cve.bz

CAPEC-80

Using UTF-8 Encoding to Bypass Validation Logic

High severityHigh likelihoodDetailed pattern

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.

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Show the promptExplain the attack pattern CAPEC-80: Using UTF-8 Encoding to Bypass Validation Logic: how it unfolds, what it needs and how to prevent and detect it in my systems. Use https://capec.mitre.org/data/definitions/80.html and https://cve.bz/cve/capec/80?via=ask-ai-copy, which lists recent CVEs with the weaknesses it exploits. Cite the sources.

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How the attack unfolds

  1. Explore

    The attacker looks for a way in.

    1. [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.

      3 techniques
      • 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.

  2. Experiment

    They probe what they found.

    1. [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.

      3 techniques
      • 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.

Prerequisites

  • 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.

Skills required

  • Low skill

    An attacker can inject different representation of a filtered character in UTF-8 format.

  • Medium skill

    An attacker may craft subtle encoding of input data by using the knowledge that they have gathered about the target host.

Signs of an attack

  • A web page that contains overly long UTF-8 codes constitute a protocol anomaly, and could be an indication that an attacker is attempting to exploit a vulnerability on the target host.

  • An attacker can use a fuzzer in order to probe for a UTF-8 encoding vulnerability. The fuzzer should generate suspicious network activity noticeable by an intrusion detection system.

  • An IDS filtering network traffic may be able to detect illegal UTF-8 characters.

Consequences

What a successful attack can lead to
ImpactAffectsNote
Bypass Protection MechanismConfidentiality, Access Control, Authorization
Execute Unauthorized CommandsConfidentiality, Integrity, AvailabilityRun Arbitrary Code
Modify DataIntegrity
Unreliable ExecutionAvailability

Mitigations

  • 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.

  • The exact response required from an UTF-8 decoder on invalid input is not uniformly defined by the standards. In general, there are several ways a UTF-8 decoder might behave in the event of an invalid byte sequence:

    • 1. Insert a replacement character (e.g. '?', '').
    • 2. Ignore the bytes.
    • 3. Interpret the bytes according to a different character encoding (often the ISO-8859-1 character map).
    • 4. Not notice and decode as if the bytes were some similar bit of UTF-8.
    • 5. Stop decoding and report an error (possibly giving the caller the option to continue).

    It is possible for a decoder to behave in different ways for different types of invalid input.

    RFC 3629 only requires that UTF-8 decoders must not decode "overlong sequences" (where a character is encoded in more bytes than needed but still adheres to the forms above). The Unicode Standard requires a Unicode-compliant decoder to "...treat any ill-formed code unit sequence as an error condition. This guarantees that it will neither interpret nor emit an ill-formed code unit sequence."

    Overlong forms are one of the most troublesome types of UTF-8 data. The current RFC says they must not be decoded but older specifications for UTF-8 only gave a warning and many simpler decoders will happily decode them. Overlong forms have been used to bypass security validations in high profile products including Microsoft's IIS web server. Therefore, great care must be taken to avoid security issues if validation is performed before conversion from UTF-8, and it is generally much simpler to handle overlong forms before any input validation is done.

    To maintain security in the case of invalid input, there are two options. The first is to decode the UTF-8 before doing any input validation checks. The second is to use a decoder that, in the event of invalid input, returns either an error or text that the application considers to be harmless. Another possibility is to avoid conversion out of UTF-8 altogether but this relies on any other software that the data is passed to safely handling the invalid data.

    Another consideration is error recovery. To guarantee correct recovery after corrupt or lost bytes, decoders must be able to recognize the difference between lead and trail bytes, rather than just assuming that bytes will be of the type allowed in their position.

  • 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.

Weaknesses it exploits

  • CWE-20 Improper Input Validation
  • CWE-73 External Control of File Name or Path
  • CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component
  • CWE-172 Improper Encoding or Decoding of Data
  • CWE-173 Improper Handling of Alternate Encoding
  • CWE-180 Incorrect Behavior Order: Validate Before Canonicalize
  • CWE-181 Incorrect Behavior Order: Validate Before Filter
  • CWE-692 Incomplete Denylist to Cross-Site Scripting
  • CWE-697 Incorrect Comparison

Recent CVEs with these weaknesses

  1. 9.6CVE-2026-108186

    Improper Input Validation in Zoom Workplace for Windows may allow an unauthenticated user to conduct an escalation of privilege via network access.

  2. 4.9CVE-2026-105427

    Improper input validation in Ivanti EPMM before version 12.10.0.1, 12.9.0.3, and 12.8.0.5 allows a remote authenticated attacker with admin privileges to permanently disable all Apache HTTPS services resulting in denial of service.

  3. 6.7CVE-2026-105426

    Improper input validation in Ivanti EPMM before version 12.10.0.1, 12.9.0.3, and 12.8.0.5 allows a remote authenticated attacker with admin privileges to set arbitrary Linux kernel parameters as root.

  4. 4.3CVE-2026-123178

    An insufficient input validation vulnerability in the listed NETGEAR Orbi Models router allows an authenticated administrator connected to the local network to execute unauthorized commands and modify the router’s software and functionalit…

  5. 6.3CVE-2026-125871

    The sourcecache expansion module in MISP Modules (misp-modules) contains a Server-Side Request Forgery (SSRF) vulnerability. When a user with permission to create or modify attributes of type 'link' or 'url' submits an attribute value, the…

  6. 5.5CVE-2026-125703

    A flaw was found in the GStreamer gst-plugins-bad MXF plugin. Insufficient bounds checks when parsing crafted VANC data can cause an out-of-bounds read in the demuxer, while an incorrect buffer size calculation when muxing multiple ancilla…

  7. 5.3CVE-2026-125113

    The XRootD project provides a high-performance, fault-tolerant, and secure solution for handling large amounts of data distributed across multiple storage resources. Prior to version 5.9.2, XRootD's rpCheck() function in src/XrdXrootd/XrdX…

  8. 5.3CVE-2026-125110

    Rack is a modular Ruby web server interface. Prior to 2.2.25, 3.1.23, and 3.2.8, Rack::Static serves a path after Utils.clean_path_info normalization but evaluates header_rules against a path that is only URL-decoded. An unauthenticated at…

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Part of
Related to
  • CAPEC-64 Using Slashes and URL Encoding Combined to Bypass Validation Logic
  • CAPEC-71 Using Unicode Encoding to Bypass Validation Logic

Examples

Perhaps the most famous UTF-8 attack was against unpatched Microsoft Internet Information Server (IIS) 4 and …

Perhaps the most famous UTF-8 attack was against unpatched Microsoft Internet Information Server (IIS) 4 and IIS 5 servers. If an attacker made a request that looked like this

http://servername/scripts/..%c0%af../winnt/system32/ cmd.exe

the server didn't correctly handle %c0%af in the URL. What do you think %c0%af means? It's 11000000 10101111 in binary; and if it's broken up using the UTF-8 mapping rules, we get this: 11000000 10101111. Therefore, the character is 00000101111, or 0x2F, the slash (/) character! The %c0%af is an invalid UTF-8 representation of the / character. Such an invalid UTF-8 escape is often referred to as an overlong sequence.

So when the attacker requested the tainted URL, they accessed

http://servername/scripts/../../winnt/system32/cmd.exe

In other words, they walked out of the script's virtual directory, which is marked to allow program execution, up to the root and down into the system32 directory, where they could pass commands to the command shell, Cmd.exe.

See also: CVE-2000-0884

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