Vulnerabilities mentioned in the table above are addressed in the following security updates. Exception is Exchange 2013 CU23 which seemingly only gets fixed for CVE-2021-26427; it is unclear if that is because of Exchange 2013’s lifecycle phase or because the problem does not exist in those builds.
More detailed information can be found at the original blog post here. Check the KB articles for any known release notes, such as the possible cross-forest Free/Busy issue and HTTP headers containing version information.
Be advised that these security updates are Cumulative Update level specific. You cannot apply the update for Exchange 2019 CU11 to Exchange 2019 CU10. Also, the security update download has the same name for different Cumulative Updates, and I would suggest tagging the file name with the CU level, e.g. Exchange2019-CU10-KBXXXXXX-x64-en.msp.
As a reminder, run the Security Update from an elevated command prompt to prevent issues during installation. In other words: Do not just double-click on the .MSP file. And on a final note, as with any patch or update, I’d recommend to apply this in a test environment first, prior to implementing it in production. However, it is not recommended to wait for regular maintenance cycles when it concerns security updates, and follow a more agile approach; the ratings are an indication of the urgency.
When looking at the MSRC information, you will notice 3 additional CVE issues addressed for July 13th. However, as far as I can see CVE-2021-34473, CVE-2021-34523 and CVE-2021-33766 were addressed in the April 2021 and eventually the May 2021 Security Updates, which also would explain MSRC’s mention of earlier CUs, such as Exchange 2019 CU8.
CVE-2021-33768 does not seem applicable to Exchange 2019 CU9 or Exchange 2016 CU20.
CVE-2021-34470 is only addressed in the security update for Exchange 2013 CU23.
More detailed information can be found at the original blog post here, which mentions some specific post-deployment instructions:
When running n-1 CU of Exchange 2019 (CU9) or Exchange 2016 (CU20), and you do not plan to upgrade to the latest CU yet but do wish to install this Security Update, you must also update the AD Schema using the CU10 or CU21 installation files.
When you are running Exchange 2013 CU23 in your organization, and no later Exchange builds are present, you need to deploy a schema update immediately after deploying the Security Update. After deploying the SU, from an elevated CMD prompt, run Setup.exe /PrepareSchema /IAcceptExchangeServerLicenseTerms from Exchange’s bin folder. You you need to separate the update from deploying the update, see end of article for a tip.
The blog also mentions some issues, which are identical to the ones mentioned with the May 2021 Security Updates:
Accounts ending in ‘$’ cannot use EMS or access the ECP.
Cross-forest Free/Busy might stop working resulting in 400 Bad Request (solution).
Running cmdlets against EMC using invoked runspace might result in no-language mode error (info).
Be advised that these security updates are Cumulative Update level specific. You cannot apply the update for Exchange 2019 CU9 to Exchange 2019 CU8. Also, the security update download has the same name for different Cumulative Updates, and I would suggest tagging the file name with the CU level, e.g. Exchange2019-CU9-KBXXXXXX-x64-en.msp.
On another note, after deploying the security updates Exchange will start reporting its version number in the HTTP response header.
As a reminder, run the Security Update from an elevated command prompt to prevent issues during installation. In other words: Do not just double-click on the .MSP file. And on a final note, as with any patch or update, I’d recommend to apply this in a acceptance environment first, prior to implementing it in production. However, it is not recommended to wait for regular maintenance cycles when it concerns security updates, and follow a more agile approach. The rating implies a form of urgency.
OWA/ECP and HMAC errors There are reports of the Security Update breaking OWA/ECP. Symptoms are browsers displaying an HMAC error:
Server Error in '/owa' Application.
ASSERT: HMACProvider.GetCertificates:protectionCertificates.Length<1
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: Microsoft.Exchange.Diagnostics.ExAssertException: ASSERT: HMACProvider.GetCertificates:protectionCertificates.Length<1
It is likely related to “Microsoft Exchange Server Auth Certificate”, which can be expired, invalid or for other reasons not being picked up. The reported solution is renewing the “Microsoft Exchange Server Auth Certificate”. This procedure can be found here. Do note that it may take an hour for the certificate to become effective. Meanwhile, you can check the comments in the original Exchange Team post, which is lively with feedback and responses.
Exchange 2013 CU23 SU & Schema Updating Because with Exchange 2013 CU23 schema preparation needs to occur immediately after deploying the SU on (the first) Exchange 2013 CU23 server, a tip might be that you could deploy Exchange 2013 CU23 Management Tools on a workstation, install the SU on that workstation, then run the PrepareSchema from there before deploying the SU on any Exchange 2013 CU23 server.
This might also be helpful in multi-domain organizations, or organizations where AD and Exchange are managed by different teams or require separate changes. Note that performing the schema update this way requires Visual C++ 2012 Runtime, otherwise you will run into a “Exchange Server setup didn’t complete the operation” and the ExchangeSetup.log will contain “Could not load file or assembly ‘Microsoft.Exchange.CabUtility.dll”.
More detailed information can be found at the original blog post here, which also mentions some known issues and workarounds which you might encounter after deploying these updates.
Be advised that these security updates are Cumulative Update level specific. You cannot apply the update for Exchange 2019 CU9 to Exchange 2019 CU8. Also, the security update download has the same name for different Cumulative Updates, and I would suggest tagging the file name with the CU level, e.g. Exchange2019-CU9-KB5003435-x64-en.msp.
Also, run the Security Update from an elevated command prompt, to prevent issues during installation (other words: Do not just double-click on the .MSP file). And on a final note, as with any patch or update, I’d recommend to apply this in a acceptance environment first, prior to implementing it in production. However, it is not recommended to wait for regular maintenance cycles when it concerns security updates, and follow a more agile approach. The rating implies a form of urgency.
15Apr2021: Added note about Pwn2Own vulnerabilities not being addressed by these updates.
A quick blog on April’s security updates for Exchange Server 2013 up to 2019. Details regarding these vulnerabilities are confidential, but organizations are recommended to install these updates based on their rating. With patching procedures still fresh in everyone’s memory, and every Exchange on-premises server being current after the Hafnium issues, that should not be a problem, right?
The fixes address the following Remote Code Execution vulnerabilities:
More detailed information can be found at the original blog post here. Note that the recently discovered at the Pwn2Own 2021 contest are not (yet) addressed by these updates, according to this blog by the contest organizers.
The exploit can be fixed by single security update, which you can find below.
Be advised that these security updates are Cumulative Update level specific. You cannot apply the update for Exchange 2016 CU20 to Exchange 2016 CU19. Also, the security update download has the same name for different Cumulative Updates, and I would suggest tagging the file name with the CU level, e.g. Exchange2019-CU9-KB5001779-x64-en.msp.
Also, run the Security Update from an elevated command prompt, to prevent issues during installation (other words: Do not just double-click on the .MSP file). And on a final note, as with any patch or update, I’d recommend to apply this in a acceptance environment first, prior to implementing it in production. However, it is not recommended to wait for regular maintenance cycles when it concerns security updates, and follow a more agile approach. The rating implies a form of urgency.
Another month, another set of security updates for Exchange Server 2016 and 2019, including out-of-band updates for Exchange 2013 CU23 and Exchange 2010 SP3 (Rollup 32). Given the risk of this vulnerability, security updates for older out-of-support CUs (Ex2016 CU8 was released December 2017) were also made available. According to the related Exchange team blog, these exploits are seen being used as part of an attack chain. After publication of this vulnerability named Hafnium, proof of concept kits were published after which variations started to appear (e.g. DearCry). Needless to say, the security update is critical and deployment should not be postponed – intermediate mitigations (with consequences) are also available.
These fixes address the following Remote Code Execution vulnerabilities:
The exploit can be fixed by security update, or in case of Exchange 2010 SP3 by applying a Rollup, which you can find in the table below per current Exchange version. Microsoft published security updates for older CUs as well on March 8th; these have been added to the table below.
You may not be prompted for a reboot, but one is required.
When manually installing the update use an elevated command prompt, don’t just double-click the .msp. To apply an .msp from an elevated prompt, e.g. msiexec.exe /p <Full Path to File>.
When you need to update to a more current Cumulative Update first, update using an elevated command prompt, e.g. setup.exe /m:upgrade /IAcceptExchangeServerLicenseTerms
Per product group feedback, Exchange 2010 is not vulnerable to the same attack chain as Exchange 2013/2016/2019, hence the Rollup mentioning a single CVE.
When running product levels earlier than the ones patched, i.e. Exchange 2016 CU17, you are at risk. There are no patches for earlier product levels, so you need to update to a recent CU after which you can install the security update.
When installing a recent CU first in order to be able to install the security update, reboot after installing the CU, then install the security update. This prevents issues caused by files being locked or updating files pending replacement during reboot.
When you are significantly behind regarding keeping your Exchange servers up to date, the blog Upgrade Paths for CU’s and .NET might help in determining an update strategy.
The statement to stay up to date with at most CU n-1 is not some random adage; apart from features and fixes, it also allows you to quickly respond to these type of emergencies.
Make sure you have configured proper Anti-Virus/Malware exclusions for Exchange server, as documented here for Exchange 2016/2019. I’ve seen significant delays or even hangs during setup of Cumulative Updates because of paths and processes not being excluded. When running Exchange virtually, any I/O inspection running on top of your hypervisor is also considered anti-virus/malware software, such as Trend Micro Deep Inspection on VMWare.
When deploying CU(n) on top of CU(n-1) when an interim update already has been installed, it is recommended to uninstall the IU prior to deploying CU(n). While it might go through, an abort is likely with mention of detecting an IU (INTERIMUPDATEDETECTED) in Exchange Setup log.
Security Updates are Cumulative Update level specific. You cannot apply the update for Exchange 2016 CU18 to Exchange 2016 CU19. Note that the security update file has the same name for different Cumulative Updates; I would suggest tagging the file name with the CU level, e.g. Exchange2016-CU18-KB5000871-x64-en.msp.
The publication of security updates for some older CUs does not remove the necessity to update and patch with current CUs.
There is also official communication to support this update, including steps to remediate issues with updates and steps to perform analysis (many people overlook the recommendation to run the update elevated for some reason). This deck can be found here: March 2021 Exchange Server Security Update – v1.2.65 – EN.pdf (thanks Chris Lehr).
Mitigations I would also recommend the official follow-up post, which not only has been updated since the original post, but also includes mitigations for organizations which cannot deploy the update yet:
A script to configure IIS rewrite rules to block cookies used in the attack (mitigates CVE-2021-26855).
Disabling UM Services (mitigates CVE-2021-26857).
Disabling ECP application pool (mitigates CVE-2021-27065).
Disabling OAB application pool (addresses CVE-2021-26858).
Needless to say, steps like disabling ECP or OAB impacts client functionality.
Finally Since some people are discovering artifacts of HAFNIUM dating before Microsoft’s official communication, people have been wondering how long this has been going on. For those interested, Krebson Security has published an article with a concise timeline of the events related to this attack.
