CWE-434 危险类型文件的不加限制上传

Unrestricted Upload of File with Dangerous Type

结构: Simple

Abstraction: Base

状态: Draft

被利用可能性: Medium

基本描述

The software allows the attacker to upload or transfer files of dangerous types that can be automatically processed within the product's environment.

相关缺陷

• cwe_Nature: ChildOf cwe_CWE_ID: 669 cwe_View_ID: 1000 cwe_Ordinal: Primary

• cwe_Nature: ChildOf cwe_CWE_ID: 669 cwe_View_ID: 1003 cwe_Ordinal: Primary

• cwe_Nature: PeerOf cwe_CWE_ID: 351 cwe_View_ID: 1000

• cwe_Nature: PeerOf cwe_CWE_ID: 436 cwe_View_ID: 1000

• cwe_Nature: PeerOf cwe_CWE_ID: 430 cwe_View_ID: 1000

适用平台

Language: [{'cwe_Name': 'ASP.NET', 'cwe_Prevalence': 'Sometimes'}, {'cwe_Name': 'PHP', 'cwe_Prevalence': 'Often'}, {'cwe_Class': 'Language-Independent', 'cwe_Prevalence': 'Undetermined'}]

Paradigm: {'cwe_Name': 'Web Based', 'cwe_Prevalence': 'Undetermined'}

Technology: {'cwe_Name': 'Web Server', 'cwe_Prevalence': 'Sometimes'}

常见的影响

检测方法

Dynamic Analysis with Automated Results Interpretation

Dynamic Analysis with Manual Results Interpretation

Manual Static Analysis - Source Code

Automated Static Analysis - Source Code

Architecture or Design Review

可能的缓解方案

Architecture and Design

策略:

Generate a new, unique filename for an uploaded file instead of using the user-supplied filename, so that no external input is used at all.[REF-422] [REF-423]

MIT-21 Architecture and Design

策略: Enforcement by Conversion

When the set of acceptable objects, such as filenames or URLs, is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames or URLs, and reject all other inputs.

Architecture and Design

策略:

Consider storing the uploaded files outside of the web document root entirely. Then, use other mechanisms to deliver the files dynamically. [REF-423]

MIT-5 Implementation

策略: Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a whitelist of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs (i.e., do not rely on a blacklist). A blacklist is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, blacklists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright. For example, limiting filenames to alphanumeric characters can help to restrict the introduction of unintended file extensions.

Architecture and Design

策略:

Define a very limited set of allowable extensions and only generate filenames that end in these extensions. Consider the possibility of XSS (CWE-79) before allowing .html or .htm file types.

Implementation

策略: Input Validation

Ensure that only one extension is used in the filename. Some web servers, including some versions of Apache, may process files based on inner extensions so that "filename.php.gif" is fed to the PHP interpreter.[REF-422] [REF-423]

Implementation

策略:

When running on a web server that supports case-insensitive filenames, perform case-insensitive evaluations of the extensions that are provided.

MIT-15 Architecture and Design

策略:

For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.

Implementation

策略:

Do not rely exclusively on sanity checks of file contents to ensure that the file is of the expected type and size. It may be possible for an attacker to hide code in some file segments that will still be executed by the server. For example, GIF images may contain a free-form comments field.

Implementation

策略:

Do not rely exclusively on the MIME content type or filename attribute when determining how to render a file. Validating the MIME content type and ensuring that it matches the extension is only a partial solution.

MIT-17 ['Architecture and Design', 'Operation']

策略: Environment Hardening

Run your code using the lowest privileges that are required to accomplish the necessary tasks [REF-76]. If possible, create isolated accounts with limited privileges that are only used for a single task. That way, a successful attack will not immediately give the attacker access to the rest of the software or its environment. For example, database applications rarely need to run as the database administrator, especially in day-to-day operations.

MIT-22 ['Architecture and Design', 'Operation']

策略: Sandbox or Jail

Run the code in a "jail" or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict which files can be accessed in a particular directory or which commands can be executed by the software. OS-level examples include the Unix chroot jail, AppArmor, and SELinux. In general, managed code may provide some protection. For example, java.io.FilePermission in the Java SecurityManager allows the software to specify restrictions on file operations. This may not be a feasible solution, and it only limits the impact to the operating system; the rest of the application may still be subject to compromise. Be careful to avoid CWE-243 and other weaknesses related to jails.

示例代码

例

The following code intends to allow a user to upload a picture to the web server. The HTML code that drives the form on the user end has an input field of type "file".

good HTML

Once submitted, the form above sends the file to upload_picture.php on the web server. PHP stores the file in a temporary location until it is retrieved (or discarded) by the server side code. In this example, the file is moved to a more permanent pictures/ directory.

bad PHP

The problem with the above code is that there is no check regarding type of file being uploaded. Assuming that pictures/ is available in the web document root, an attacker could upload a file with the name:

attack

Since this filename ends in ".php" it can be executed by the web server. In the contents of this uploaded file, the attacker could use:

attack PHP

Once this file has been installed, the attacker can enter arbitrary commands to execute using a URL such as:

attack

which runs the "ls -l" command - or any other type of command that the attacker wants to specify.

例

The following code demonstrates the unrestricted upload of a file with a Java servlet and a path traversal vulnerability. The HTML code is the same as in the previous example with the action attribute of the form sending the upload file request to the Java servlet instead of the PHP code.

good HTML

When submitted the Java servlet's doPost method will receive the request, extract the name of the file from the Http request header, read the file contents from the request and output the file to the local upload directory.

bad Java

As with the previous example this code does not perform a check on the type of the file being uploaded. This could allow an attacker to upload any executable file or other file with malicious code.

Additionally, the creation of the BufferedWriter object is subject to relative path traversal (CWE-22, CWE-23). Depending on the executing environment, the attacker may be able to specify arbitrary files to write to, leading to a wide variety of consequences, from code execution, XSS (CWE-79), or system crash.

分析过的案例

Notes

Relationship

Research Gap PHP applications are most targeted, but this likely applies to other languages that support file upload, as well as non-web technologies. ASP applications have also demonstrated this problem.

分类映射

相关攻击模式

• CAPEC-1

引用

• REF-422 Dynamic File Uploads, Security and You

• REF-423 8 Basic Rules to Implement Secure File Uploads

• REF-424 Top 25 Series - Rank 8 - Unrestricted Upload of Dangerous File Type

• REF-76 Least Privilege

• REF-62 The Art of Software Security Assessment

• REF-962 Automated Source Code Security Measure (ASCSM)