File Upload Vulnerabilities

Abuse archives

There are weaknesses that exist when a file upload functionality accepts and extracts archives without proper security measures in place.

tar and zip allow you to include symlinks in tarballs/archives they generated. If an application does not properly validate the content of the archives, it can lead to arbitrary reading/writing of files.


Abusing tar permissions

If an application uses Unix tar command to extract .tar files, removes symlinks and accesses subdirectory directly, you can try to bypass the symlink removing process with tar permissions. Unix tar command preserves the unix permissions assigned to it while creating the archive. If you create a parent directory which no one have read permissions (set chmod to 300) while creating the subdirectory with the complete permissions (set the chmod to 700), you can include symlinks inside the subdirectory that will not be found during the symlink removing process, but will be found when accessing directly since the subdirectory has read permissions.

$ mkdir parent
$ cd parent
$ tar cf a.tar . --mode=300
$ mkdir sub
$ cd sub
$ ln -s /etc/passwd file.txt
$ cd ..
$ tar -rf a.tar sub

Zip Slip

The Zip Slip takes advantage of zips that may contain files with specifically placed payloads set to the names, that once extracted, lead to a path traversal, and can write any file to any directory the webserver has access to. It can affect numerous archive formats, including tar, jar, war, cpio, apk, rar and 7z.

Abuse filename

Path as filename

Try to use different kinds of path as a filename:

  • Absolute path, for example filename=/etc/passwd

  • Relative path, for example filename=../../../../../../etc/passwd

  • UNC path, for example filename=\\\file.png

Injections via filename

Try to exploit command injection or sqli via filename, for example a$(whoami)z.png, a`whoami`z.png or a';select+sleep(10);--z.png

SSRF via filename

Try to send URL as filename to get blind SSRF, for example filename= You can also try to change type="file" to type="url" within a request.

DoS via large filename

Try to upload a file with large name, sometimes it leads to DoS.


Bypass restrictions


Try to change a Content-Type value:

  • Allowed MIME-type + disallowed extension

  • Disallowed MIME-type + allowed extension

  • Remove Content-Type

  • Send Content-Type twice within request with allowed and disallowed MIME-types

Magic bytes

If an application use a file's magic bytes to deduce the Content-type, you can try to bypass security measures by forging the magic bytes of an allowed file. For example, if GIF images are allowed, you can forge a GIF image's magic bytes GIF89a to make the server think we are sending it a valid GIF.



Try to change a file extension:

  • Less-common extension, such as .phtml

  • Double extension, such as .jpg.svg or .svg.jpg

  • Extension with a delimeter, such as %0a, %09, %0d, %00, #, etc. For example, file.png%00.svg or file.png\x0d\x0a.svg

  • Empty extension, for example file.

  • Extension with varied capitalization, such as .sVG

  • Try to cut allowed extension with max filename length.

  • Empty filename, for example .svg

  • Right-to-left override, for example file.%E2%80%AEphp.jpg, see Report: RTL override symbol not stripped from file names

  • Send filename twice within request with allowed and disallowed extensions, for example filename="file.png";filename="file.svg"

Invalid regex

Windows dots

Within Windows, when a file is created with a trailing full-stop, the file is saved without said trailing character, leading to potential blacklist bypasses on Windows file uploads.

For example, if an application is rejecting files that end in .aspx, you can upload a file called shell.aspx.. Now this filename will bypass the blacklist, as .aspx != .aspx., but upon saving the file to the server, Windows will cut out the trailing ., leaving shell.aspx.

Windows ADS

An Alternate Data Stream (ADS) is a little-known feature of the NTFS file system. It has the ability of forking data into an existing file without changing its file size or functionality. In other words, ADS allows you to hide a file inside another one.

The following example hides copy of calc.exe inside file.txt:

C:> echo Somedata > file.txt
C:> type file.txt
C:> type c:\windows\system32\calc.exe > file.txt:calc.exe

To start the hidden calc.exe copy, you can run the following command:

C:> start c:\file.txt:calc.exe


Third-party vulnerabilities

Vulnerabilities in image processors




ExifTool versions 7.44 through 12.23 inclusive are vulnerable to a local command execution vulnerability when processing djvu files. If an application is accepting uploaded files, which are passed to ExifTool, it can lead to RCE.


Configuration files

Some servers/frameworks work with configuration files at runtime to define various settings and restrictions. The most famous examples are the the Apache httpd/Tomcat .htaccess and the ASP.NET/IIS web.config files. You can check your server/framework and try to upload particular config to bypass some security measures or even execute code.


Potentially dangerous files


Try to upload on an IIS server files with the asp, ashx, asmx, asa, aspx, cer or xamlx extensions to get RCE.


Adobe ColdFusion

Try to upload ColdFusion files with the cfm, cfml, cfc or dbm extensions to get RCE.

Adobe ColdFusion SSRF


Try to upload JSP files with the jsp, jspx, jsw, jsv, or jspf extensions to get RCE.


Try to upload perl files with the pl, pm, cgi, or lib extensions to get RCE.



Try to upload valid XML file with external entities to get XXE.


Race condition

File upload race condition

If an application uploads a file directly to a target folder before the file passes validation, you can abuse this behavior by using race condition.

Suppose file upload has the following flow:

  1. Upload file to a target folder

  2. Validate file

  3. If the validation fails, remove the file. Otherwise, send the link to a user

You can use race condition to fetch the file between steps 1 and 3 while the validation is in progress.


URL-based file upload race condition

If an application allows users to upload a file by providing a URL and fetches the file for validation to a user-accessible folder, you can abuse this behavior by using race condition.

Suppose file upload has the following flow:

  1. Receive the URL from a user

  2. Create a local copy for validation within a user-accessible folder

  3. Validate file

  4. If the validation fails, reject the URL. Otherwise, send the link to a user

You can use race condition to fetch the file between steps 2 and 4 while the validation is in progress.


SSRF via HTTP range requests

If an application download a file from a user-provided link with HTTP range requests you can try to redirect the request one of the chunks to an internal server.



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