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Log entries created from user input

ID: go/log-injection
Kind: path-problem
Security severity: 7.8
Severity: error
Precision: medium
Tags:
   - security
   - external/cwe/cwe-117
Query suites:
   - go-security-extended.qls
   - go-security-and-quality.qls

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If unsanitized user input is written to a log entry, a malicious user may be able to forge new log entries.

Forgery can occur if a user provides some input with characters that are interpreted when the log output is displayed. If the log is displayed as a plain text file, then new line characters can be used by a malicious user. If the log is displayed as HTML, then arbitrary HTML may be included to spoof log entries.

Recommendation

User input should be suitably encoded before it is logged.

If the log entries are plain text then line breaks should be removed from user input, using strings.Replace or similar. Care should also be taken that user input is clearly marked in log entries, and that a malicious user cannot cause confusion in other ways.

For log entries that will be displayed in HTML, user input should be HTML encoded using html.EscapeString or similar before being logged, to prevent forgery and other forms of HTML injection.

Example

In the following example, a user name, provided by the user, is logged using a logging framework without any sanitization.

package main

import (
	"log"
	"net/http"
)

// BAD: A user-provided value is written directly to a log.
func handler(req *http.Request) {
	username := req.URL.Query()["username"][0]
	log.Printf("user %s logged in.\n", username)
}

In the next example, strings.Replace is used to ensure no line endings are present in the user input.

package main

import (
	"log"
	"net/http"
	"strings"
)

// GOOD: The user-provided value is escaped before being written to the log.
func handlerGood(req *http.Request) {
	username := req.URL.Query()["username"][0]
	escapedUsername := strings.ReplaceAll(username, "\n", "")
	escapedUsername = strings.ReplaceAll(escapedUsername, "\r", "")
	log.Printf("user %s logged in.\n", escapedUsername)
}

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