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Microsoft announced on Friday that it had made additional improvements to the mitigation mechanism available to thwart exploitation efforts against the recently discovered unpatched security weaknesses in Exchange Server.

To that end, Microsoft updated the blocking rule in IIS Manager from “.*autodiscover.json.*Powershell.*” to “(?=.*autodiscover.json)(?=.*powershell).”

The new instructions for adding the URL Rewrite rule are listed below as per the Microsoft bulletin:

  1. Launch IIS Manager.
  2. Choose Default Web Site.
  3. Click URL Rewrite in the Feature View.
  4. Click Add Rule(s) on the right-hand side of the Actions pane.
  5. Click OK after selecting Request Blocking.
  6. Include the phrase “(?=.*autodiscover.json)(?=.*powershell)” (excluding quotes).
  7. Under Using, choose Regular Expression.
  8. Select Abort Request from the How to Block menu, then click OK.
  9. Expand the rule and choose the rule that contains the pattern: (?=.*autodiscover.json)(?=.*powershell) and select Edit from the Conditions menu.
  10. Change the URL condition entry to UrlDecode:REQUEST URI and then click OK.

Users can also achieve the required security measures by running the PowerShell-based Exchange On-premises Mitigation Tool (EOMTv2.ps1), which has been upgraded to account for the aforementioned URL pattern.

The actively exploited vulnerabilities, known as ProxyNotShell (CVE-2022-41040 and CVE-2022-41082), have yet to be fixed by Microsoft. However, with Patch Tuesday approaching, the wait may not be long.

If the weaknesses are successfully weaponized, an authorized attacker might use the two vulnerabilities to gain remote code execution on the underlying server.

The IT titan admitted this week that a single state-sponsored attacker could have exploited the vulnerabilities as early as August 2022 in limited, targeted attacks on fewer than ten organizations worldwide.

Update: Microsoft said over the weekend that it had made another change to the URL string – “(?=.*autodiscover)(?=.*powershell)” – which will be added to the blocking rule in IIS Manager to prevent exploitation attempts.

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