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.

ThreatScore

Threat Mapped score: 1.8

Industry: Finiancial

Threat priority: P4 - Informational (Low)

Observed Examples (CVEs)

CVE: 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: CVE-1999-0168

RPC portmapper could redirect service requests from an attacker to another entity, which thinks the requests came from the portmapper.
CVE: 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: CVE-2002-1484

Web server allows attackers to request a URL from another server, including other ports, which allows proxied scanning.
CVE: CVE-2004-2061

CGI script accepts and retrieves incoming URLs.
CVE: CVE-2001-1484

Bounce attack allows access to TFTP from trusted side.
CVE: CVE-2010-1637

Web-based mail program allows internal network scanning using a modified POP3 port number.
CVE: CVE-2009-0037

URL-downloading library automatically follows redirects to file:// and scp:// URLs

Related Attack Patterns (CAPEC)

Attack TTPs

T1090.001 — Internal Proxy (command-and-control)

Malware

APTs (Intrusion Sets)

Modes of Introduction

Phase	Note
Architecture and Design	REALIZATION: This weakness is caused during implementation of an architectural security tactic.

Common Consequences

Impact: Gain Privileges or Assume Identity, Hide Activities, Execute Unauthorized Code or Commands — Notes:

Potential Mitigations

Architecture and Design: Enforce the use of strong mutual authentication mechanism between the two parties. (N/A)
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. (N/A)

Applicable Platforms

None (Not Language-Specific, Undetermined)

Demonstrative Examples

Intro: A SoC contains a microcontroller (running ring-3 (least trusted ring) code), a Memory Mapped Input Output (MMIO) mapped IP core (containing design-house secrets), and a Direct Memory Access (DMA) controller, among several other compute elements and peripherals. The SoC implements access control to protect the registers in the IP core (which registers store the design-house secrets) from malicious, ring-3 (least trusted ring) code executing on the microcontroller. The DMA controller, however, is not blocked off from accessing the IP core for functional reasons.

Body: The weakness here is that the intermediary or the proxy agent did not ensure the immutability of the identity of the microcontroller initiating the transaction.

The code in ring-3 (least trusted ring) of the
                     microcontroller attempts to directly read the protected
                     registers in IP core through MMIO transactions. However,
                     this attempt is blocked due to the implemented access
                     control. Now, the microcontroller configures the DMA core
                     to transfer data from the protected registers to a memory
                     region that it has access to. The DMA core, which is
                     acting as an intermediary in this transaction, does not
                     preserve the identity of the microcontroller and, instead,
                     initiates a new transaction with its own identity. Since
                     the DMA core has access, the transaction (and hence, the
                     attack) is successful.

Notes

Relationship: This weakness has a chaining relationship with CWE-668 (Exposure of Resource to Wrong Sphere) because the proxy effectively provides the attacker with access to the target's resources that the attacker cannot directly obtain.
Maintenance: This could possibly be considered as an emergent resource.
Theoretical: It could be argued that the "confused deputy" is a fundamental aspect of most vulnerabilities that require an active attacker. Even for common implementation issues such as buffer overflows, SQL injection, OS command injection, and path traversal, the vulnerable program already has the authorization to run code or access files. The vulnerability arises when the attacker causes the program to run unexpected code or access unexpected files.

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