@logtape/syslog contains two related output-encoding bugs in the structured data formatting code. Both only affect deployments with includeStructuredData: true, which is non-default.

1. Unescaped C0 control characters in structured data values

escapeStructuredDataValue() in packages/syslog/src/syslog.ts escapes \, ", and ] per RFC 5424 but does not escape newline (\n), carriage return (\r), or any other C0 control characters (U+0000–U+001F):

function escapeStructuredDataValue(value: string): string {
  return value
    .replace(/\\/g, "\\\\")
    .replace(/"/g, '\\"')
    .replace(/]/g, "\\]");
  // \n, \r, and other C0 control characters are not escaped
}

TCP syslog commonly uses \n as a frame delimiter (RFC 6587, non-transparent framing). If an attacker-controlled value contains a literal newline, that newline terminates the current syslog frame. Bytes following the newline begin a new frame, and if they form a valid RFC 5424 header (<PRI>1 …), a downstream collector will accept them as a separate, authentic-looking syslog record.

2. Unvalidated SD-NAME keys

Structured data parameter keys are inserted into the message without validation or escaping:

elements.push(`${key}="${escapedValue}"`);

RFC 5424 defines SD-NAME as printable US-ASCII characters excluding =, ], ", and space, with a maximum length of 32. A key containing any of those characters, control characters, or exceeding the length limit will produce malformed structured data. If the key itself contains an embedded ], it can prematurely close the structured-data element.

In typical usage, property keys are developer-defined string literals and therefore safe. However, if an application forwards attacker-controlled keys as log properties—for example by spreading request headers or arbitrary metadata into a log record—this becomes a second injection path.

Proof of concept

The following Node.js snippet (no dependencies, no network required) demonstrates that the escaped value still contains a literal newline:

function escapeStructuredDataValue(value) {
  return value
    .replace(/\\/g, "\\\\")
    .replace(/"/g, '\\"')
    .replace(/]/g, "\\]");
}

const payload =
  'normal\n<134>1 2026-01-01T00:00:00Z forged evil - - - INJECTED';

const result = escapeStructuredDataValue(payload);
console.log("Newline present after escape:", result.includes("\n")); // true

Tested with Node.js 22.17.1.

Impact

An attacker who controls log property values can:

• forge syslog records attributed to arbitrary hosts, applications, or process IDs;
• insert records with arbitrary severity or facility levels;
• obscure malicious activity by injecting misleading entries around legitimate ones;
• break downstream log parsers or SIEM correlation rules that rely on log integrity.

Affected downstream collectors include rsyslog, syslog-ng, Splunk, Elastic Stack, and any other system using RFC 6587 non-transparent framing.

Suggested fix

Structured data values

Escape all C0 control characters (U+0000–U+001F) in addition to \, ", and ]. RFC 5424 does not define an escape sequence for control characters in PARAM-VALUE; the most interoperable approach is to strip or replace them:

function escapeStructuredDataValue(value: string): string {
  return value
    .replace(/\\/g, "\\\\")
    .replace(/"/g, '\\"')
    .replace(/]/g, "\\]")
    .replace(/[\x00-\x1f]/g, (c) =>
      `\\x${c.charCodeAt(0).toString(16).padStart(2, "0")}`
    );
}

Alternatively, strip them entirely: .replace(/[\x00-\x1f]/g, ""). The right choice depends on whether downstream consumers need some representation of the original value.

SD-NAME keys

Validate each key against the RFC 5424 SD-NAME grammar before including it. Keys that fail validation should be skipped or sanitized:

// SD-NAME: printable US-ASCII, excluding '=', ']', '"', SP; max 32 chars
const SD_NAME_RE = /^[!-<>-Z\\^-z|~]{1,32}$/;

for (const [key, value] of Object.entries(record.properties)) {
  if (!SD_NAME_RE.test(key)) continue;
  const escapedValue = escapeStructuredDataValue(String(value));
  elements.push(`${key}="${escapedValue}"`);
}

References

• GHSA-8h6h-x5pq-56fq
• dahlia/logtape@7a6e5b9
• https://github.com/dahlia/logtape/releases/tag/1.3.11
• https://github.com/dahlia/logtape/releases/tag/2.0.14
• https://github.com/dahlia/logtape/releases/tag/2.1.5