Impact

Any user who can edit any page like their profile can create a custom skin with a template override that is executed with programming right, thus allowing remote code execution.

To reproduce, as a user without edit, script or admin right, add an object of class XWiki.XWikiSkins to your profile. Name it whatever you want and set the Base Skin to flamingo.
Add an object of class XWikiSkinFileOverrideClass and set the path to macros.vm and the content to:

#macro(mediumUserAvatar $username)
  #resizedUserAvatar($username 50)
  $services.logging.getLogger('Skin').error("I got programming: $services.security.authorization.hasAccess('programming')")
#end

Back to your profile, click Test this skin. Force a refresh, just in case.
If the error "Skin - I got programming: true" gets logged, the installation is vulnerable.

Patches

This has been patched in XWiki 14.10.19, 15.5.4 and 15.10RC1.

Workarounds

We're not aware of any workaround except upgrading.

References

• https://jira.xwiki.org/browse/XWIKI-21478
• 3d4dbb4 (>= 15.8 RC1)
• da177c3 (< 15.8 RC1)