Second, the library fails to validate SD-NAME keys according to RFC 5424 specifications. If an application forwards attacker-controlled metadata - such as HTTP headers or user-supplied parameters - as structured data keys, an attacker can inject structural characters (like ]) to break the log format or cause further injection. These vulnerabilities allow attackers to forge logs, manipulate severity/facility levels, and undermine the integrity of downstream SIEM systems like Splunk or Elastic Stack.

Impact

Successful exploitation allows an attacker to inject arbitrary log entries into downstream infrastructure. This can be used to forge audit logs, obscure malicious activity, or break log ingestion pipelines. Any organization relying on @logtape/syslog to forward application logs to a centralized collector is at risk if they allow user-controlled input to influence log properties.

Recommendation

• Update @logtape/syslog to the latest patched version immediately (v1.3.11, v2.0.14, or v2.1.5+).
• Review applications using the includeStructuredData: true configuration to identify and sanitize any user-controlled input being passed as log properties.
• Audit downstream logging infrastructure (rsyslog, syslog-ng, SIEM collectors) to ensure they are configured to ignore or sanitize records that do not strictly adhere to expected RFC 5424 structured data formats.