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Skype squishes cross-zone scripting bug

Underlying weakness nixed

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Skype has fixed a cross-zone scripting weakness in its voice over IP client software which spawned a couple of security bugs over recent weeks.

Problems have arisen for Windows users because Skype uses Internet Explorer web controls to render internal and external HTML pages.

Skype is running these web controls in Local Zone and, worse, accessing HTML pages in an unlocked Local Zone mode, an approach that opens the door to so-called cross-zone scripting exploits.

For example, the use of vulnerable controls made it possible to inject a malicious script into the "Add video to chat" dialogue on video-sharing sites such as Skype partner DailyMotion. Skype was initially obliged to block the feature after the vulnerability was discovered last month.

A similar vulnerability in the SkypeFind feature, which lets users recommend businesses to others running the VoIP client, also stemmed from the same underlying cross-zone scripting weakness. Skype patched the feature when the SkypeFind problem came up last week, but the underlying issue remained.

Tuesday brought the arrival of a more complete fix that addresses the underlying architectural weakness involved in both the SkypeFind and DailyMotion security flaps.

Skype said it fixed the core vulnerability by setting IE control security context to Internet Zone (instead of local zone, as previously implemented). Windows users need to update to Skype for Windows version 3.6.*.248 or later, as explained in an advisory here.

More background on cross-zone scripting vulnerabilities can be found in postings by security researcher Aviv Raff, who's kept a close watching brief on the issue over recent weeks. ®

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