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morgan vulnerable to Log Forging via unneutralized control characters in :remote-user

Moderate severity GitHub Reviewed Published Jun 2, 2026 in expressjs/morgan • Updated Jul 10, 2026

Package

npm morgan (npm)

Affected versions

>= 1.2.0, <= 1.10.1

Patched versions

1.11.0

Description

Impact

Morgan's :remote-user token extracts the Basic auth username from the Authorization header and writes it to the log stream without neutralizing control characters. An attacker can send a crafted Authorization: Basic header containing CR/LF characters to inject forged log lines, corrupting the one-request-per-line structure of access logs.

The built-in combined, common, default, and short formats are affected, as well as any custom format that includes :remote-user.

Patches

Users should upgrade to version 1.11.0.

Workarounds

Use a custom format string that does not include :remote-user.

References

@UlisesGascon UlisesGascon published to expressjs/morgan Jun 2, 2026
Published by the National Vulnerability Database Jun 3, 2026
Published to the GitHub Advisory Database Jul 10, 2026
Reviewed Jul 10, 2026
Last updated Jul 10, 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-2026-5078

GHSA ID

GHSA-4vj7-5mj6-jm8m

Source code

Credits

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