Double encoding is the act of encoding data twice in a row using the same encoding scheme. It is usually used as an attack technique to bypass authorization schemes or security filters that intercept user input. In double encoding attacks against security filters, characters of the payload that are treated as illegal by those filters are replaced with their double-encoded form.

Double URI-encoding is a special type of double encoding in which data is URI-encoded twice in a row. It has been used to bypass authorization schemes and security filters against code injection, directory traversal, cross-site scripting (XSS) and SQL injection.

Description

In double encoding, data is encoded twice in a row using the same encoding scheme, that is, double-encoded form of data X is Encode(Encode(X)) where Encode is an encoding function.^[1]

Double encoding is usually used as an attack technique to bypass authorization schemes or security filters that intercept user input.^[2] In double encoding attacks against security filters, characters of the payload that are treated as illegal by those filters are replaced with their double-encoded form.^[3] Security filters might treat data X and its encoded form as illegal.^[4] However, it is still possible for Encode(Encode(X)), which is the double-encoded form of data X, to not to be treated as illegal by security filters and hence pass through them, but later on, the target system might use the double-decoded form of Encode(Encode(X)), which is X, something that the filters would have been treated as illegal.^[5]

Double URI-encoding

Double URI-encoding, also referred to as double percent-encoding, is a special type of double encoding in which data is URI-encoded twice in a row.^[6] In other words, double-URI-encoded form of data X is URI-encode(URI-encode(X)).^[7] For example for calculating double-URI-encoded form of <, first < is URI-encoded as %3C which then in turn is URI-encoded as %253C, that is, double-URI-encode(<) = URI-encode(URI-encode(<)) = URI-encode(%3C) = %253C.^[8] As another example, for calculating double-URI-encoded form of ../, first ../ is URI-encoded as %2E%2E%2F which then in turn is URI-encoded as %252E%252E%252F, that is, double-URI-encode(../) = URI-encode(URI-encode(../)) = URI-encode(%2E%2E%2F) = %252E%252E%252F.^[9]

Double URI-encoding is usually used as an attack technique against web applications and web browsers to bypass authorization schemes and security filters that intercept user input.^[10][11] For example because . and its URI-encoded form %2E are used in some directory traversal attacks, they are usually treated as illegal by security filters.^[12] However, it is still possible for %252E, which is the double-URI-encoded form of ., to not to be treated as illegal by security filters and hence pass through them, but later on, when the target system is building the path related to the directory traversal attack it might use the double-URI-decoded form of %252E, which is ., something that the filters would have been treated as illegal.^[13]

Double URI-encoding attacks have been used to bypass authorization schemes and security filters against code injection, directory traversal, XSS and SQL injection.^[14]

Prevention

Decoding some user input twice using the same decoding scheme, once before a security measure and once afterwards, may allow double encoding attacks to bypass that security measure.^[15] Thus, to prevent double encoding attacks, all decoding operations on user input should occur before authorization schemes and security filters that intercept user input.^[16]

Examples

<?php
/* Note that $_GET is already URI-decoded */
$path = $_GET["file"];

/* Security filter */
/* Exit if user input contains directory traversal sequence */
if (strstr($path, "../") or strstr($path,  "..\\"))
{
    exit("Directory traversal attempt detected.");
}

/* URI-decode user input once again */
$path = urldecode($path);

/* Build file path to be sent using user input */
echo htmlentities(file_get_contents("uploads/" . $path));

<?php
/* Note that $_GET is already URI-decoded */
$name = $_GET["name"];

/* Security filter */
/* Sanitize user input via htmlentity */
$name = htmlentities($name);

/* URI-decode user input once again */
$name = urldecode($name);

/* Build message to be shown using user input */
echo "Hello " . $name;

Sources

• CAPEC (2022). "CAPEC-120: Double Encoding". capec.mitre.org. 3.7. Retrieved 23 July 2022.
• CWE (2022). "CWE-174: Double Decoding of the Same Data". cwe.mitre.org. 4.8. Retrieved 23 July 2022.
• Imperva (2022). "Double URL Encoding". docs.imperva.com. Retrieved 23 July 2022.
• OWASP (2022). "Double Encoding". owasp.org. Retrieved 23 July 2022.
• PHP (2022). "urldecode". php.net. Retrieved 23 July 2022.
• PortSwigger (2022). "Obfuscating attacks using encodings". portswigger.net. Obfuscation via double URL encoding. Retrieved 23 July 2022.
• Prasad, Prakhar (2016). "Double encoding". Mastering Modern Web Penetration Testing. Packt Publishing. pp. 11–14. ISBN 978-1785284588.

References

External links

• OWASP entry for double encoding attacks
• CAPEC entry for double encoding attacks
• CWE entry for the weakness exploited by double encoding attacks