Migrating to newer, supported applications where possible is the best approach to addressing security issues in legacy web applications. Legacy web applications are web applications that are outdated, unsupported, or incompatible with the current technologies and standards. Legacy web applications may have various security issues, such as:
* Vulnerabilities and bugs that are not fixed or patched by the developers or vendors
* Weak or obsolete encryption and authentication mechanisms that are easily broken or bypassed by attackers
* Lack of compliance with the security policies and regulations that are applicable to the web applications
* Incompatibility or interoperability issues with the newer web browsers, operating systems, or platforms that are used by the users or clients Migrating to newer, supported applications where possible is the best approach to addressing security issues in legacy web applications, because it can provide several benefits, such as:
* Enhancing the security and performance of the web applications by using the latest technologies and standards that are more secure and efficient
* Reducing the risk and impact of the web application attacks by eliminating or minimizing the vulnerabilities and bugs that are present in the legacy web applications
* Increasing the compliance and alignment of the web applications with the security policies and regulations that are applicable to the web applications
* Improving the compatibility and interoperability of the web applications with the newer web browsers, operating systems, or platforms that are used by the users or clients The other options are not the best approaches to addressing security issues in legacy web applications, but rather approaches that can mitigate or remediate the security issues, but not eliminate or prevent them.
Debugging the security issues is an approach that can mitigate the security issues in legacy web applications, but not the best approach, because it involves identifying and fixing the errors or defects in the code or logic of the web applications, which may be difficult or impossible to do for the legacy web applications that are outdated or unsupported. Conducting a security assessment is an approach that can remediate the security issues in legacy web applications, but not the best approach, because it involves evaluating and testing the security effectiveness and compliance of the web applications, using various techniques and tools, such as audits, reviews, scans, or penetration tests, and identifying and reporting any security weaknesses or gaps, which may not be sufficient or feasible to do for the legacy web applications that are incompatible or obsolete.
Protecting the legacy application with a web application firewall is an approach that can mitigate the security issues in legacy web applications, but not the best approach, because it involves deploying and configuring a web application firewall, which is a security device or software that monitors and filters the web traffic between the web applications and the users or clients, and blocks or allows the web requests or responses based on the predefined rules or policies, which may not be effective or efficient to do for the legacy web applications that have weak or outdated encryption or authentication mechanisms.

A Public Key is also known as an asymmetric algorithm and the use of a secret key would be a symmetric algorithm.
The following answers are incorrect:
Use of the recipient's public key for encryption and decryption based on the recipient's private key. Is incorrect this would be known as an asymmetric algorithm.
Use of software encryption assisted by a hardware encryption accelerator. This is incorrect, it is a distractor.
Use of Elliptic Curve Encryption. Is incorrect this would use an asymmetric algorithm.

A federated identity standard is Security Assertion Markup Language (SAML). SAML is a standard that enables the exchange of authentication and authorization information between different parties, such as service providers and identity providers, using XML-based messages called assertions. SAML can facilitate the single sign-on (SSO) process, which allows a user to access multiple services or applications with a single login session, without having to provide their credentials multiple times. SAML can also support the federated identity management, which allows a user to use their identity or credentials from one domain or organization to access the services or applications from another domain or organization, without having to create or maintain separate accounts. 802.11i, Kerberos, and LDAP are not federated identity standards, as they are related to the wireless network security, the network authentication protocol, or the directory service protocol, not the exchange of authentication and authorization information between different parties. References: CISSP All-in-One Exam Guide, Eighth Edition, Chapter 5, Identity and Access Management, page 692. Official (ISC)2 CISSP CBK Reference, Fifth Edition, Chapter 5, Identity and Access Management, page 708.

Allowing objects to be used sequentially by multiple users without a refresh of the
objects can lead to disclosure of residual data. It is important that steps be taken to eliminate the
chance for the disclosure of residual data.
Object reuse refers to the allocation or reallocation of system resources to a user or, more
appropriately, to an application or process. Applications and services on a computer system may
create or use objects in memory and in storage to perform programmatic functions. In some
cases, it is necessary to share these resources between various system applications. However,
some objects may be employed by an application to perform privileged tasks on behalf of an
authorized user or upstream application. If object usage is not controlled or the data in those
objects is not erased after use, they may become available to unauthorized users or processes.
Disclosure of residual data and Unauthorized obtaining of a privileged execution state are both a
problem with shared memory and resources. Not clearing the heap/stack can result in residual
data and may also allow the user to step on somebody's session if the security token/identify was
maintained in that space. This is generally more malicious and intentional than accidental though.
The MOST common issue would be Disclosure of residual data.
The following answers are incorrect:
Unauthorized obtaining of a privileged execution state. Is incorrect because this is not a problem
with Object Reuse.
Data leakage through covert channels. Is incorrect because it is not the best answer. A covert
channel is a communication path. Data leakage would not be a problem created by Object Reuse.
In computer security, a covert channel is a type of computer security attack that creates a capability to transfer information objects between processes that are not supposed to be allowed to communicate by the computer security policy. The term, originated in 1973 by Lampson is defined as "(channels) not intended for information transfer at all, such as the service program's effect on system load." to distinguish it from Legitimate channels that are subjected to access controls by COMPUSEC. Denial of service through a deadly embrace. Is incorrect because it is only a detractor.
References: Hernandez CISSP, Steven (2012-12-21). Official (ISC)2 Guide to the CISSP CBK, Third Edition ((ISC)2 Press) (Kindle Locations 4174-4179). Auerbach Publications. Kindle Edition. and https://www.fas.org/irp/nsa/rainbow/tg018.htm and http://en.wikipedia.org/wiki/Covert_channel