cve.bz

CAPEC-43

Exploiting Multiple Input Interpretation Layers

High severityMedium likelihoodDetailed pattern

An attacker supplies the target software with input data that contains sequences of special characters designed to bypass input validation logic. This exploit relies on the target making multiples passes over the input data and processing a "layer" of special characters with each pass. In this manner, the attacker can disguise input that would otherwise be rejected as invalid by concealing it with layers of special/escape characters that are stripped off by subsequent processing steps. The goal is to first discover cases where the input validation layer executes before one or more parsing layers. That is, user input may go through the following logic in an application: <parser1> --> <input validator> --> <parser2>. In such cases, the attacker will need to provide input that will pass through the input validator, but after passing through parser2, will be converted into something that the input validator was supposed to stop.

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Show the promptExplain the attack pattern CAPEC-43: Exploiting Multiple Input Interpretation Layers: how it unfolds, what it needs and how to prevent and detect it in my systems. Use https://capec.mitre.org/data/definitions/43.html and https://cve.bz/cve/capec/43?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. [Determine application/system inputs where bypassing input validation is desired] The attacker first needs to determine all of the application's/system's inputs where input validation is being performed and where they want to bypass it.

      1 technique
      • While using an application/system, the attacker discovers an input where validation is stopping them from performing some malicious or unauthorized actions.

  2. Experiment

    They probe what they found.

    1. [Determine which character encodings are accepted by the application/system] The attacker then needs to provide various character encodings to the application/system and determine which ones are accepted. The attacker will need to observe the application's/system's response to the encoded data to determine whether the data was interpreted properly.

      5 techniques
      • Determine which escape characters are accepted by the application/system. A common escape character is the backslash character, '\'
      • Determine whether URL encoding is accepted by the application/system.
      • Determine whether UTF-8 encoding is accepted by the application/system.
      • Determine whether UTF-16 encoding is accepted by the application/system.
      • Determine if any other encodings are accepted by the application/system.

    2. [Combine multiple encodings accepted by the application.] The attacker now combines encodings accepted by the application. The attacker may combine different encodings or apply the same encoding multiple times.

      2 techniques
      • Combine same encoding multiple times and observe its effects. For example, if special characters are encoded with a leading backslash, then the following encoding may be accepted by the application/system: "\\\.". With two parsing layers, this may get converted to "\." after the first parsing layer, and then, to "." after the second. If the input validation layer is between the two parsing layers, then "\\\.\\\." might pass a test for ".." but still get converted to ".." afterwards. This may enable directory traversal attacks.
      • Combine multiple encodings and observe the effects. For example, the attacker might encode "." as "\.", and then, encode "\." as "&#92;&#46;", and then, encode that using URL encoding to "%26%2392%3B%26%2346%3B"

  3. Exploit

    They carry out the attack.

    1. [Leverage ability to bypass input validation] Attacker leverages their ability to bypass input validation to gain unauthorized access to system. There are many attacks possible, and a few examples are mentioned here.

      4 techniques
      • Gain access to sensitive files.
      • Perform command injection.
      • Perform SQL injection.
      • Perform XSS attacks.

Prerequisites

  • User input is used to construct a command to be executed on the target system or as part of the file name.

  • Multiple parser passes are performed on the data supplied by the user.

Skills required

  • Medium skill

    Knowledge of various escaping schemes, such as URL escape encoding and XML escape characters.

Signs of an attack

  • Control characters are being detected by the filters repeatedly.

Consequences

What a successful attack can lead to
ImpactAffectsNote
Modify DataIntegrity
Gain PrivilegesConfidentiality, Access Control, Authorization
Read DataConfidentiality

Mitigations

  • An iterative approach to input validation may be required to ensure that no dangerous characters are present. It may be necessary to implement redundant checking across different input validation layers. Ensure that invalid data is rejected as soon as possible and do not continue to work with it.

  • Make sure to perform input validation on canonicalized data (i.e. data that is data in its most standard form). This will help avoid tricky encodings getting past the filters.

  • 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 would not be permitted to enter into the system.

Weaknesses it exploits

  • CWE-20 Improper Input Validation
  • CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component
  • CWE-77 Improper Neutralization of Special Elements used in a Command
  • CWE-78 Improper Neutralization of Special Elements used in an OS Command
  • CWE-179 Incorrect Behavior Order: Early Validation
  • CWE-181 Incorrect Behavior Order: Validate Before Filter
  • CWE-183 Permissive List of Allowed Inputs
  • CWE-184 Incomplete List of Disallowed Inputs
  • CWE-697 Incorrect Comparison
  • CWE-707 Improper Neutralization

Recent CVEs with these weaknesses

  1. 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.

  2. 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.

  3. 7.2CVE-2026-105425

    OS command injection in Ivanti Sentry before versions 10.7.4, 10.8.3, and 11.0.0 as well as 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 achieve remote code…

  4. 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.

  5. 7.0CVE-2026-16478

    Permissive list of allowed inputs vulnerability in TÜBİTAK BİLGEM Software Technologies Research Institute pardus-nvidia-installer allows Privilege Escalation. This issue affects pardus-nvidia-installer: before 2.0.1.

  6. 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…

  7. 8.8CVE-2026-12573

    Zoho Corporation ManageEngine DDI Central versions 6200 and below are vulnerable to Cisco IOS command injection via DHCP pool name leading to arbitrary commands on managed Cisco routers.

  8. 8.8CVE-2026-123717

    OS command injection in the pass and bitwarden credential providers in Eclipse Foundation Eclipse Otterdog through 1.7.1 allows a user able to modify an organization configuration to execute arbitrary commands with the privileges of the op…

The newest records naming one of them. Each weakness's page lists them all.

Part of

Examples

The backslash character provides a good example of the multiple-parser issue. A backslash is used to escape c…

The backslash character provides a good example of the multiple-parser issue. A backslash is used to escape characters in strings, but is also used to delimit directories on the NT file system. When performing a command injection that includes NT paths, there is usually a need to "double escape" the backslash. In some cases, a quadruple escape is necessary.

Original String: C:\\\\winnt\\\\system32\\\\cmd.exe /c

<parsing layer>

Interim String: C:\\winnt\\system32\\cmd.exe /c

<parsing layer>

Final String: C:\winnt\system32\cmd.exe /c

This diagram shows each successive layer of parsing translating the backslash character. A double backslash becomes a single as it is parsed. By using quadruple backslashes, the attacker is able to control the result in the final string.

[REF-1]

MITRE's words, unchanged: CAPEC-43 at MITRE. CAPEC is Copyright © 2007–2026, The MITRE Corporation, used under the CAPEC terms of use: “The MITRE Corporation (MITRE) hereby grants you a non-exclusive, royalty-free license to use Common Attack Pattern Enumeration and Classification (CAPEC™) for research, development, and commercial purposes. Any copy you make for such purposes is authorized provided that you reproduce MITRE’s copyright designation and this license in any such copy.” CAPEC and the CAPEC logo are trademarks of The MITRE Corporation. Credits. All attack patterns.