Détail du CAPEC-80

This attack is a specific variation on leveraging alternate encodings to bypass validation logic. This attack leverages the possibility to encode potentially harmful input in UTF-8 and submit it to applications not expecting or effective at validating this encoding standard making input filtering difficult. UTF-8 (8-bit UCS/Unicode Transformation Format) is a variable-length character encoding for Unicode. Legal UTF-8 characters are one to four bytes long. However, early version of the UTF-8 specification got some entries wrong (in some cases it permitted overlong characters). UTF-8 encoders are supposed to use the "shortest possible" encoding, but naive decoders may accept encodings that are longer than necessary. According to the RFC 3629, a particularly subtle form of this attack can be carried out against a parser which performs security-critical validity checks against the UTF-8 encoded form of its input, but interprets certain illegal octet sequences as characters.

Flux d'exécution

[Survey the application for user-controllable inputs] Using a browser or an automated tool, an attacker follows all public links and actions on a web site. They record all the links, the forms, the resources accessed and all other potential entry-points for the web application.

• Use a spidering tool to follow and record all links and analyze the web pages to find entry points. Make special note of any links that include parameters in the URL.
• Use a proxy tool to record all user input entry points visited during a manual traversal of the web application.
• Use a browser to manually explore the website and analyze how it is constructed. Many browsers' plugins are available to facilitate the analysis or automate the discovery.

[Probe entry points to locate vulnerabilities] The attacker uses the entry points gathered in the "Explore" phase as a target list and injects various UTF-8 encoded payloads to determine if an entry point actually represents a vulnerability with insufficient validation logic and to characterize the extent to which the vulnerability can be exploited.

• Try to use UTF-8 encoding of content in Scripts in order to bypass validation routines.
• Try to use UTF-8 encoding of content in HTML in order to bypass validation routines.
• Try to use UTF-8 encoding of content in CSS in order to bypass validation routines.

Conditions préalables

The application's UTF-8 decoder accepts and interprets illegal UTF-8 characters or non-shortest format of UTF-8 encoding.
Input filtering and validating is not done properly leaving the door open to harmful characters for the target host.

Compétences requises

An attacker can inject different representation of a filtered character in UTF-8 format.
An attacker may craft subtle encoding of input data by using the knowledge that they have gathered about the target host.

Atténuations

The Unicode Consortium recognized multiple representations to be a problem and has revised the Unicode Standard to make multiple representations of the same code point with UTF-8 illegal. The UTF-8 Corrigendum lists the newly restricted UTF-8 range (See references). Many current applications may not have been revised to follow this rule. Verify that your application conform to the latest UTF-8 encoding specification. Pay extra attention to the filtering of illegal characters.
Array
For security reasons, a UTF-8 decoder must not accept UTF-8 sequences that are longer than necessary to encode a character. If you use a parser to decode the UTF-8 encoding, make sure that parser filter the invalid UTF-8 characters (invalid forms or overlong forms).
Look for overlong UTF-8 sequences starting with malicious pattern. You can also use a UTF-8 decoder stress test to test your UTF-8 parser (See Markus Kuhn's UTF-8 and Unicode FAQ in reference section)
Assume all input is malicious. Create an allowlist that defines all valid input to the software system based on the requirements specifications. Input that does not match against the allowlist should not be permitted to enter into the system. Test your decoding process against malicious input.

Faiblesses connexes

Références

REF-1

Exploiting Software: How to Break Code
G. Hoglund, G. McGraw.

REF-112

Secure Programming for Linux and Unix HOWTO
David Wheeler.
http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/character-encoding.html

REF-530

Writing Secure Code
Michael Howard, David LeBlanc.

REF-531

Security Risks of Unicode
Bruce Schneier.
https://www.schneier.com/crypto-gram/archives/2000/0715.html

REF-532

Wikipedia
http://en.wikipedia.org/wiki/UTF-8

REF-533

RFC 3629 - UTF-8, a transformation format of ISO 10646
F. Yergeau.
http://www.faqs.org/rfcs/rfc3629.html

REF-114

IDS Evasion with Unicode
Eric Hacker.
http://www.securityfocus.com/infocus/1232

REF-535

Corrigendum #1: UTF-8 Shortest Form
http://www.unicode.org/versions/corrigendum1.html

REF-525

UTF-8 and Unicode FAQ for Unix/Linux
Markus Kuhn.
http://www.cl.cam.ac.uk/~mgk25/unicode.html

REF-537

UTF-8 decoder capability and stress test
Markus Kuhn.
http://www.cl.cam.ac.uk/%7Emgk25/ucs/examples/UTF-8-test.txt

Soumission

Modifications