eBay is racing to fix a second serious security flaw that may allow criminals to spread malware through files seemingly hosted by the online tat bazaar.

The bug is closely related to an earlier one discovered by David Sopas, a researcher at security firm WebSegura in Portugal. The hole uncovered by Sopas, since closed by eBay, could have been exploited to fool an eBay user into downloading and opening a file from a trusted eBay website – a file that could be an executable that installs malware. Users of IE 8 and 9 are particularly vulnerable, we're told.

"When using eBay and inspecting its requests, I noticed a call to a JSON file that made me wonder a bit about a security vulnerability – Reflected Filename Download," he said in an advisory, which has since been taken offline.

"To the victim the entire process looked like a file was offered for download by eBay trusted domain and it would not raise any suspicious. A malicious user could gain total control over a victim's computer and launch multiple attacks."

Sopas told the ThreatPost blog that he'd spotted similar bugs in Facebook and Instagram, vulnerabilities that have since been fixed.

Reflected Filename Download flaws are fairly common, he opined, but fixing them can be tricky. The attack typically involves smuggling a malicious executable batch file in a semi-legit-looking URL, which could be sent in phishing email spam. Clicking on the link downloads the data to disk, which when opened by a naive user will starts the installation of software nasties.

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Sonas told The Register that he had informed eBay of the problem with one of its web pages, and held back from fully disclosing the vulnerability until the online souk had fixed the hole and given him the go-ahead. eBay told him he was fine to publish, but then asked him to pull the advisory because it had found a "major security issue related to [the original] discovery."

There's no response from eBay so far as to when the new vulnerability will be fixed, so for the moment avoid running any executable file seemingly downloaded from the e-swap-meet. We'll keep you posted. ®