The open letter is thought to have been circulated on pro-ISIS websites, despite the fact the woman could be killed for voicing discontent from within the terror organisation's stronghold in Syria.
Anyone who dares to criticise ISIS from the inside is branded 'murtad', meaning apostate, and face being whipped, tortured and even being executed.
The woman, who is believed to be a Westerner, calls herself al-Muhajirahm in the letter which was posted online under the title 'A Reminder to the Leaders of the ISIS on January 27.
Voice of America reports she complains about the treatment of some widows within ISIS, who she says are not given the support they need after their husbands kill themselves.
She wrote: 'Imagine that you've helped a sister who requested zakat (charity) two days ago, but you have ignored the sister who has been waiting a month before.
'She cries every night, concerned about how to feed her children as her husband is killed.
Dissatisfaction from within ISIS is rarely voiced, presumably because of the ramifications faced by those who speak out.
The letter was posted on JustPaste.It and was spotted by Washington-based Middle East Media Research Institute (MEMRI), a group that monitors online terror activity.
'Such criticism is almost never found in such a widely circulated document,' Anat Agron, a MEMRI researcher told media. 'In the past, ISIS members have publicly criticized aspects of life in the ISIS, however, normally such posts were swiftly deleted.'
Meanwhile, ISIS have unveiled a new terrorists training camp in the Egyptian desert as the group continues to pose a threat in the Sinai peninsula.
The images were taken at the Abu Hajr al-Masri training camp, named after a former Egyptian terrorist and is situated in the remote Sinai desert, Daily Mail reported.
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