Vulnerability Note VU#102014
Optimistic TCP acknowledgements can cause denial of service
A vulnerability in the TCP congestion control mechanism could be leveraged by an attacker to cause a denial of service.
The Transmission Control Protocol (TCP) is described in RFC 793 as a means to provide reliable host-to-host transmission between hosts in a packet-switched computer network. Numerous Internet protocols such as HTTP, SMTP, and FTP rely on TCP as their underlying transport protocol. Several different TCP congestion control mechanisms are specified in RFC 2581.
In the course of normal operation a TCP client acknowledges (ACKs) the receipt of packets sent to it by the server. A TCP sender varies its transmission rate based on receiving ACKs of the packets it sends. An optimistic ACK is an ACK sent by a client for a data segment that it has not yet received. A vulnerability exists in the potential for a client to craft optimistic ACKs timed in such a way that they correspond to legitimate packets that the sender has already injected into the network (often referred to as "in-flight" packets). As a result, the sender believes that the transfer is progressing better than it actually is and may increase the rate at which it sends packets. An important side effect of this condition is the amplification factor that it introduces. An attacker exploiting this vulnerability can potentially cause victims to transmit much more data than the bandwidth available to the attacker.
A remote attacker can cause a TCP sender to transmit packets faster than it would in the course of a normal connection. The victim could potentially exhaust its network bandwidth, thereby resulting in a denial of service. In an attack involving multiple victims, the aggregate volume of generated traffic may cause congestion or a bandwidth exhaustion denial of service to intermediate transit network providers as well.
The CERT/CC is currently unaware of a practical solution to this problem.
Systems Affected (Learn More)
|Vendor||Status||Date Notified||Date Updated|
|Sun Microsystems, Inc.||Affected||02 Sep 2005||03 Nov 2005|
|3com, Inc.||Unknown||02 Sep 2005||02 Sep 2005|
|Alcatel||Unknown||02 Sep 2005||02 Sep 2005|
|Apple Computer, Inc.||Unknown||02 Sep 2005||02 Sep 2005|
|AT&T||Unknown||02 Sep 2005||02 Sep 2005|
|Avaya, Inc.||Unknown||02 Sep 2005||02 Sep 2005|
|Avici Systems, Inc.||Unknown||02 Sep 2005||02 Sep 2005|
|Borkerware Technologies||Unknown||02 Sep 2005||02 Sep 2005|
|Charlotte's Web Networks||Unknown||02 Sep 2005||02 Sep 2005|
|Check Point Software Technologies||Unknown||02 Sep 2005||02 Sep 2005|
|Chiaro Networks, Inc.||Unknown||02 Sep 2005||02 Sep 2005|
|Cisco Systems, Inc.||Unknown||02 Sep 2005||02 Sep 2005|
|Computer Associates||Unknown||02 Sep 2005||02 Sep 2005|
|Conectiva Inc.||Unknown||26 Oct 2005||26 Oct 2005|
|Cray Inc.||Unknown||02 Sep 2005||02 Sep 2005|
CVSS Metrics (Learn More)
Thanks to Rob Sherwood of the University of Maryland for reporting this vulnerability and researching its associated exploitation methods. The CERT/CC acknowledges Stefan Savage, Neal Cardwell, David Wetherall, and Tom Anderson for the original publication of the underlying protocol issue that causes the vulnerability.
This document was written by Chad Dougherty. Additional technical feedback and assistance with this vulnerability was provided by Hal Burch and Art Manion.
- CVE IDs: Unknown
- Date Public: 10 Nov 2005
- Date First Published: 10 Nov 2005
- Date Last Updated: 10 Nov 2005
- Severity Metric: 10.50
- Document Revision: 27
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