Double Decoding of the Same Data

Description

This vulnerability occurs when an application decodes the same piece of data twice in sequence. This double processing can bypass or neutralize security checks that happen after the first decode, leaving the system exposed.

Extended Description

Double decoding is dangerous because it breaks the expected data flow. Security mechanisms like input validation, sanitization, or intrusion detection are often placed after an initial decoding step, assuming the data is now in its canonical form. When a second, unexpected decode happens, it can transform the data again, rendering those intermediate protections useless and allowing malicious payloads to slip through. Developers can prevent this by establishing a strict, one-time decoding policy in a centralized location within the application's data pipeline. All incoming data should be decoded, validated, and sanitized once into a trusted, internal format before being passed to other components. This ensures security controls are applied to the final, operational form of the data and cannot be circumvented by further transformations.

Common Consequences 1

Scope: Access ControlConfidentialityAvailabilityIntegrityOther

Impact: Bypass Protection MechanismExecute Unauthorized Code or CommandsVaries by Context

Potential Mitigations 4

Phase: Architecture and Design

Strategy: Input Validation

Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.

Phase: Implementation

Strategy: Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

Phase: Implementation

Strategy: Output Encoding

Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component.

Phase: Implementation

Strategy: Input Validation

Inputs should be decoded and canonicalized to the application's current internal representation before being validated (Incorrect Behavior Order: Validate Before Canonicalize). Make sure that the application does not decode the same input twice (Double Decoding of the Same Data). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.

Observed Examples 6

CVE-2004-1315Forum software improperly URL decodes the highlight parameter when extracting text to highlight, which allows remote attackers to execute arbitrary PHP code by double-encoding the highlight value so that special characters are inserted into the result.

CVE-2004-1939XSS protection mechanism attempts to remove "/" that could be used to close tags, but it can be bypassed using double encoded slashes (%252F)

CVE-2001-0333Directory traversal using double encoding.

CVE-2004-1938"%2527" (double-encoded single quote) used in SQL injection.

CVE-2005-1945Double hex-encoded data.

CVE-2005-0054Browser executes HTML at higher privileges via URL with hostnames that are double hex encoded, which are decoded twice to generate a malicious hostname.

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Modes of Introduction

Implementation

Related Weaknesses

ChildOf:

Encoding Error (CWE-172)

ChildOf:

Multiple Operations on Resource in Single-Operation Context (CWE-675)

Taxonomy Mapping

PLOVER

Notes

Research GapProbably under-studied.