The absence of authentication means everyone is able to access this service.
A service that does not authenticate its clients allows access to all comers.
[1] Standards Mapping - OWASP Top 10 2004 - (OWASP 2004) A3 Broken Authentication and Session Management
[2] Standards Mapping - OWASP Top 10 2010 - (OWASP 2010) A3 Broken Authentication and Session Management
[3] Standards Mapping - OWASP Top 10 2007 - (OWASP 2007) A7 Broken Authentication and Session Management
[4] Standards Mapping - Security Technical Implementation Guide Version 3 - (STIG 3) APP3480.2 CAT II
[5] Common Security Scenarios Microsoft
[6] Standards Mapping - Common Weakness Enumeration - (CWE) CWE ID 285
[7] Standards Mapping - FIPS200 - (FISMA) IA
[8] Standards Mapping - Web Application Security Consortium 24 + 2 - (WASC 24 + 2) Insufficient Authentication
[9] Message Security with an Anonymous Client Microsoft
[10] Standards Mapping - SANS Top 25 2009 - (SANS 2009) Porous Defenses - CWE ID 285
[11] Standards Mapping - SANS Top 25 2010 - (SANS 2010) Porous Defenses - CWE ID 285
[12] Standards Mapping - Payment Card Industry Data Security Standard Version 1.1 - (PCI 1.1) Requirement 6.5.3, Requirement 7.2
[13] Standards Mapping - Payment Card Industry Data Security Standard Version 1.2 - (PCI 1.2) Requirement 6.5.7, Requirement 7.2
[14] Standards Mapping - Payment Card Industry Data Security Standard Version 2.0 - (PCI 2.0) Requirement 6.5.8, Requirement 7.2