String header has no maximum length defined
Issue ID: response-header-string-maxlength
Average severity: Medium
Description
A string header does not specify the maximum length for the accepted strings.
For more details, see the OpenAPI Specification.
Example
The following is an example of how this type of risk could look in your API definition:
{
"responses": {
"200": {
"description": "OK",
"headers": {
"x-ids": {
"type": "string"
}
}
}
}
}
Possible exploit scenario
Attackers strive to make your APIs behave in an unexpected way to learn more about your system or to cause a data breach. We highly recommend that you minimize any risks and clearly specify the data that your API produces in each method.
Remediation
Set the maxLength
property for string headers. This provides an extra layer of safety ensuring that your API only returns data that you expect it to return.
{
"responses": {
"200": {
"description": "OK",
"headers": {
"x-ids": {
"type": "string",
"maxLength": 8
}
}
}
}
}
We also recommend that you specify a regular expression to further lock down the expected string format.
Remember to include the anchors ^
and $
in your regular expression, otherwise the overall length of the pattern could be considered infinite. If you include the anchors in the regular expression and the pattern only has fixed or constant quantifiers (like {10,64}
, for example), you do not have to define the property maxLength
separately for the object, as the length is fully constrained by the pattern. However, if the regular expression does not include the anchors or its quantifiers are not fixed (like in ^a.*b$
), it can be considered to be just a part of a longer string and the property maxLength
is required to constrain the length.
For more information on regular expressions, see the following:
- Language-agnostic information on regular expressions at Base Definitions page on regular expressions
- OWASP Validation Regex Repository
- RegExr, an online tool for building and testing regular expressions
The following are examples of regular expressions for some common elements:
Element | Examples of regular expressions | Examples with escape |
---|---|---|
Alphanumeric string |
|
— |
Base64‑encoding (for an image) |
|
^data:image\\/(?:gif|png|jpeg|bmp|webp)(?:;charset=utf-8)?;base64,(?:[A-Za-z0-9]|[+/])+={0,2}$
|
Date and time |
|
|
Duration |
|
^\\d+:\\d{2}:\\d{2}$
|
Email address (common format) |
|
^([a-z0-9_\\.-]+)@([\\da-z\\.-]+)\\.([a-z\\.]{2,5})$
|
File |
|
|
IP address |
|
|
Numbers |
|
|
Password constraints |
Password that has:
|
^(?=.*[0-9])(?=.*[a-z])(?=.*[A-Z])(?=.*[*.!@$%^&(){}[]:;<>,.?/~_+-=|\\]).{10,64}$
|
Phone number |
International phone number, country code optional: Use libraries instead or regular expressions to validate phone numbers whenever possible. |
^(?:(?:\\(?(?:00|\\+)([1-4]\\d\\d|[1-9]\\d?)\\)?)?[\\-\\.\\ \\\\\\/]?)?((?:\\(?\\d{1,}\\)?[\\-\\.\\ \\\\\\/]?){0,})(?:[\\-\\.\\ \\\\\\/]?(?:#|ext\\.?|extension|x)[\\-\\.\\ \\\\\\/]?(\\d+))?$
|
URL/URI (with protocol optional) |
|
^(https?:\\/\\/)?(www\\.)?[-a-zA-Z0-9@:%._\\+~#=]{2,256}\\.[a-z]{2,6}\\b([-a-zA-Z0-9@:%_\\+.~#?&//=]*)$
|
UUID |
|
— |