Description
The product decodes the same input twice, which can limit the effectiveness of any protection mechanism that occurs in between the decoding operations.
Potential Impact
Access Control, Confidentiality, Availability, Integrity, Other
Bypass Protection Mechanism, Execute Unauthorized Code or Commands, Varies by Context
Mitigations & Prevention
Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.
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 relat
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
Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.
Real-World CVE Examples
| CVE ID | Description |
|---|---|
| CVE-2004-1315 | Forum 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 t |
| CVE-2004-1939 | XSS protection mechanism attempts to remove "/" that could be used to close tags, but it can be bypassed using double encoded slashes (%252F) |
| CVE-2001-0333 | Directory traversal using double encoding. |
| CVE-2004-1938 | "%2527" (double-encoded single quote) used in SQL injection. |
| CVE-2005-1945 | Double hex-encoded data. |
| CVE-2005-0054 | Browser executes HTML at higher privileges via URL with hostnames that are double hex encoded, which are decoded twice to generate a malicious hostname. |
Related Weaknesses
Taxonomy Mappings
- PLOVER: — Double Encoding
Frequently Asked Questions
What is CWE-174?
CWE-174 (Double Decoding of the Same Data) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Variant-level weakness. The product decodes the same input twice, which can limit the effectiveness of any protection mechanism that occurs in between the decoding operations.
How can CWE-174 be exploited?
Attackers can exploit CWE-174 (Double Decoding of the Same Data) to bypass protection mechanism, execute unauthorized code or commands, varies by context. This weakness is typically introduced during the Implementation phase of software development.
How do I prevent CWE-174?
Key mitigations include: Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.
What is the severity of CWE-174?
CWE-174 is classified as a Variant-level weakness (Low-Medium abstraction). It has been observed in 6 real-world CVEs.