Allowing users jump between HTTP and HTTPS leaves the application vulnerable to session hijacking.
Many applications use cookies to communicate a user's session identifier. When the application is accessed over HTTPS, cookies are protected (attackers are unable to sniff them). But if developers allow HTTP access to non-sensitive parts of the application, the session identifier can be stolen. When a user browses to an unprotected part of the site, the cookie, containing the session ID, is sent in the clear. If attackers sniff the traffic, they can see the session ID and use it to take control of the user's session.
The following example shows a configuration that mixes insecure and secure channels:
<property name="filterInvocationDefinitionSource">
<value>
CONVERT_URL_TO_LOWERCASE_BEFORE_COMPARISON
\A/secure/.*\Z=REQUIRES_SECURE_CHANNEL
\A/acegilogin.jsp.*\Z=REQUIRES_SECURE_CHANNEL
\A/j_acegi_security_check.*\Z=REQUIRES_SECURE_CHANNEL
\A.*\Z=REQUIRES_INSECURE_CHANNEL
</value>
</property>
[1] Standards Mapping - OWASP Top 10 2004 - (OWASP 2004) A3 Broken Authentication and Session Management
[2] Standards Mapping - OWASP Top 10 2007 - (OWASP 2007) A7 Broken Authentication and Session Management
[3] Standards Mapping - OWASP Top 10 2010 - (OWASP 2010) A9 Insufficient Transport Layer Protection
[4] Ben Alex Acegi Security - Channel Security
[5] Standards Mapping - Security Technical Implementation Guide Version 3 - (STIG 3) APP3405 CAT I
[6] Standards Mapping - Common Weakness Enumeration - (CWE) CWE ID 5
[7] Standards Mapping - Web Application Security Consortium 24 + 2 - (WASC 24 + 2) Information Leakage
[8] Standards Mapping - Payment Card Industry Data Security Standard Version 1.1 - (PCI 1.1) Requirement 6.5.3
[9] Standards Mapping - Payment Card Industry Data Security Standard Version 1.2 - (PCI 1.2) Requirement 6.5.7
[10] Standards Mapping - Payment Card Industry Data Security Standard Version 2.0 - (PCI 2.0) Requirement 6.5.8