VMware vSphere 8.0 vCenter Appliance Management Interface (VAMI) Security Technical Implementation Guide

Denial of service (DoS) is one threat against web servers. Many DoS attacks attempt to consume web server resources in such a way that no more resources are available to satisfy legitimate requests. Mitigation against these threats is to take steps to limit the number of resources that can be consumed in certain ways. VAMI provides the "maxConnections" attribute of the to limit the number of concurrent Transmission Control Protocol (TCP) connections. This comes preconfigured with a tested, supported value that must be verified and maintained.

Data exchanged between the user and the web server can range from static display data to credentials used to log in the hosted application. Even when data appears to be static, the nondisplayed logic in a web page may expose business logic or trusted system relationships. The integrity of all the data being exchanged between the user and web server must always be trusted. To protect the integrity and trust, encryption methods should be used to protect the complete communication session. To protect the integrity and confidentiality of the remote sessions, VAMI uses Secure Sockets Layer (SSL)/Transport Layer Security (TLS). Satisfies: SRG-APP-000015-WSR-000014, SRG-APP-000315-WSR-000003

Remote access can be exploited by an attacker to compromise the server. By recording all remote access activities, it will be possible to determine the attacker's location, intent, and degree of success. VAMI uses the "mod_accesslog" module to log information relating to remote requests. These logs can then be piped to external monitoring systems.

Web server logging capability is critical for accurate forensic analysis. Without sufficient and accurate information, a correct replay of the events cannot be determined. Ascertaining the correct type of event that occurred is important during forensic analysis. The correct determination of the event and when it occurred is important in relation to other events that happened at that same time. Without sufficient information establishing what type of log event occurred, investigation into the cause of event is severely hindered. Log record content that may be necessary to satisfy the requirement of this control includes, but is not limited to, time stamps, source and destination IP addresses, user/process identifiers, event descriptions, application-specific events, success/fail indications, file names involved, access control, or flow control rules invoked. Satisfies: SRG-APP-000095-WSR-000056, SRG-APP-000096-WSR-000057, SRG-APP-000097-WSR-000058, SRG-APP-000098-WSR-000059, SRG-APP-000099-WSR-000061, SRG-APP-000100-WSR-000064, SRG-APP-000374-WSR-000172, SRG-APP-000375-WSR-000171

Log data is essential in the investigation of events. If log data were to become compromised, then competent forensic analysis and discovery of the true source of potentially malicious system activity would be difficult, if not impossible, to achieve. In addition, access to log records provides information an attacker could potentially use to their advantage since each event record might contain communication ports, protocols, services, trust relationships, user names, etc. The web server must protect the log data from unauthorized read, write, copy, etc. This can be done by the web server if the web server is also doing the logging function. The web server may also use an external log system. In either case, the logs must be protected from access by nonprivileged users. Satisfies: SRG-APP-000118-WSR-000068, SRG-APP-000119-WSR-000069, SRG-APP-000120-WSR-000070

Protection of log data includes assuring log data is not accidentally lost or deleted. Backing up log records to an unrelated system or onto separate media than the system the web server is actually running on helps to assure that, in the event of a catastrophic system failure, the log records will be retained. Satisfies: SRG-APP-000125-WSR-000071, SRG-APP-000358-WSR-000063, SRG-APP-000358-WSR-000163

Controlling what a user of a hosted application can access is part of the security posture of the web server. Any time a user can access more functionality than is needed for the operation of the hosted application poses a security issue. A user with too much access can view information that is not needed for the user's job role, or the user could use the function in an unintentional manner. A MIME tells the web server what type of program various file types and extensions are and what external utilities or programs are needed to execute the file type. A limited number of MIME types must be configured manually, and automatic mapping must be disabled.

Resource mapping is the process of tying a particular file type to a process in the web server that can serve that type of file to a requesting client and to identify which file types are not to be delivered to a client. By not specifying which files can and which files cannot be served to a user, VAMI could deliver sensitive files.

A web server can be installed with functionality that, by its nature, is not secure. WebDAV is an extension to the HTTP protocol that, when developed, was meant to allow users to create, change, and move documents on a server, typically a web server or web share. Allowing this functionality, development, and deployment is much easier for web authors. WebDAV is not widely used and has serious security concerns because it may allow clients to modify unauthorized files on the web server.

Most of the attention to denial-of-service (DoS) attacks focuses on ensuring that systems and applications are not victims of these attacks. However, these systems and applications must also be secured against use to launch such an attack against others. A variety of technologies exist to limit or, in some cases, eliminate the effects of DoS attacks. Limiting system resources that are allocated to any user to a bare minimum may also reduce the ability of users to launch some DoS attacks. One DoS mitigation is to prevent VAMI from keeping idle connections open for too long.

The web server's private key is used to prove the identity of the server to clients and securely exchange the shared secret key used to encrypt communications between the web server and clients. By gaining access to the private key, an attacker can pretend to be an authorized server and decrypt the Secure Sockets Layer (SSL) traffic between a client and the web server.

In UNIX and related computer operating systems, a file descriptor is an indicator used to access a file or other input/output resource, such as a pipe or network connection. File descriptors index into a per-process file descriptor table maintained by the kernel, which in turn indexes into a systemwide table of files opened by all processes, called the file table. As a single-threaded server, Lighttpd must be limited in the number of file descriptors that can be allocated. This will prevent Lighttpd from being used in a form of DoS attack against the operating system.

Invalid user input occurs when a user inserts data or characters into a hosted application's data entry field and the hosted application is unprepared to process that data. This results in unanticipated application behavior, potentially leading to an application compromise. Invalid user input is one of the primary methods employed when attempting to compromise an application. An attacker can also enter Unicode into hosted applications in an effort to break out of the document home or root home directory or to bypass security checks.

The goal is to completely control the web user's experience in navigating any portion of the web document root directories. Ensuring all web content directories have at least the equivalent of an "index.html" file is a significant factor to accomplish this end. Enumeration techniques, such as Uniform Resource Locator (URL) parameter manipulation, rely on being able to obtain information about the web server's directory structure by locating directories without default pages. In this scenario, the web server will display to the user a listing of the files in the directory being accessed. By having a default hosted application web page, the anonymous web user will not obtain directory browsing information or an error message that reveals the server type and version.

Any application providing too much information in error logs and in administrative messages to the screen risks compromising the data and security of the application and system. VAMI must only generate error messages that provide information necessary for corrective actions without revealing sensitive or potentially harmful information in error logs and administrative messages. The "mod_status" module generates the status overview of the webserver. The information covers the following: - Uptime. - Average throughput. - Current throughput. - Active connections and their state. While this information is useful on a development system, production systems must not have "mod_status" enabled.

Information needed by an attacker to begin looking for possible vulnerabilities in a web server includes any information about the web server and plug-ins or modules being used. When debugging or trace information is enabled in a production web server, information about the web server, such as web server type, version, patches installed, plug-ins and modules installed, type of code being used by the hosted application, and any backends being used for data storage may be displayed. Because this information may be placed in logs and general messages during normal operation of the web server, an attacker does not need to cause an error condition to gain this information.

During a Transport Layer Security (TLS) session negotiation, when choosing a cipher during a handshake, normally the client's preference is used. This is potentially problematic as a malicious, dated, or poorly configured client could select the most insecure cipher offered by the server, even if it supports stronger ones. If "ssl.honor-cipher-order" is enabled, the "ssl.cipher-list" setting will be treated as an ordered list of cipher values from most preferred to least, left to right.

All versions of the Secure Sockets Layer (SSL) and TLS protocols (up to and including TLS 1.2) are vulnerable to a man-in-the-middle attack (CVE-2009-3555) during a renegotiation. This vulnerability allows an attacker to "prefix" a chosen plaintext to the HTTP request as seen by the web server. The protocols have since been amended by RFC 5746, but the fix must be supported by both client and server to be effective. While Lighttpd and the underlying OpenSSL libraries are no longer vulnerable, steps must be taken to account for older clients that do not support RFC 5746. To this end, Lighttpd disables client-initiated renegotiation entirely by default. This configuration must be validated and maintained.

Web servers will often display error messages to client users, displaying enough information to aid in the debugging of the error. The information given back in error messages may display the web server type, version, patches installed, plug-ins and modules installed, type of code being used by the hosted application, and any backends being used for data storage. This information could be used by an attacker to blueprint what type of attacks might be successful. Therefore, VAMI must be configured to hide the server version at all times.

HSTS instructs web browsers to only use secure connections for all future requests when communicating with a website. Doing so helps prevent SSL protocol attacks, SSL stripping, cookie hijacking, and other attempts to circumvent SSL protection.

Clickjacking, also known as a “UI redress attack”, is when an attacker uses multiple transparent or opaque layers to trick a user into clicking on a button or link on another page when they were intending to click on the top level page. Thus, the attacker is “hijacking” clicks meant for their page and routing them to another page, most likely owned by another application, domain, or both. Using a similar technique, keystrokes can also be hijacked. With a carefully crafted combination of stylesheets, iframes, and text boxes, a user can be led to believe they are typing in the password to their email or bank account but are instead typing into an invisible frame controlled by the attacker.

MIME sniffing was, and still is, a technique used by some web browsers to examine the content of a particular asset. This is done for the purpose of determining an asset's file format. This technique is useful in the event that there is not enough metadata information present for a particular asset, thus leaving the possibility that the browser interprets the asset incorrectly. Although MIME sniffing can be useful to determine an asset's correct file format, it can also cause a security vulnerability. This vulnerability can be quite dangerous both for site owners as well as site visitors. This is because an attacker can leverage MIME sniffing to send an XSS (Cross Site Scripting) attack.

A Content Security Policy (CSP) requires careful tuning and precise definition of the policy. If enabled, CSP has significant impact on the way browsers render pages (e.g., inline JavaScript is disabled by default and must be explicitly allowed in the policy). CSP prevents a wide range of attacks, including cross-site scripting and other cross-site injections.