CWE-441: Unintended Proxy or Intermediary (‘Confused Deputy’)

Description

The product receives a request, message, or directive from an upstream component, but the product does not sufficiently preserve the original source of the request before forwarding the request to an external actor that is outside of the product’s control sphere. This causes the product to appear to be the source of the request, leading it to act as a proxy or other intermediary between the upstream component and the external actor.

Extended Description

If an attacker cannot directly contact a target, but the product has access to the target, then the attacker can send a request to the product and have it be forwarded to the target. The request would appear to be coming from the product’s system, not the attacker’s system. As a result, the attacker can bypass access controls (such as firewalls) or hide the source of malicious requests, since the requests would not be coming directly from the attacker.

Since proxy functionality and message-forwarding often serve a legitimate purpose, this issue only becomes a vulnerability when:

• The product runs with different privileges or on a different system, or otherwise has different levels of access than the upstream component;
• The attacker is prevented from making the request directly to the target; and
• The attacker can create a request that the proxy does not explicitly intend to be forwarded on the behalf of the requester. Such a request might point to an unexpected hostname, port number, hardware IP, or service. Or, the request might be sent to an allowed service, but the request could contain disallowed directives, commands, or resources.

Alternate Terms

• Confused Deputy: This weakness is sometimes referred to as the “Confused deputy” problem, in which an attacker misuses the authority of one victim (the “confused deputy”) to use that victim’s legitimate (restricted) capabilities to target another victim.

Related Weaknesses

Modes of Introduction

Applicable Platforms

Languages

Class: Not Language-Specific

Technologies

Class: Not Technology-Specific

Common Consequences

Detection Methods

Automated Static Analysis

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect “sources” (origins of input) with “sinks” (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Effectiveness: High

Potential Mitigations

Architecture and Design

Enforce the use of strong mutual authentication mechanism between the two parties.

Architecture and Design

Whenever a product is an intermediary or proxy for transactions between two other components, the proxy core should not drop the identity of the initiator of the transaction. The immutability of the identity of the initiator must be maintained and should be forwarded all the way to the target.

Observed Examples

• CVE-1999-0017: FTP bounce attack. The design of the protocol allows an attacker to modify the PORT command to cause the FTP server to connect to other machines besides the attacker’s.
• CVE-1999-0168: RPC portmapper could redirect service requests from an attacker to another entity, which thinks the requests came from the portmapper.
• CVE-2005-0315: FTP server does not ensure that the IP address in a PORT command is the same as the FTP user’s session, allowing port scanning by proxy.
• CVE-2002-1484: Web server allows attackers to request a URL from another server, including other ports, which allows proxied scanning.
• CVE-2004-2061: CGI script accepts and retrieves incoming URLs.
• CVE-2001-1484: Bounce attack allows access to TFTP from trusted side.
• CVE-2010-1637: Web-based mail program allows internal network scanning using a modified POP3 port number.
• CVE-2009-0037: URL-downloading library automatically follows redirects to file:// and scp:// URLs