Configure custom rules language attributes

Each Google Cloud Armor security policy rule has a priority, a match condition, and an action. Google Cloud Armor performs the action of the highest priority rule that matches a request. Rules with a lower priority than the highest priority matching rule are not evaluated, even if they have the same match conditions.

Each security policy rule supports two types of match conditions:

• A basic match condition contains lists of IP addresses or lists of IP address ranges. Basic match conditions are defined by using the --src-ip-ranges flag when creating a rule using the Google Cloud CLI.
• An advanced match condition contains an expression with up to five subexpressions that can match a variety of attributes of an incoming request. Advanced match conditions are defined using the --expression flag when creating a rule using the Google Cloud CLI.

This page discusses advanced match conditions and the Google Cloud Armor custom rules language that you use to write expressions in the advanced match conditions of security policy rules. The Google Cloud Armor custom rules language is a subset of the Common Expression Language (CEL). Expressions written in the Google Cloud Armor custom rules language require two components:

• The attribute: the data to inspect
• The operation: how to use the data

For example, the following expression uses the attributes origin.ip and 9.9.9.0/24 in the operation inIpRange(). In this case, the expression returns true if origin.ip is within the 9.9.9.0/24 IP address range.

inIpRange(origin.ip, '9.9.9.0/24')

Even though the previous example expression only matches on source IP address, when you use the example expression in a Google Cloud Armor security policy rule, the rule is considered a rule with advanced match conditions from a quota perspective. For more information, see Google Cloud Armor quotas and limits.

Attributes

Attributes represent information from an incoming request, such as the origin IP address or the requested URL path.

Field	Type	Field description
origin.ip	string	The source IP address of the request.
origin.user_ip	string	The IP address of the originating client, which is included in the HTTP-HEADER by an upstream proxy. Before you use this attribute, you must configure the userIpRequestHeaders[] option in the security policy's advancedOptionsConfig field to match a source like True-Client-IP, X-Forwarded-For, or X-Real-IP. If you do not configure the userIpRequestHeaders[] option, if the configured header contains invalid IP address values, or if the configured header is not present, origin.user_ip defaults to origin.ip. For more information, see the securityPolicy resource reference.
origin.tls_ja3_fingerprint	string	JA3 TLS/SSL fingerprint if the client connects using HTTPS, HTTP/2, or HTTP/3. If not available, an empty string is returned.
request.headers	map	A string-to-string map of the HTTP request headers. If a header contains multiple values, the value in this map would be a comma-separated string of all of the values of the header. The keys in this map are all lowercase. All headers accepted by external Application Load Balancers are inspected, and the same header limitations apply. Recommended approach is to first check for availability using has(), such as has(request.headers['header-key']) && request.headers['header-key'] != 'header-value'.
request.method	string	The HTTP request method, such as GET or POST.
request.path	string	The requested HTTP URL path.
request.scheme	string	The HTTP URL scheme such as http or https. Values for this attribute are all lowercase.
request.query	string	The HTTP URL query in the format of name1=value&name2=value2, as it appears in the first line of the HTTP request. No decoding is performed.
origin.region_code	string	The Unicode country code that is associated with the origin IP, such as US. If you are creating a rule or expression that uses ISO 3166-1 alpha 2 country or region codes, Google Cloud Armor treats each code independently. Google Cloud Armor rules and expressions explicitly use those region codes to allow or deny requests. For more information, see unicode_region_subtag in the Unicode Technical Standard.
origin.asn	integer	The autonomous system number (ASN) that is associated with the origin IP address. The globally unique ASN is determined based on the network operator supporting the IP address prefixes that contain the origin IP address.

Note: Edge security policies only support filtering based on source IP address and geography attributes, which are origin.ip and origin.region_code.

reCAPTCHA attributes

This section lists attributes that are only applicable to reCAPTCHA tokens or exemption cookies. A subexpression based on these attributes returns false if the reCAPTCHA token or exemption cookie to be evaluated is not available or is invalid for one of the following reasons:

• The token is malformed and cannot be decoded.
• The token contains invalid attributes. For example, the token was generated using a reCAPTCHA key that does not match the rule's associated reCAPTCHA keys.
• The token is expired.

Exemption cookie attributes

Field	Type	Field description
token.recaptcha_exemption.valid	bool	The presence of a valid reCAPTCHA exemption cookie.

Action-token attributes

Field	Type	Field description
token.recaptcha_action.score	float	The score from a reCAPTCHA action-token. A valid score ranges from 0.0 to 1.0, with 0.0 being very likely an illegitimate user, and 1.0 being very likely a legitimate user.
token.recaptcha_action.captcha_status	string	The captcha status from a reCAPTCHA action-token. A valid status is NONE, PASS, or FAIL, where NONE refers to when there are no challenges involved during reCAPTCHA assessment, such that the captcha field is missing in the action-token.
token.recaptcha_action.action	string	The action name (up to 100 characters) from a reCAPTCHA action-token. See Action names.
token.recaptcha_action.valid	bool	The presence of a valid reCAPTCHA action-token.

Session-token attributes

Field	Type	Field description
token.recaptcha_session.score	float	The score from a reCAPTCHA session-token. A valid score ranges from 0.0 to 1.0, with 0.0 being very likely an illegitimate user, and 1.0 being very likely a legitimate user.
token.recaptcha_session.valid	bool	The presence of a valid reCAPTCHA session-token.

Operations

The following reference describes the operators that you can use with attributes (represented by x, y, and k) to define rule expressions.

Expressions	Description
x == "foo"	Returns true if x is equal to the given constant string literal.
x == R"fo'o"	Returns true if x is equal to the given raw string literal that does not interpret escape sequences. Raw string literals are convenient for expressing strings that themselves must use escape sequence characters.
x == y	Returns true if x is equal to y.
x != y	Returns true if x is not equal to y.
x + y	Returns the concatenated string xy.
x && y	Returns true if both x and y are true.
x || y	Returns true if x, y, or both are true.
!x	Returns true if the Boolean value x is false, or returns false if the Boolean value x is true.
x.contains(y)	Returns true if the string x contains the substring y.
x.startsWith(y)	Returns true if the string x begins with the substring y.
x.endsWith(y)	Returns true if the string x ends with the substring y.
x.matches(y)	Returns true if the string x is partially matched by the specified RE2 pattern y. The RE2 pattern is compiled by using the RE2::Latin1 option that disables Unicode features.
inIpRange(x, y)	Returns true if the IP address x is contained within the IP range y.
x.lower()	Returns the lowercase value of the string x.
x.upper()	Returns the uppercase value of the string x.
x.base64Decode()	Returns the base64 decoded value of x; the characters _ - are first replaced with / + respectively. Returns "" (empty string) if x is not a valid base64 value.
has(m['k'])	Returns true if key k is available in the map m.
m['k']	Returns the value at key k in the string-to-string map m if k is available; otherwise, returns an error. Recommended approach is to first check for availability by using "has(m['k'])==true".
int(x)	Converts the string result of x to an int type. It can then be used to do an integer comparison by using standard arithmetic operators such as > and <=. This works only for values that are supposed to be integers.
size(x)	Returns the length of string x.
x.urlDecode()	Returns the url-decoded value of x; character sequences in %## format are replaced with the non-ASCII equivalents, and + is replaced with a space. Invalid encodings are returned as-is.
x.urlDecodeUni()	Returns the url-decoded value of x; in addition to urlDecode(), this also handles unicode character sequences in %u### format. Invalid encodings are returned as-is.
x.utf8ToUnicode()	Returns the lowercase Unicode representation of a UTF-8 encoded x.

Example expressions

For each of these expressions, the action taken depends on whether the expression is included in a deny rule or an allow rule.

Allow or deny access based on an IP address range in IPv4 or IPv6

• The following expression matches with requests from the 198.51.100.0/24 IP address range:

  inIpRange(origin.ip, '198.51.100.0/24')
  

• The following expression matches with requests from the 2001:db8::/32 IP address range:

  inIpRange(origin.ip, '2001:db8::/32')
  

Allow or deny access based on a custom client IP address range behind an upstream proxy

If you have configured the origin.user_ip operator, you can match based on the header values that you specified in your advancedOptionsConfig.userIpRequestHeaders[] field.

• The following expression matches with requests that originated from the 192.0.2.0/24 IP address range:

  inIpRange(origin.user_ip, '192.0.2.0/24')
  

• The following expression matches with requests that originated from the 2001:db8::/32 IP address range:

  inIpRange(origin.user_ip, '2001:db8::/32')
  

Allow or deny traffic with a specific cookie

• The following expression matches with requests that have a cookie containing 80=BLAH:

  has(request.headers['cookie']) && request.headers['cookie'].contains('80=BLAH')
  

Allow or deny traffic with a non-empty referer header

• The following expression matches with requests that have a non-empty referer header:

  has(request.headers['referer']) && request.headers['referer'] != ""
  

Allow or deny traffic based on host URL in header

• The following expression matches with requests to a specific URL:

  request.headers['host'].lower().contains('test.example.com')
  

Allow or deny traffic from a specific region

If your web application isn't available in the AU region, then all requests from that region must be blocked.

• In a deny rule, use the following expression, which matches requests from the AU region:

  origin.region_code == 'AU'
  

Alternatively, if your web application is only available in the AU region, then requests from all other regions must be blocked.

• In a deny rule, use the following expression, which matches requests from all regions other than the AU region:

  origin.region_code != 'AU'
  

The region codes are based on the ISO 3166-1 alpha 2 codes. In some cases, a region corresponds to a country, but this is not always the case. For example, the US code includes all states of the United States, one district, and six outlying areas.

Note: We recommend that you don't use one statement to compare origin.region_code to a list of regions. Instead, we recommend that you use multiple statements separated by AND or OR operators (like origin.region_code != 'AU' || origin.region_code != 'US').

Allow or deny traffic from a specific ASN

If your web application needs to be blocked to customers serviced by a specific network operator, you can use the ASN number of the network operator to block.

• In a deny rule, use the following expression, which matches requests from a specific ASN:

  origin.asn == 123
  

Alternatively, if your web application is to be only available to customers behind a specific network operator, then requests from all other network operators must be blocked.

• In a deny rule, use the following expression, which matches all other network operators other than the one that you are interested in allowing:

  origin.asn != 123
  

Multiple expressions

To include multiple conditions in a single rule, combine multiple subexpressions.

• In the following example, requests from 1.2.3.0/24 (such as your alpha testers) in the AU region match the following expression:

  origin.region_code == "AU" && inIpRange(origin.ip, '1.2.3.0/24')
  

• The following expression matches requests from 1.2.3.4 where a user agent contains the string WordPress:

  inIpRange(origin.ip, '1.2.3.4/32') &&
  has(request.headers['user-agent']) && request.headers['user-agent'].contains('WordPress')
  

Allow or deny traffic for a request URI that matches a regular expression

• The following expression matches with requests that contain the string example_path in the URI:

  request.path.matches('/example_path/')
  

• The following expression matches with requests that have Chrome in the User-Agent header field:

  request.headers['user-agent'].matches('Chrome')
  

• The following expression shows case-insensitive matching for the User-Agent header containing wordpress; it matches with User-Agent:WordPress/605.1.15, User-Agent:wordPress, and other variations of wordpress:

  request.headers['user-agent'].matches('(?i:wordpress)')
  

Allow or deny traffic that contains a specific base64 decoded value

• The following expression matches with requests that have a base64 decoded value of myValue for the user-id header:

  has(request.headers['user-id']) && request.headers['user-id'].base64Decode().contains('myValue')
  

Allow or deny traffic that contains a string value of a specific length

• The following expression matches requests that have a URL length greater than 10 characters:

  size(request.path) > 10
  

• The following expression matches requests that have a header x-data length greater than or equal to 1024 characters:

  size(request.headers['x-data']) >= 1024
  

Allow or deny traffic that has zero content-length in the HTTP body

• The following expression matches requests that have a zero content-length in the HTTP body:

  int(request.headers["content-length"]) == 0
  

Allow or deny traffic that contains a specific URL encoded value

• The following expression matches with requests that have a cookie value containing %3c:

  has(request.headers['cookie']) && request.headers['cookie'].urlDecode().contains('<')
  

Allow or deny traffic that contains a specific URL encoded value of a Unicode string

• The following expression matches with requests that have a cookie value equal to Match%2BValue or Match%u002BValue:

  has(request.headers['cookie']) && request.headers['cookie'].urlDecodeUni() == 'Match+Value'
  

Allow or deny traffic that contains a specific Unicode string of a UTF-8 text

• The following expression matches with requests that have a cookie value equal to ¬:

  has(request.headers['cookie']) && request.headers['cookie'].utf8ToUnicode() == '%u00ac'
  

Allow or deny traffic based on a known JA3 fingerprint

• The following expression matches with requests that have a JA3 fingerprint equal to e7d705a3286e19ea42f587b344ee6865:

  origin.tls_ja3_fingerprint == 'e7d705a3286e19ea42f587b344ee6865'
  

Allow or deny traffic based on a list of JA3 fingerprints

• The following expression matches with requests that have a JA3 fingerprint equal to any of the following JA3 fingerprints:

  • e7d705a3286e19ea42f587b344ee6865
  • f8a5929f8949e846267b582072e35f84
  • 8f8b62163873a62234c14f15e7b88340

  origin.tls_ja3_fingerprint == 'e7d705a3286e19ea42f587b344ee6865' || origin.tls_ja3_fingerprint == 'f8a5929f8949e846267b582072e35f84' || origin.tls_ja3_fingerprint == '8f8b62163873a62234c14f15e7b88340'
  

Preconfigured WAF rules

Preconfigured WAF rules use preconfigured static signatures, regular expressions, or both to match on the HTTP POST body, HTTP request headers, and query parameters. The available preconfigured WAF rules are based on the OWASP Modsecurity core rule set version 3.3. Google Cloud Armor provides several predefined preconfigured WAF rules. For a complete list of preconfigured WAF rules, see the Google Cloud Armor preconfigured WAF rules overview.

To list all of the available preconfigured WAF rules, see List available preconfigured WAF rules.

For more information about preconfigured WAF rules, see the use case Mitigate application layer attacks by using preconfigured WAF rules.

Preconfigured WAF rule names

Preconfigured WAF rule names have the format <attack category>-<ModSecurity CRS version>-<version field>. The attack category specifies the type of attacks that you want to protect against, such as xss (cross-site scripting) or sqli (SQL injection).

The supported version fields are stable and canary. Additions and modifications to the rules are released in the canary version first. When additions and modifications are considered safe and stable, they are promoted to the stable version.

Preconfigured WAF rule member IDs

A preconfigured WAF rule contains several expressions, each with its signature. For example, the preconfigured WAF rule xss-v33-stable includes an expression called owasp-crs-v030301-id941100-xss, which corresponds to rule ID id941100 for version 3.3. You can use the signatures to exclude specific expressions from being used, which is useful if a particular expression consistently triggers a false positive. For more information, see the false positives troubleshooting information.

For information about the core rule set and tuning at different sensitivity levels, see Tuning Google Cloud Armor WAF rules.

Operator for preconfigured WAF rules

Expressions	Description
evaluatePreconfiguredWaf(string, MAP<string, dyn>)	Returns true if any one of the WAF signatures inside the specified WAF rule set returns true. The first argument is the name of the WAF rule set, such as xss-v33-stable. The second argument (optional) is a map in which the key is a string and the value is dynamically typed depending on the key. The purpose of this argument is to fine tune which WAF signatures are evaluated. Accepted keys include the following: "sensitivity": This corresponds to the ModSecurity Core Rule Set paranoia level, which has 4 levels, ranging from 1 to 4. Its value is an integer with a valid range from 0 to 4. Note that 0 is reserved as a valid value when used in combination with "opt_in_rule_ids" (described later). When specifying a sensitivity of x (x >= 1), all the associated WAF signatures with a sensitivity value from 1 to x are evaluated. When omitted, 4 is used as the sensitivity value. "opt_out_rule_ids": WAF signatures (represented by rule IDs) to be opted out from evaluation, where the base set is determined by the sensitivity value. Its value is a list of strings. The maximum number of rule IDs allowed is 128. "opt_in_rule_ids": WAF signatures (represented by rule IDs) to be opted in for evaluation, where the base set is empty. Its value is a list of strings. The maximum number of rule IDs allowed is 128. When using this, a "sensitivity" of 0 must be specified. The keys "opt_out_rule_ids" and "opt_in_rule_ids" are mutually exclusive. You can choose to use "opt_in_rule_ids" if you want to review and manually opt in new WAF signatures that are added later into an existing rule set.
evaluatePreconfiguredExpr(string, LIST)	Returns true if any one of the expressions inside the specified preconfigured WAF rule returns true. The first argument is the name of the preconfigured WAF rule, such as xss-stable. The second argument (optional) is a comma-separated string list of IDs that should be excluded from evaluation. The exclusion list is useful when a given member of the preconfigured WAF rule triggers a false positive.

Preconfigured WAF rule examples

Note: The following examples use the evaluatePreconfiguredExpr() expression. For full WAF tuning capabilities, we recommend that you use evaluatePreconfiguredWaf(). For more information about preconfigured WAF rule tuning capabilities, see Tune WAF rules.

• The following expression uses the xss-v33-stable preconfigured WAF rule to mitigate XSS attacks:

  evaluatePreconfiguredExpr('xss-v33-stable')
  

• The following expression uses all the expressions from the xss-v33-stable preconfigured WAF rule except for member IDs 941100 and 941110:

  evaluatePreconfiguredExpr('xss-v33-stable', ['owasp-crs-v030301-id941100-xss',
  'owasp-crs-v030301-id941110-xss'])
  

• The following expression uses a preconfigured WAF rule to mitigate SQLi attacks from the 198.51.100.0/24 IP address range:

  inIpRange(origin.ip, '198.51.100.0/24') && evaluatePreconfiguredExpr('sqli-v33-stable')
  

What's next

• Configure security policies, rules, and expressions
• Tune web application firewall (WAF) rules
• Troubleshoot issues