CVE-2026-87859: morgan vulnerable to Log Injection via unescaped double quote in quoted log fields
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.
Other sources
morgan is an HTTP request logger middleware for Node.js. In versions before 1.12.1, its escapeLogField() function does not escape the double quote character, which delimits the quoted fields of the Apache combined log format that morgan emits. An unauthenticated remote attacker who controls a value written to a quoted field, such as the User-Agent or Referer header, can include a double quote to close that field early, so a log consumer that parses the log by field position reads attacker-supplied text as the following field. In the built-in formats this makes the recorded value differ from the value that was sent, and in custom formats that quote an attacker-controlled token before a server-controlled one it can forge values such as the response status. No newline is injected, so record separation stays intact. The issue is fixed in morgan 1.12.1, which escapes the double quote. Users should upgrade to morgan 1.12.1 or later.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/morganto a version that resolves this vulnerability.Fixed in 1.12.1 - Upgrade
Upgrade
morganto a version that resolves this vulnerability.Fixed in 1.12.1
Event History
Frequently Asked Questions
Who is exposed to this issue?
Applications using morgan before 1.12.1 are exposed when they log attacker-controlled values in quoted fields. This includes built-in formats that log headers such as User-Agent or Referer, and custom formats that quote attacker-controlled tokens.
What does an attacker need to exploit it?
An unauthenticated remote attacker only needs to send a request containing a double quote in a value that morgan writes to a quoted log field, such as User-Agent or Referer. Exploitation does not require newline injection or breaking log record separation.
What is the practical impact on log analysis?
A parser that reads Apache combined-style logs by field position can interpret attacker-supplied text as the next field after the quote closes the original field early. In custom formats where an attacker-controlled quoted token precedes a server-controlled token, this can forge logged values such as the response status.
How can this be remediated?
Upgrade morgan to version 1.12.1 or later. That version escapes double quote characters in escapeLogField().