The cross-site scripting flaw was found by Shahin Ramezany, who goes by the nickname "Abysssec." The vulnerability can allow an attacker to harvest a victim's cookie for their Yahoo account if the victim is successfully tricked into clicking on a malicious link.
The vulnerability was patched by Yahoo on Monday, but penetration testing company Offensive Security and Ramezany say that the patch did not fix the problem.
"With little modification to the original proof-of-concept code written by Abysssec, it is still possible to exploit the original Yahoo vulnerability, allowing an attacker to completely take over a victim's account," Offensive Security wrote on its blog.
A Yahoo spokeswoman did not have an immediate comment when contacted Wednesday.
Offensive Security hosted a video showing how the attack works but left out details that might allow attackers to replicate it. The company said XSS filters provide little defense against an attack and warned that people should be wary of clicking on links within emails until Yahoo fixes the vulnerability.