Unrestricted File Upload OWASPThis is a Vulnerability. To view all vulnerabilities, please see the Vulnerability Category page. Last revision mmddyy 0. Description. Uploaded files represent a significant risk to applications. The first step in many attacks is to get some code to the system to be attacked. Then the attack only needs to find a way to get the code executed. Using a file upload helps the attacker accomplish the first step. The consequences of unrestricted file upload can vary, including complete system takeover, an overloaded file system or database, forwarding attacks to back end systems, client side attacks, or simple defacement. It depends on what the application does with the uploaded file and especially where it is stored. Pdfcrowd is a WebHTML to PDF online service. Convert HTML to PDF online in the browser or in your PHP, Python, Ruby. NET, Java apps via the REST API. Uploaded files represent a significant risk to applications. The first step in many attacks is to get some code to the system to be attacked. Then the. Online file conversion, covering a wide range of different image, document, music, video and compression formats. Free for files up to 100MB, and premium services. There are really two classes of problems here. Download Air Force Commissioned Officer Training Manual. The first is with the file metadata, like the path and file name. These are generally provided by the transport, such as HTTP multi part encoding. This data may trick the application into overwriting a critical file or storing the file in a bad location. You must validate the metadata extremely carefully before using it. The other class of problem is with the file size or content. How To Upload A Pdf File To Html' title='How To Upload A Pdf File To Html' />The range of problems here depends entirely on what the file is used for. See the examples below for some ideas about how files might be misused. To protect against this type of attack, you should analyse everything your application does with files and think carefully about what processing and interpreters are involved. Risk Factors The impact of this vulnerability is high, supposed code can be executed in the server context or on the client side. The likelihood of detection for the attacker is high. The prevalence is common. As a result the severity of this type of vulnerability is high. It is important to check a file upload modules access controls to examine the risks properly. Server side attacks The web server can be compromised by uploading and executing a web shell which can run commands, browse system files, browse local resources, attack other servers, or exploit the local vulnerabilities, and so forth. Client side attacks Uploading malicious files can make the website vulnerable to client side attacks such as XSS or Cross site Content Hijacking. T.I Why You Wanna'>T.I Why You Wanna. Uploaded files can be abused to exploit other vulnerable sections of an application when a file on the same or a trusted server is needed can again lead to client side or server side attacks Uploaded files might trigger vulnerabilities in broken librariesapplications on the client side e. Phone Mobile. Safari Lib. TIFF Buffer Overflow. Uploaded files might trigger vulnerabilities in broken librariesapplications on the server side e. Image. Magick flaw that called Image. Tragick. Uploaded files might trigger vulnerabilities in broken real time monitoring tools e. Symantec antivirus exploit by unpacking a RAR file A malicious file such as a Unix shell script, a windows virus, an Excel file with a dangerous formula, or a reverse shell can be uploaded on the server in order to execute code by an administrator or webmaster later on the victims machine. An attacker might be able to put a phishing page into the website or deface the website. The file storage server might be abused to host troublesome files including malwares, illegal software, or adult contents. Uploaded files might also contain malwares command and control data, violence and harassment messages, or steganographic data that can be used by criminal organisations. Uploaded sensitive files might be accessible by unauthorised people. File uploaders may disclose internal information such as server internal paths in their error messages. Examples. Attacks on application platform Upload. Upload. gif file to be resized image library flaw exploited Upload huge files file space denial of service Upload file using malicious path or name overwrite a critical file Upload file containing personal data other users access it Upload file containing tags tags get executed as part of being included in a web page Upload. Attacks on other systems Upload. Upload virus infected file victims machines infected Upload. Cross site Scripting XSS Upload. Flash object victim experiences Cross site Content Hijacking. Upload. rar file to be scanned by antivirus command executed on a client running the vulnerable antivirus software. Weak Protections and Bypassing Methods. Blacklisting File Extensions. This protection might be bypassed by. Finding missed extensions that can be executed on the server side or can be dangerous on the client side e. Finding flaws in a web server configuration when it parses files with double extensions or it executes them by providing a sensitive extension after a delimiter such as or character e. PHP code and has been uploaded. In Apache, a php file might be executed using the double extension technique such as file. In IIS6 or prior versions, a script file can be executed by using one of these two methods. In Windows, it is possible to create a directory by using a file uploader and ADS Alternate Data Stream. In this method, a filename that ends with IndexAllocation or I3. IndexAllocation makes the file uploader to create a directory rather than a file e. IndexAllocation creates folder. Changing a number of letters to their capital forms to bypass case sensitive rules e. Sp or file. PHp. Using Windows 8. HTACCE1 Finding characters that are converted to other useful characters during the file upload process. For instance, when running PHP on IIS, the, lt, and double quote characters respectively convert to, and. In order to include the double quote character in the filename in a normal file upload request, the filename in the Content Disposition header should use single quotes e. Finding neutral characters after a filename such as trailing spaces and dots in Windows filesystem or dot and slash characters in a Linux filesystem. These characters at the end of a filename will be removed automatically e. Although slash or backslash characters are also normally problematic characters, they can be ignored in a normal file upload request as anything before these characters may count as the directory name on the server side that said, they should be tried for a thorough test e. Finding flaws in extension detection techniques. A web server may use the first extension after the first dot. Using control characters such as null character 0x. In this method, all the strings after the Null character will be discarded when saving the files. Both URL encoded and decoded version of the null character should be tried in a file upload request for a thorough test. Using NTFS alternate data stream ADS in Windows. In this case, a colon character will be inserted after a forbidden extension and before a permitted one. As a result, an empty file with the forbidden extension will be created on the server e. This file might be edited later using other techniques such as using its short filename. The data pattern can also be used to create non empty files. Therefore, adding a dot character after this pattern might also be useful to bypass further restrictions.