GHSA-jxfw-x594-9x9m: Medium severity npm/morgan vulnerability
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 Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/morganto a version that resolves this vulnerability.Fixed in 1.12.0 - Upgrade
Upgrade
Morganto a version that resolves this vulnerability.Fixed in 1.12.0
Event History
Frequently Asked Questions
Which Morgan logging configurations are affected?
The built-in combined, common, default, and short formats are affected. Custom formats are also affected when they log attacker-controlled request data through tokens such as :url, request-header tokens, :req[header], or :remote-user.
What does an attacker need to exploit this issue?
An attacker only needs to send a request containing Unicode line-separator characters in a logged value. NEL (U+0085) can be placed in an ordinary HTTP request header because it is valid obs-text in header values; affected fields also include URLs, User-Agent, Referer, arbitrary request headers, and Basic authentication usernames.
What is the practical impact?
The injected characters can cause downstream ECMAScript-aware or Unicode-aware log processors to interpret one physical access-log record as multiple logical records. This can let an attacker forge log entries.
Is there a workaround if upgrading is not immediately possible?
No workaround is provided. The recommended remediation is to upgrade Morgan to version 1.12.0.