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The feature of code signing was introduced in Java to provide an alternative mechanism Java uses code signing as a requirement for granting elevated privileges to code depending on the security policy in effect. Many security policies permit signed code to operate with elevated privileges. For example, Java applets that need to read system properties from a user's machine can escape the default sandbox restrictions by prompting the user with a security dialog, about whether the party that signed the code is considered trustworthy. This element of trusting the signature allows applets to escape the default security sandbox restrictions. On the other hand, with applications that use a custom security policy, explicit permissions need to be granted to the particular codebase and optionally, the signer. This has the benefit of ensuring that only trusted signed code runs with the specified privileges.when signed. Consequently, users can grant explicit permissions either to a particular codebase or to all code signed by a particular signer. This approach places control of security in the hands of the user, who can choose whether to run an application with full or restricted permissions.

Signing code, however, has its own problems. According to Bruce Schneier [Schneier 2000 Wiki MarkupSigning code, however, has its own demerits. According to Schneier \[[Schneier 00|AA. Java References#Schneier 00]\]:

First, users have no idea how to decide if a particular signer is trusted or not. Second, just because a component is signed doesn't mean that it is safe. Third, just because two components are individually signed does not mean that using them together is safe; lots of accidental harmful interactions can be exploited. Fourth, "safe" is not an all-or-nothing thing; there are degrees of safety. And fifth, the fact that the evidence of attack (the signature on the code) is stored on the computer under attack is mostly useless: The attacker could delete or modify the signature during the attack, or simply reformat the drive where the signature is stored."

Code signing is designed primarily to authenticate the origin of the code as well as to verify authenticity of origin and the integrity of the code. It relies on the Certification Authority a certification authority (CA) to duly confirm the identity of the principal signer. Naive users should not be expected to understand how certificates and the public key infrastructure (PKI) work. All too often, they

Users commonly associate digital signatures with safety of code execution, and trust trusting the code to cause them no harm. In general, there is a misconception that signed code is safe to be executed. The problem manifests itself arises when a vulnerability is discovered in signed code. As many users choose the option of permanently trusting the organizations that they have full confidence in, they are not notified if an adversary offers them the vulnerable software with the intentions of exploiting it. Because many systems are configured to permanently trust certain signing organizations, those systems fail to notify their users when downloading content signed by the trusted organization, even when that content contains vulnerabilities. An attacker can offer the users legitimately signed vulnerable content with the intention of exploiting that content.

Consider, for example, signed Java applets. Whenever When a certificate is verified to be correct (as opposed to being self-signed or tampered), on widely used platforms, the user is confronted presented with a security dialog that has in which the check box option "Always trust the content from the publisher" turned on. Unfortunately, this is selected by default. The dialog primarily asks whether or not the signed code should be executed. Unfortunately, if the user confirms the dialog with the check box selected, the "Always trust..." setting overrides any future warning dialogs about potentially vulnerable software versions from the respective organization.

In addition, McGraw and Felten \[[McGraw 00|AA. Java References#McGraw 00]\] stress that only privileged code should be signed. They further recommend that, since all development is not in-house, an organization that signs its code must not vouch for code acquired from a third party. Anything that is not comprehensible must not be signed (Also see the related guideline [SEC32-J. Create and sign a SignedObject before creating a SealedObject]). They suggest using the {{AccessController}} mechanism wherein only the privileged code ({{doPrivileged() section}}) ought to be signed. The other code can be left unsigned, restricting it to the sandbox. 

It follows that unprivileged code is not required to be digitally signed and therefore should not be. This conviction adequately respects the guideline SEC00-J. Follow the principle of least privilege.

Exceptions

. An attacker can take advantage of this mechanism by exploiting vulnerable code signed by the trusted organization. In this case, the code will execute with the user's implied permission and can be freely exploited.

An organization that signs its own code should not vouch for code acquired from a third party without carefully auditing the third-party code. When signing privileged code, ensure that all of the signed code is confined to a single JAR file (see ENV01-J. Place all security-sensitive code in a single JAR and sign and seal it for more information) and also that any code invoked from the privileged code is also contained in that JAR file. Nonprivileged code must be left unsigned, restricting it to the sandbox. For example, unsigned applets and Java Network Launching Protocol (JNLP) applications are granted the minimum set of privileges and are restricted to the sandbox. Finally, never sign any code that is incomprehensible or unaudited.

Exceptions

ENV00-J-EX1: An organization that has an internal PKI and uses code signing for internal development activities (such as to facilitate facilitating code - check-in and track developerstracking developer activity) may sign unprivileged code. This codebase should however, not be carried forward to the a production environment. The keys used for internal signing must not be distinct from those used to ship the productsto sign externally available code.

ENV00-J-EX2: Oracle has deprecated the use of unsigned applets and will soon cease to support them. Applets that are signed have traditionally been run with full privileges. Since Java 1.7.0 update 21, Oracle has provided mechanisms to allow applets to be signed and yet run without full permissions. This enables applets that are today unsigned to continue to run in a security sandbox despite being signed. Signing an applet that runs with restricted privileges under versions of Java at least as recent as update 21 constitutes an exception to this rule. For more information, see Signed Java Applet Security Improvements on the CERT/CC blog.

Risk Assessment

Signing unprivileged code violates the principle of least privilege as because it can circumvent security restrictions defined by the security policies of applets and JNLP applications, for example.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[Schneier 00|AA. Java References#Schneier 00]\]
\[[McGraw 00|AA. Java References#McGraw 00]\] Appendix C: Sign Only Privileged Code
\[[Dormann 08|AA. Java References#Dormann 08]\]

Detecting code that should be considered privileged or sensitive requires programmer assistance. Given identified privileged code as a starting point, automated tools could compute the closure of all code that can be invoked from that point. Such a tool could plausibly determine whether a body of signed code both includes that entire closure and excludes all other code.

Related Guidelines

Android Implementation Details

The Android system uses code signing as a means to identify the author of an application and establish trust relationships between applications, not as a means to grant elevated privileges to code.

Bibliography

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Image Added Image Added Image AddedSEC09-J. Prefer using SSLSockets over Sockets for secure data exchange      01. Platform Security (SEC)      ENV30-J. Always use a Security Manager