CVE-2026-5078: morgan vulnerable to Log Forging via unneutralized control characters in :remote-user

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-03 · updated 2026-07-22
Weakness: CWE-117

Description

Impact: The morgan logging middleware's :remote-user token extracts the Basic auth username from the Authorization request header and writes it to the log stream without neutralizing control characters. An unauthenticated attacker can send a crafted Authorization Basic header containing CR or LF bytes to inject forged log lines, breaking the one-request-per-line structure of access logs and enabling log forgery against downstream log consumers. The built-in combined, common, default, and short formats are affected, as well as any custom format that references :remote-user. Affected versions: morgan 1.2.0 through 1.10.1. Patches: upgrade to morgan 1.11.0, which neutralizes control characters in the :remote-user token output. Workarounds: use a custom format string that does not include :remote-user.

Affected products

morgan: from 1.2.0 before 1.11.0 — fixed in 1.11.0

How to fix

Upgrade to a fixed version: morgan 1.11.0. Check the vendor advisory for mitigations.

References

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