Skip to content

morgan vulnerable to Log Injection via unescaped double quote in quoted log fields

Moderate severity GitHub Reviewed Published Sep 11, 2026 in expressjs/morgan • Updated Sep 28, 2026

Package

npm morgan (npm)

Affected versions

< 1.12.1

Patched versions

1.12.1

Description

Impact

Morgan writes attacker-controlled request data to the access log. Its escaping (added in 1.11.0 and 1.12.0) neutralizes control characters, the Unicode line separators, and backslash, but not the double quote (0x22), which is the field delimiter of the Apache combined log format morgan emits. An attacker who controls a quoted field (:user-agent, :referrer, the request URL, or the Basic auth :remote-user) can inject a double quote to close the field early and forge additional fields in the log record. The combined, common, and default formats, and any custom format that quotes a token, are affected. This is an incomplete fix of CVE-2026-5078 and CVE-2026-15603.

Patches

Users should upgrade to version 1.12.1.

Workarounds

Update to version 1.12.1.

References

@UlisesGascon UlisesGascon published to expressjs/morgan Sep 11, 2026
Published by the National Vulnerability Database Sep 11, 2026
Published to the GitHub Advisory Database Sep 28, 2026
Reviewed Sep 28, 2026
Last updated Sep 28, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
Low
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(33rd percentile)

Weaknesses

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. Learn more on MITRE.

CVE ID

CVE-2026-87859

GHSA ID

GHSA-9f6g-j8ch-79g4

Source code

Credits

Loading Checking history
See something to contribute? Suggest improvements for this vulnerability.