url decoding is performed early on x-www-form-urlencoded buffer

[innodron]

Description

Any text input field containing '&' (encoded %26) is wrongly parsed as further arguments because parsing.cpp is performing urlDecode() on the entire buffer for "application/x-www-form-urlencoded" type header. Instead, url decoding should be executed on the parsed arguments themselves in _parseArguments().
(Indeed 2.4.0-rc.1 _parseArguments() is already performing urlDecode() on the arguments parsed)

ESP8266WebServer/src/Parsing.cpp
185:      if (contentLength > 0) {
186:        if (searchStr != "") searchStr += '&';
187:        if(isEncoded){
188:          //url encoded form
189:          String decoded = urlDecode(plainBuf); <-- Cause of error

hence 
plainBuf: arg0=arg0_value&arg1=arg1_value&arg2=inner_arg0%3Dinner_arg0_value%26inner_arg1%3Dinner_arg1_value%26inner_arg2%3Dinner_arg2_value
becomes
decoded : arg0=arg0_value&arg1=arg1_value&arg2=inner_arg0=inner_arg0_value&inner_arg1=inner_arg1_value&inner_arg2=inner_arg2_value

which then parsed in _parseArguments() as:

arg0       : arg0_value
arg1       : arg1_value
arg2       : inner_arg0=inner_arg0_value
inner_arg1 : inner_arg1_value
inner_arg2 : inner_arg2_value

though it should have been parsed to:

arg0       : arg0_value
arg1       : arg1_value
arg2       : inner_arg0=inner_arg0_value&inner_arg1=inner_arg1_value&inner_arg2=inner_arg2_value

I fixed the issue temporarily as following:

185:      if (contentLength > 0) {
186:        if (searchStr != "") searchStr += '&';
187:        //if(isEncoded){
188:          //url encoded form
189:        //  String decoded = urlDecode(plainBuf); --> plainBuf should not be decoded. Parameters should be decoded instead
190:        //  size_t decodedLen = decoded.length();
191:        //  memcpy(plainBuf, decoded.c_str(), decodedLen);
192:        //  plainBuf[decodedLen] = 0;
193:          searchStr += plainBuf;
194:        //}

though it may need a finer resolution by the original author(s)

Hardware

Hardware: Sparkfun ESP8266 Thing Developer
Core Version: 2.4.0-rc.1

Settings in IDE

Module: Sparkfun ESP8266 Thing Developer
Flash Size: 512
CPU Frequency: 80Mhz
Flash Mode: qio
Flash Frequency: 40Mhz
Upload Using: SERIAL
Reset Method: nodemcu

[innodron]

Issue moved to esp8266/Arduino/issues as esp8266/Arduino#3669