Microsoft Security Bulletin MS02-051

Technical description:

The Remote Data Protocol (RDP) provides the means by which Windows systems can provide remote terminal sessions to clients. The protocol transmits information regarding a terminal sessions' keyboard, mouse and video to the remote client, and is used by Terminal Services in Windows NT 4.0 and Windows 2000, and by Remote Desktop in Windows XP. Two security vulnerabilities, both of which are eliminated by this patch, have been discovered in various RDP implementations.

The first involves how session encryption is implemented in certain versions of RDP. All RDP implementations allow the data in an RDP session to be encrypted. However, in the versions in Windows 2000 and Windows XP, the checksums of the plaintext session data are sent without being encrypted themselves. An attacker who was able to eavesdrop on and record an RDP session could conduct a straightforward cryptanalytic attack against the checksums and recover the session traffic.

The second involves how the RDP implementation in Windows XP handles data packets that are malformed in a particular way. Upon receiving such packets, the Remote Desktop service would fail, and with it would fail the operating system. It would not be necessary for an attacker to authenticate to an affected system in order to deliver packets of this type to an affected system.

Mitigating factors:

Cryptographic Flaw in RDP Protocol:

• An attacker would need the ability to capture an RDP session in order to exploit this vulnerability. In most cases, this would require that the attacker have physical access to the network media.
• Because encryption keys are negotiated on a per-session basis, a successful attack would allow an attacker to decrypt only a single session and not multiple sessions. Thus, the attacker would need to conduct a separate cryptanalytic attack against each session he or she wished to compromise.

Denial of Service in Remote Desktop:

• Remote Desktop service in Windows XP is not enabled by default.
• Even if Remote Desktop service were enabled, a successful attack would require that the attacker be able to deliver packets to the Remote Desktop port on an affected system. Customers who block port 3389 at the firewall would be protected against attempts to exploit this vulnerability. (By default Internet Connection Firewall does block port 3389).

Severity Rating:

The above assessment is based on the types of systems affected by the vulnerability, their typical deployment patterns, and the effect that exploiting the vulnerability would have on them. Successfully exploiting the encryption vulnerability requires the attacker to have access to the network media; the denial of service vulnerability only affects Windows XP systems, and only if Remote Desktop Sharing has been enabled.

Vulnerability identifier:

• Weak Encryption in RDP Protocol:CVE-CAN-2002-0863
• Denial of Service in Remote Desktop:CVE-CAN-2002-0864

Tested Versions:

Microsoft tested Windows NT 4.0 Terminal Services Edition, Windows 2000 and Windows XP to assess whether they are affected by these vulnerabilities. Previous versions are no longer supported, and may or may not be affected by these vulnerabilities.

What vulnerabilities are eliminated by this patch?
This patch eliminates two vulnerabilities affecting the implementation of the RDP protocol:

• The first involves a cryptographic flaw affecting the Windows 2000 and Windows XP implementations.
• The second involves denial of service vulnerability affecting the Windows XP implementation only.

What is the RDP protocol?
Remote Desktop Protocol is a networking protocol that supports remote Windows terminal sessions between a client and a server. It transmit all of the information usually associated with a local console session - keystrokes, video and mouse data, and so forth - across a network, allowing users to have full, interactive logon sessions with remote systems. Microsoft Knowledge Base article Q186607 provides detailed technical information about the protocol.

In which Microsoft products is RDP implemented?
In general, RDP is the underlying protocol for Windows features that allow remote desktop sessions. For instance:

• Windows NT 4.0, Terminal Server Edition implements RDP 4.0.
• Terminal Services in Windows 2000 implements RDP 5.0
• Remote Desktop Sharing in Windows XP implements RDP 5.1

Are RDP sessions enabled by default in Windows?
RDP sessions are enabled by default in Windows NT 4.0, Terminal Server Edition, but not in any other version of Windows. (Please note, however, that neither of these vulnerabilities occur in Windows NT 4.0 Terminal Server Edition).

Cryptographic Flaw in RDP Protocol (CAN-2002-0863):

What's the scope of first vulnerability?
This vulnerability could enable an attacker to read the contents of an encrypted RDP session, thereby compromising any data within it. This could include information such as usernames and passwords, as well as any data the user entered into an application or which an application displayed for the user.
To exploit the vulnerability, the attacker would need the ability to eavesdrop on and record an RDP session. In most cases, this would require the attacker to have physical access to the network media itself. It would also require the attacker to have the technical ability to mount a cryptanalytic attack on the recorded data (the attack is, however, straightforward). Only the RDP implementations in Windows 2000 and Windows XP are affected.

What causes the vulnerability?
The vulnerability results because, although session data is encrypted in RDP 5.0 (the version that ships with Windows 2000) and RDP 5.1 (the version that ships with Windows XP), the checksums of the session data are not.

What is RDP Encryption?
Because RDP packets can sometimes be sent across uncontrolled or untrusted networks like the Internet or an extranet, RDP encrypts the data in a remote session using the RC4 cryptoalgorithm. In versions prior to Windows XP, the server administrator could select the key size to use; in Windows XP, all sessions are protected using 128-bit key.
The vulnerability, however, has nothing to do with the cryptoalgorithm used by RDP, nor with the key size. Instead, it results because of an implementation error involving the handling of checksum data.

What is checksum data?
Checksums are frequently used in networking applications as a way of detecting and correcting errors that occur during transmission. Before one computer sends a data packet, it performs a mathematical operation on the data, and sends the result of the operation along with the data itself. Upon receiving the data and checksum, the other computer performs the same mathematical operation on the data it received, and confirms that the result matches what it received. If they match, it's a good indicator that the data wasn't corrupted in transit.

What's wrong with the way checksums are handled in encrypted RDP sessions?
The checksums, like the session data itself, should be encrypted. However, in the Windows 2000 and Windows XP implementations of RDP, they aren't. The session data is encrypted, but before it's encrypted a checksum is calculated - and that checksum is sent in plaintext.

Why does this constitute a security vulnerability? After all, the checksums aren't the same as the session data, they're just information about the session data.
True, but there are straightforward cryptanalytic techniques that would enable an attacker to recover the session data from the checksums. Having broken the encryption, the attacker would be able to see the user's entire RDP session. Any information - from the information the user entered at the keyboard, to the movements of his or her mouse, to the information displayed on the screen - could be read.

How could an attacker exploit this vulnerability?
To exploit this vulnerability, an attacker would first have to have the means to capture a user's encrypted network traffic, most likely through the use of a network packet tracer. This is an important point, because in most cases it would require the attacker to have physical access to the network cabling that carries the session data.
Having captured the data, the attacker would need to subject it to a cryptanalytic attack in order to "crack" the encryption on the session data. This would require some technical knowledge about the RDP data format and cryptanalytic techniques, but these are not insurmountable hurdles.

If an attacker managed to decrypt one session, would that make it easier to decrypt future ones?
No. Unique session keys are negotiated at the start of every RDP session, so each attempt to compromise a session would need to start from scratch.

Does the vulnerability affect Windows NT 4.0?
No. Only the RDP implementations in Windows 2000 and Windows XP are affected.

What does the patch do?
The patch eliminates the vulnerability by encrypting the checksums as well as the session data.

Denial of Service in Remote Desktop (CAN-2002-0864):

What's the scope of the second vulnerability?
This is a denial of service vulnerability. An attacker who successfully exploited this vulnerability could cause a system hosting remote sessions to fail, with the loss of any unsaved data.
This vulnerability only affects Windows XP, but even then, the affected feature is not enabled by default. Even if it were enabled, an attacker would need the ability to deliver data to an affected system in order to exploit the vulnerability, so users who have observed normal firewalling precautions would not at risk from Internet-based attacks.

What causes the vulnerability?
The vulnerability results because of a flaw in the way RDP 5.1 (the version implemented in Windows XP) handles certain types of invalid data packets. Instead of handling them gracefully, RDP - and with the operating system itself - would fail upon processing them.

What's wrong with the way the RDP implementation in Windows XP handles the invalid data involved in the vulnerability?
By design, RDP should always check the validity of all incoming data packets before trying to act upon them. However, the RDP implementation in Windows XP doesn't check for one particular type of flaw in the incoming packets. Because of this, it would be possible to create a packet that, when processed, would create a series of failures that would culminate in the failure of the operating system itself.

What could an attacker do via this vulnerability?
An attacker could cause a Windows XP system to fail, if it's been configured to allow Remote Desktop sessions. The operator would need to reboot the machine in order to restore normal service.

Who could exploit the vulnerability?
Any user who could deliver the specific type of packets involved in this vulnerability to an affected Windows XP system could exploit it.

Are default installations of Windows XP affected by the vulnerability?
No. Remote Desktop Service does not run by default.

I don't know if I've enabled Remote Desktop. How can I tell?
Select Start, then Control Panel, then System. In the System Properties dialog, select the Remote tab, and inspect the checkbox in the Remote Desktop section. If it's selected, Remote Desktop is enabled; if it's not, Remote Desktop is disabled.

Would the attacker need to be able to establish a Remote Desktop session in order to exploit this vulnerability? 
No. The attacker would only need to send the correct set of packets to the correct port.

Could the vulnerability be exploited from the Internet?
It depends on whether the attacker were able to deliver packets to the port on which RDP operates, port 3389. If standard best practices have been followed, this port will be blocked at the firewall. (For instance, this port is blocked by default by Internet Connection Firewall).

Could a user inadvertently exploit this vulnerability?
No. The specific series of packets needed to cause the server to fail cannot be generated as part of a normal Remote Desktop session.

What does the patch do?
The patch addresses the vulnerability by ensuring that the Remote Desktop service handles malformed packets gracefully.

Download locations for this patch

• Windows 2000:

  http://www.microsoft.com/downloads/details.aspx?FamilyId=C9A38D45-5145-4844-B62E-C69D32AC929B&displaylang=en

• Windows XP:

  http://www.microsoft.com/downloads/details.aspx?FamilyId=84FC577D-F2D5-47B8-AB98-77BA7501B00B&displaylang=en

• Windows XP 64 bit Edition:

  http://www.microsoft.com/downloads/details.aspx?FamilyId=97945A5D-DB0B-40F8-9A2E-DE93CBB5CB3A&displaylang=en