An attacker could exploit the vulnerability to compromise the victim accounts, change their email settings and to perform other malicious activities.
The vulnerability resides in the webmail version of the Yahoo service that failed to filter the malicious code in HTML emails.
Pynnönen reported the bug to Yahoo in early December 2018 and the bug was fixed by Yahoo in January and the researcher doesn’t disclose any technical details and he was awarded $10,000 for this bug.
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This is not the first time Pynnönen find’s stored XSS in Yahoo mail, early he discovered a stored XSS in December 2015, which allows an attacker to send a
One year later he found another stored XSS vulnerability with the web version of the Yahoo Mail service due to improper sanitization of codes in HTML emails.