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mime/multipart: Allow limiting maximum amount of part's header data #26339

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andrius4669 opened this issue Jul 11, 2018 · 2 comments
Open

mime/multipart: Allow limiting maximum amount of part's header data #26339

andrius4669 opened this issue Jul 11, 2018 · 2 comments
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@andrius4669
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@andrius4669 andrius4669 commented Jul 11, 2018

This is feature request, to allow user of mime/multipart package's Reader to specify limit of header data loaded into memory when calling NewPart() function.
Reader.NewPart() calls net/textproto package's Reader.ReadMIMEHeader() function which without any limitation loads all header data into memory.
mime/multipart's Reader is also used in net/http package to parse multipart/form-data POST requests, which means that functions like Request.ParseMultipartForm, Request.FormValue, Request.FormFile, Request.MultipartReader are affected by this.
I believe this can be used as remote denial of service attack vector.
Possible solution for this would be limiting HTTP request body size (done by default in some servers like nginx), but this is unsuitable for cases where server needs to accept POST requests with big files (for example, file hosting service).
Similar case like this is net/http package's Server's MaxHeaderBytes field, which limits ammount of header data, but not body.

@andrius4669
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@andrius4669 andrius4669 commented Jul 11, 2018

To illustrate issue, example client code exploiting this weakness: https://www.zerobin.net/?8db1a70f1608fb27#TzogmA1w8sPRNSnByLRcRrwHY0D7urscLFil6AbDdQA=
And test server's code affected by it: https://www.zerobin.net/?6f1057f7dc78ba55#4/v39opPJuwZyqEF8xft8Iqvmkv0QPyhWC189EZy+HE=
This can cause out of memory conditions therefore I recommend turning off swap on machines which run test server and running test server with nice -19.

@ianlancetaylor ianlancetaylor changed the title mime/multipart: Allow limiting maximum ammount of part's header data mime/multipart: Allow limiting maximum amount of part's header data Jul 11, 2018
@ianlancetaylor ianlancetaylor added the NeedsInvestigation label Jul 11, 2018
@ianlancetaylor ianlancetaylor added this to the Unplanned milestone Jul 11, 2018
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@ianlancetaylor ianlancetaylor commented Jul 11, 2018

See also #20169.

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