0
1.12.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/expressjs/morgan/security/advisories/GHSA-jxfw-x594-9x9m
NVD — CVE-2026-15603 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-15603
Security Fix Commit FIX COMMIT
https://github.com/expressjs/morgan/commit/fbf93833186920cc39bb19b247e4a86b61bef4fe
Morgan Security Information SECURITY
https://cna.openjsf.org/security-advisories.html
Morgan Source Repository REPOSITORY
https://github.com/expressjs/morgan
Morgan Release RELEASE
https://github.com/expressjs/morgan/releases/tag/1.12.0

Frequently Asked Questions

What is CVE-2026-15603?
CVE-2026-15603 is a security vulnerability in Morgan involving morgan vulnerable to Log Forging via unescaped Unicode line separators under CWE-117: Improper Output Neutralization for Logs.
Which Morgan versions are affected?
Affected versions include releases prior to 1.12.0 across affected packages: morgan.
What version fixes CVE-2026-15603?
Upgrading to 1.12.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is morgan affected?
Yes, morgan is affected in releases prior to 1.12.0. Upgrade your dependency to 1.12.0 or later.
What is the CVSS score of CVE-2026-15603?
CVE-2026-15603 has a CVSS v3.1 base score of 5.3 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N.
What CWE is associated with CVE-2026-15603?
CVE-2026-15603 is classified under CWE-117: Improper Output Neutralization for Logs (The 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.).
How can I check npm dependencies for CVE-2026-15603?
Run the dependency tree command: 'npm ls morgan'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-jxfw-x594-9x9m",
  "modified" : "2026-09-08T21:28:26Z",
  "published" : "2026-09-08T21:28:26Z",
  "aliases" : [ "CVE-2026-15603" ],
  "summary" : "morgan vulnerable to Log Forging via unescaped Unicode line separators",
  "details" : "### Impact\n\nMorgan 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.\n\nWhen 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.\n\n### Patches\n\nUsers should upgrade to version 1.12.0.\n\n### Workarounds\n\nNone. 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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "morgan"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.12.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/expressjs/morgan/security/advisories/GHSA-jxfw-x594-9x9m"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-15603"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/expressjs/morgan/commit/fbf93833186920cc39bb19b247e4a86b61bef4fe"
  }, {
    "type" : "WEB",
    "url" : "https://cna.openjsf.org/security-advisories.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/expressjs/morgan"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/expressjs/morgan/releases/tag/1.12.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-117" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-08T21:28:26Z",
    "nvd_published_at" : "2026-08-28T16:17:07Z"
  }
}