CVE-2026-54511

LogTape is an unobtrusive logging library. Prior to 1.3.11, 2.0.14, and 2.1.5, the @logtape/syslog package's escapeStructuredDataValue() function in packages/syslog/src/syslog.ts does not neutralize C0 control characters from U+0000 through U+001F in structured data values, and formatStructuredData() inserts property keys without validating the RFC 5424 SD-NAME grammar. When includeStructuredData is true, an attacker-controlled newline can terminate an RFC 6587 non-transparent TCP syslog frame and make following bytes appear as a forged RFC 5424 record, while a key containing a closing bracket or other forbidden character can terminate or corrupt the structured-data element. Applications that forward attacker-controlled property values or keys can therefore allow forged records with arbitrary hosts, applications, process identifiers, facilities, or severity levels, undermining downstream collector and SIEM integrity. This issue is fixed in versions 1.3.11, 2.0.14, and 2.1.5.

LogTape is an unobtrusive logging library. Prior to 1.3.11, 2.0.14, and 2.1.5, the @logtape/syslog package's escapeStructuredDataValue() function in packages/syslog/src/syslog.ts does not neutralize C0 control characters from U+0000 through U+001F in structured data values, and formatStructuredData() inserts property keys without validating the RFC 5424 SD-NAME grammar. When includeStructuredData is true, an attacker-controlled newline can terminate an RFC 6587 non-transparent TCP syslog frame and make following bytes appear as a forged RFC 5424 record, while a key containing a closing bracket or other forbidden character can terminate or corrupt the structured-data element. Applications that forward attacker-controlled property values or keys can therefore allow forged records with arbitrary hosts, applications, process identifiers, facilities, or severity levels, undermining downstream collector and SIEM integrity. This issue is fixed in versions 1.3.11, 2.0.14, and 2.1.5.

@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…

LogTape is an unobtrusive logging library. Prior to 1.3.11, 2.0.14, and 2.1.5, the @logtape/syslog package's escapeStructuredDataValue() function in packages/syslog/src/syslog.ts does not neutralize C0 control characters from U+0000 through U+001F in structured data values, and formatStructuredData() inserts property keys without validating the RFC 5424 SD-NAME grammar. When includeStructuredData is true, an attacker-controlled newline can terminate an RFC 6587 non-transparent TCP syslog frame and make following bytes appear as a forged RFC 5424 record, while a key containing a closing bracket or other forbidden character can terminate or corrupt the structured-data element. Applications that forward attacker-controlled property values or keys can therefore allow forged records with arbitrary hosts, applications, process identifiers, facilities, or severity levels, undermining downstream collector and SIEM integrity. This issue is fixed in versions 1.3.11, 2.0.14, and 2.1.5.

LogTape is an unobtrusive logging library. Prior to 1.3.11, 2.0.14, and 2.1.5, the @logtape/syslog package's escapeStructuredDataValue() function in packages/syslog/src/syslog.ts does not neutralize C0 control characters from U+0000 through U+001F in structured data values, and formatStructuredData() inserts property keys without validating the RFC 5424 SD-NAME grammar. When includeStructuredData is true, an attacker-controlled newline can terminate an RFC 6587 non-transparent TCP syslog frame and make following bytes appear as a forged RFC 5424 record, while a key containing a closing bracket or other forbidden character can terminate or corrupt the structured-data element. Applications that forward attacker-controlled property values or keys can therefore allow forged records with arbitrary hosts, applications, process identifiers, facilities, or severity levels, undermining downstream collector and SIEM integrity. This issue is fixed in versions 1.3.11, 2.0.14, and 2.1.5.

@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…