Unintended Proxy or Intermediary ('Confused Deputy')

Description

A confused deputy vulnerability occurs when a system receives a request from a client and forwards it to an external destination without properly identifying the original source. This makes the system appear to be the originator of the request, effectively turning it into an unintended proxy for the client.

Extended Description

Attackers exploit this flaw when they cannot reach a target system directly. By sending a malicious request to your vulnerable application, they can have it relayed to the protected target. Since the request now originates from your system's IP address and context, it can bypass network firewalls, IP-based allowlists, and hide the attacker's true location, enabling unauthorized access or actions. This becomes a critical security issue only under specific conditions: your application must have higher privileges or different network access than the initial requester; the attacker must be blocked from contacting the target directly; and the attacker must be able to craft a request that your application forwards unintentionally. This often involves specifying an unexpected host, port, internal IP, or a restricted command within an otherwise permitted service request.

Common Consequences 1

Scope: Non-RepudiationAccess Control

Impact: Gain Privileges or Assume IdentityHide ActivitiesExecute Unauthorized Code or Commands

Detection Methods 1

Automated Static AnalysisHigh

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.)

Potential Mitigations 2

Phase: Architecture and Design

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

Phase: 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.

Demonstrative Examples 1

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.

Code Example:

Bad

Other

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.

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.

Code Example:

Good

Other

The DMA core forwards this transaction with the identity of the code executing on the microcontroller, which is the original initiator of the end-to-end transaction. Now the transaction is blocked, as a result of forwarding the identity of the true initiator which lacks the permission to access the confidential MMIO mapped IP core.

Observed Examples 8

CVE-1999-0017FTP 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-1999-0168RPC portmapper could redirect service requests from an attacker to another entity, which thinks the requests came from the portmapper.

CVE-2005-0315FTP 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-2002-1484Web server allows attackers to request a URL from another server, including other ports, which allows proxied scanning.

CVE-2004-2061CGI script accepts and retrieves incoming URLs.

CVE-2001-1484Bounce attack allows access to TFTP from trusted side.

CVE-2010-1637Web-based mail program allows internal network scanning using a modified POP3 port number.

CVE-2009-0037URL-downloading library automatically follows redirects to file:// and scp:// URLs

References 2

The Confused Deputy (or why capabilities might have been invented)

Norm Hardy

1988

http://www.cap-lore.com/CapTheory/ConfusedDeputy.html

ID: REF-432

Validation Vulnerabilities

moparisthebest

05-06-2015

https://mailarchive.ietf.org/arch/msg/acme/s6Q5PdJP48LEUwgzrVuw_XPKCsM/

ID: REF-1125

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Technologies:

Not Technology-Specific : Undetermined

Modes of Introduction

Architecture and Design

Related Attack Patterns

Alternate Terms

Confused Deputy

This weakness is sometimes referred to as the "Confused deputy" problem, in which an attacker misused the authority of one victim (the "confused deputy") when targeting another victim.

Related Weaknesses

ChildOf:

Externally Controlled Reference to a Resource in Another Sphere (CWE-610)

CanPrecede:

Exposure of Resource to Wrong Sphere (CWE-668)

Taxonomy Mapping

PLOVER
PLOVER
WASC

Notes

RelationshipThis weakness has a chaining relationship with Exposure of Resource to Wrong Sphere (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.

MaintenanceThis could possibly be considered as an emergent resource.

TheoreticalIt 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.