CVE-2022-41725

A denial of service is possible from excessive resource consumption in
net/http and mime/multipart. Multipart form parsing with
mime/multipart.Reader.ReadForm can consume largely unlimited amounts of
memory and disk files. This also affects form parsing in the net/http
package with the Request methods FormFile, FormValue, ParseMultipartForm,
and PostFormValue. ReadForm takes a maxMemory parameter, and is documented
as storing “up to maxMemory bytes +10MB (reserved for non-file parts) in
memory”. File parts which cannot be stored in memory are stored on disk in
temporary files. The unconfigurable 10MB reserved for non-file parts is
excessively large and can potentially open a denial of service vector on
its own. However, ReadForm did not properly account for all memory consumed
by a parsed form, such as map entry overhead, part names, and MIME headers,
permitting a maliciously crafted form to consume well over 10MB. In
addition, ReadForm contained no limit on the number of disk files created,
permitting a relatively small request body to create a large number of disk
temporary files. With fix, ReadForm now properly accounts for various forms
of memory overhead, and should now stay within its documented limit of 10MB

• maxMemory bytes of memory consumption. Users should still be aware that
  this limit is high and may still be hazardous. In addition, ReadForm now
  creates at most one on-disk temporary file, combining multiple form parts
  into a single temporary file. The mime/multipart.File interface type’s
  documentation states, “If stored on disk, the File’s underlying concrete
  type will be an *os.File.”. This is no longer the case when a form contains
  more than one file part, due to this coalescing of parts into a single
  file. The previous behavior of using distinct files for each form part may
  be reenabled with the environment variable GODEBUG=multipartfiles=distinct.
  Users should be aware that multipart.ReadForm and the http.Request methods
  that call it do not limit the amount of disk consumed by temporary files.
  Callers can limit the size of form data with http.MaxBytesReader.

Bugs

• <https://go.dev/issue/58006&gt;

Notes