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[R1] Microsoft Windows SMB v1 Service Principal Name Handling Remote Buffer Overflow

High

Synopsis

Microsoft Windows 2008 and Vista (and possible Windows 2003) contain a buffer overflow condition in the SMB v1 functionality. The issue is triggered as user-supplied input is not properly handled when processing a long Service Principal Name (SPN) in [MS-NLMP].AUTHENTICATE_MESSAGE.Payload.NtChallengeResponse.NTLMv2_CLIENT_CHALLENGE.AvPairs.MsvAvTargetName such that ((sizeof(SPN) + 2) * 2) % 256) is in range of (0x30, ERROR_LOG_MAXIMUM_SIZE). With a specially crafted NTLMSSP_AUTHENTICATE message embedded in a SMBv1 SessionSetupAndX request, a remote authenticated attacker can cause a buffer overflow, resulting in a denial of service or potentially allowing the execution of arbitrary code.

When a Server SPN is supplied by the client, the SmbServerNameHardeningLevel registry key is set to 1 or 2, and user credentials are correct, the SrvAdminCheckSpn() function in srvnet.sys is called to check the SPN. When the SPN check fails, a SMB version 1 server calls the SrvLibLogSpnError() logging function (using the Eventlog message template in netevent.dll.mui), which in turn calls the SrvLibLogError(), the vulnerable function.

On newer OSes (i.e., Windows 7), it seems that message template is not available, and an SPN error is handled differently.

Requirements:

  • Regular unprivileged Windows user credentials (i.e., AcceptSecurityContext() returns SEC_E_OK)
  • HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\LanmanServer\Parameters\smbservernamehardeninglevel >= 1
  • Windows Platforms (i.e., Windows 2008 and older OSes) supporting Eventlog message:
    "Authentication failed for SMB v %4. The server detected an invalid or NULL SPN (%2) while authenticating SPN at hardening level %3."

Solution

Microsoft has released a patch (MS15-083) to address this vulnerability. There are no known upgrades to correct this issue.

Proof of Concept

Example (in unicode): <pre><code>cifs/fe80::5efe:123.123.123.123%AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA</code></pre>

Disclosure Timeline

2015-05-06 - Issue discovered
2015-05-07 - Vendor Informed
2015-05-08 - Vendor Acknowledged, MSRC Case 30,196 TRK:0001321 Assigned
2015-06-23 - Asked for update from MSRC
2015-06-24 - Confirmed vulnerability, working on root cause analysis. Late summer / early fall rough ETA for patch.
2015-08-11 - Vendor issues patch (MS15-083)

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