feat(checker): add CEL for matching complicated expressions (#421)

* feat(lib/policy): add support for CEL checkers

This adds the ability for administrators to use Common Expression
Language[0] (CEL) for more advanced check logic than Anubis previously
offered.

These can be as simple as:

```yaml
- name: allow-api-routes
  action: ALLOW
  expression:
    and:
    - '!(method == "HEAD" || method == "GET")'
    - path.startsWith("/api/")
```

or get as complicated as:

```yaml
- name: allow-git-clients
  action: ALLOW
  expression:
    and:
    - userAgent.startsWith("git/") || userAgent.contains("libgit") || userAgent.startsWith("go-git") || userAgent.startsWith("JGit/") || userAgent.startsWith("JGit-")
    - >
      "Git-Protocol" in headers && headers["Git-Protocol"] == "version=2"
```

Internally these are compiled and evaluated with cel-go[1]. This also
leaves room for extensibility should that be desired in the future. This
will intersect with #338 and eventually intersect with TLS fingerprints
as in #337.

[0]: https://cel.dev/
[1]: https://github.com/google/cel-go

Signed-off-by: Xe Iaso <me@xeiaso.net>

* feat(data/apps): add API route allow rule for non-HEAD/GET

Signed-off-by: Xe Iaso <me@xeiaso.net>

* docs: document expression syntax

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix: fixes in review

Signed-off-by: Xe Iaso <me@xeiaso.net>

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/policy/celchecker.go

@@ -0,0 +1,108 @@
+package policy
+
+import (
+	"fmt"
+	"net/http"
+
+	"github.com/TecharoHQ/anubis/internal"
+	"github.com/TecharoHQ/anubis/lib/policy/config"
+	"github.com/TecharoHQ/anubis/lib/policy/expressions"
+	"github.com/google/cel-go/cel"
+	"github.com/google/cel-go/common/types"
+)
+
+type CELChecker struct {
+	src     string
+	program cel.Program
+}
+
+func NewCELChecker(cfg *config.ExpressionOrList) (*CELChecker, error) {
+	env, err := expressions.NewEnvironment()
+	if err != nil {
+		return nil, err
+	}
+
+	var src string
+	var ast *cel.Ast
+
+	if cfg.Expression != "" {
+		src = cfg.Expression
+		var iss *cel.Issues
+		interm, iss := env.Compile(src)
+		if iss != nil {
+			return nil, iss.Err()
+		}
+
+		ast, iss = env.Check(interm)
+		if iss != nil {
+			return nil, iss.Err()
+		}
+	}
+
+	if len(cfg.All) != 0 {
+		ast, err = expressions.Join(env, expressions.JoinAnd, cfg.All...)
+	}
+
+	if len(cfg.Any) != 0 {
+		ast, err = expressions.Join(env, expressions.JoinOr, cfg.Any...)
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	program, err := expressions.Compile(env, ast)
+	if err != nil {
+		return nil, fmt.Errorf("can't compile CEL program: %w", err)
+	}
+
+	return &CELChecker{
+		src:     src,
+		program: program,
+	}, nil
+}
+
+func (cc *CELChecker) Hash() string {
+	return internal.SHA256sum(cc.src)
+}
+
+func (cc *CELChecker) Check(r *http.Request) (bool, error) {
+	result, _, err := cc.program.ContextEval(r.Context(), &CELRequest{r})
+
+	if err != nil {
+		return false, err
+	}
+
+	if val, ok := result.(types.Bool); ok {
+		return bool(val), nil
+	}
+
+	return false, nil
+}
+
+type CELRequest struct {
+	*http.Request
+}
+
+func (cr *CELRequest) Parent() cel.Activation { return nil }
+
+func (cr *CELRequest) ResolveName(name string) (any, bool) {
+	switch name {
+	case "remoteAddress":
+		return cr.Header.Get("X-Real-Ip"), true
+	case "host":
+		return cr.Host, true
+	case "method":
+		return cr.Method, true
+	case "userAgent":
+		return cr.UserAgent(), true
+	case "path":
+		return cr.URL.Path, true
+	case "query":
+		return expressions.URLValues{Values: cr.URL.Query()}, true
+	case "headers":
+		return expressions.HTTPHeaders{Header: cr.Header}, true
+	default:
+		return nil, false
+	}
+}

lib/policy/config/config.go

@@ -55,9 +55,11 @@ type BotConfig struct {
 	UserAgentRegex *string           `json:"user_agent_regex"`
 	PathRegex      *string           `json:"path_regex"`
 	HeadersRegex   map[string]string `json:"headers_regex"`
-	Action         Rule              `json:"action"`
 	RemoteAddr     []string          `json:"remote_addresses"`
-	Challenge      *ChallengeRules   `json:"challenge,omitempty"`
+	Expression     *ExpressionOrList `json:"expression"`
+
+	Action    Rule            `json:"action"`
+	Challenge *ChallengeRules `json:"challenge,omitempty"`
 }
 
 func (b BotConfig) Zero() bool {
@@ -85,7 +87,12 @@ func (b BotConfig) Valid() error {
 		errs = append(errs, ErrBotMustHaveName)
 	}
 
-	if b.UserAgentRegex == nil && b.PathRegex == nil && len(b.RemoteAddr) == 0 && len(b.HeadersRegex) == 0 {
+	allFieldsEmpty := b.UserAgentRegex == nil &&
+		b.PathRegex == nil &&
+		len(b.RemoteAddr) == 0 &&
+		len(b.HeadersRegex) == 0
+
+	if allFieldsEmpty && b.Expression == nil {
 		errs = append(errs, ErrBotMustHaveUserAgentOrPath)
 	}
 
@@ -137,6 +144,12 @@ func (b BotConfig) Valid() error {
 		}
 	}
 
+	if b.Expression != nil {
+		if err := b.Expression.Valid(); err != nil {
+			errs = append(errs, err)
+		}
+	}
+
 	switch b.Action {
 	case RuleAllow, RuleBenchmark, RuleChallenge, RuleDeny:
 		// okay

lib/policy/config/expressionorlist.go

@@ -0,0 +1,62 @@
+package config
+
+import (
+	"encoding/json"
+	"errors"
+	"slices"
+)
+
+var (
+	ErrExpressionOrListMustBeStringOrObject = errors.New("config: this must be a string or an object")
+	ErrExpressionEmpty                      = errors.New("config: this expression is empty")
+	ErrExpressionCantHaveBoth               = errors.New("config: expression block can't contain multiple expression types")
+)
+
+type ExpressionOrList struct {
+	Expression string   `json:"-"`
+	All        []string `json:"all"`
+	Any        []string `json:"any"`
+}
+
+func (eol ExpressionOrList) Equal(rhs *ExpressionOrList) bool {
+	if eol.Expression != rhs.Expression {
+		return false
+	}
+
+	if !slices.Equal(eol.All, rhs.All) {
+		return false
+	}
+
+	if !slices.Equal(eol.Any, rhs.Any) {
+		return false
+	}
+
+	return true
+}
+
+func (eol *ExpressionOrList) UnmarshalJSON(data []byte) error {
+	switch string(data[0]) {
+	case `"`: // string
+		return json.Unmarshal(data, &eol.Expression)
+	case "{": // object
+		type RawExpressionOrList ExpressionOrList
+		var val RawExpressionOrList
+		if err := json.Unmarshal(data, &val); err != nil {
+			return err
+		}
+		eol.All = val.All
+		eol.Any = val.Any
+
+		return nil
+	}
+
+	return ErrExpressionOrListMustBeStringOrObject
+}
+
+func (eol *ExpressionOrList) Valid() error {
+	if len(eol.All) != 0 && len(eol.Any) != 0 {
+		return ErrExpressionCantHaveBoth
+	}
+
+	return nil
+}

lib/policy/config/expressionorlist_test.go

@@ -0,0 +1,73 @@
+package config
+
+import (
+	"encoding/json"
+	"errors"
+	"testing"
+)
+
+func TestExpressionOrListUnmarshal(t *testing.T) {
+	for _, tt := range []struct {
+		name     string
+		inp      string
+		err      error
+		validErr error
+		result   *ExpressionOrList
+	}{
+		{
+			name: "simple",
+			inp:  `"\"User-Agent\" in headers"`,
+			result: &ExpressionOrList{
+				Expression: `"User-Agent" in headers`,
+			},
+		},
+		{
+			name: "object-and",
+			inp: `{
+			"all": ["\"User-Agent\" in headers"]
+			}`,
+			result: &ExpressionOrList{
+				All: []string{
+					`"User-Agent" in headers`,
+				},
+			},
+		},
+		{
+			name: "object-or",
+			inp: `{
+			"any": ["\"User-Agent\" in headers"]
+			}`,
+			result: &ExpressionOrList{
+				Any: []string{
+					`"User-Agent" in headers`,
+				},
+			},
+		},
+		{
+			name: "both-or-and",
+			inp: `{
+			"all": ["\"User-Agent\" in headers"],
+			"any": ["\"User-Agent\" in headers"]
+			}`,
+			validErr: ErrExpressionCantHaveBoth,
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			var eol ExpressionOrList
+
+			if err := json.Unmarshal([]byte(tt.inp), &eol); !errors.Is(err, tt.err) {
+				t.Errorf("wanted unmarshal error: %v but got: %v", tt.err, err)
+			}
+
+			if tt.result != nil && !eol.Equal(tt.result) {
+				t.Logf("wanted: %#v", tt.result)
+				t.Logf("got:    %#v", &eol)
+				t.Fatal("parsed expression is not what was expected")
+			}
+
+			if err := eol.Valid(); !errors.Is(err, tt.validErr) {
+				t.Errorf("wanted validation error: %v but got: %v", tt.err, err)
+			}
+		})
+	}
+}

lib/policy/config/testdata/bad/multiple_expression_types.json

@@ -0,0 +1,17 @@
+{
+  "bots": [
+    {
+      "name": "multiple-expression-types",
+      "action": "ALLOW",
+      "expression": {
+        "all": [
+          "userAgent.startsWith(\"git/\") || userAgent.contains(\"libgit\")",
+          "\"Git-Protocol\" in headers && headers[\"Git-Protocol\"] == \"version=2\"\n"
+        ],
+        "any": [
+          "userAgent.startsWith(\"evilbot/\")"
+        ]
+      }
+    }
+  ]
+}

lib/policy/config/testdata/bad/multiple_expression_types.yaml

@@ -0,0 +1,10 @@
+bots:
+- name: multiple-expression-types
+  action: ALLOW
+  expression:
+    all:
+    - userAgent.startsWith("git/") || userAgent.contains("libgit")
+    - >
+      "Git-Protocol" in headers && headers["Git-Protocol"] == "version=2"
+    any:
+    - userAgent.startsWith("evilbot/")

lib/policy/config/testdata/good/git_client.json

@@ -0,0 +1,14 @@
+{
+  "bots": [
+    {
+      "name": "allow-git-clients",
+      "action": "ALLOW",
+      "expression": {
+        "all": [
+          "userAgent.startsWith(\"git/\") || userAgent.contains(\"libgit\")",
+          "\"Git-Protocol\" in headers && headers[\"Git-Protocol\"] == \"version=2\""
+        ]
+      }
+    }
+  ]
+}

lib/policy/config/testdata/good/git_client.yaml

@@ -0,0 +1,8 @@
+bots:
+- name: allow-git-clients
+  action: ALLOW
+  expression:
+    all:
+    - userAgent.startsWith("git/") || userAgent.contains("libgit")
+    - >
+      "Git-Protocol" in headers && headers["Git-Protocol"] == "version=2"

lib/policy/expressions/README.md

@@ -0,0 +1,3 @@
+# Expressions support
+
+The expressions support is based on ideas from [go-away](https://git.gammaspectra.live/git/go-away) but with different opinions about how things should be done.

lib/policy/expressions/environment.go

@@ -0,0 +1,45 @@
+package expressions
+
+import (
+	"github.com/google/cel-go/cel"
+	"github.com/google/cel-go/ext"
+)
+
+// NewEnvironment creates a new CEL environment, this is the set of
+// variables and functions that are passed into the CEL scope so that
+// Anubis can fail loudly and early when something is invalid instead
+// of blowing up at runtime.
+func NewEnvironment() (*cel.Env, error) {
+	return cel.NewEnv(
+		ext.Strings(
+			ext.StringsLocale("en_US"),
+			ext.StringsValidateFormatCalls(true),
+		),
+
+		// default all timestamps to UTC
+		cel.DefaultUTCTimeZone(true),
+
+		// Variables exposed to CEL programs:
+		cel.Variable("remoteAddress", cel.StringType),
+		cel.Variable("host", cel.StringType),
+		cel.Variable("method", cel.StringType),
+		cel.Variable("userAgent", cel.StringType),
+		cel.Variable("path", cel.StringType),
+		cel.Variable("query", cel.MapType(cel.StringType, cel.StringType)),
+		cel.Variable("headers", cel.MapType(cel.StringType, cel.StringType)),
+
+		// Functions exposed to CEL programs:
+	)
+}
+
+// Compile takes CEL environment and syntax tree then emits an optimized
+// Program for execution.
+func Compile(env *cel.Env, ast *cel.Ast) (cel.Program, error) {
+	return env.Program(
+		ast,
+		cel.EvalOptions(
+			// optimize regular expressions right now instead of on the fly
+			cel.OptOptimize,
+		),
+	)
+}

lib/policy/expressions/http_headers.go

@@ -0,0 +1,75 @@
+package expressions
+
+import (
+	"net/http"
+	"reflect"
+	"strings"
+
+	"github.com/google/cel-go/common/types"
+	"github.com/google/cel-go/common/types/ref"
+	"github.com/google/cel-go/common/types/traits"
+)
+
+// HTTPHeaders is a type wrapper to expose HTTP headers into CEL programs.
+type HTTPHeaders struct {
+	http.Header
+}
+
+func (h HTTPHeaders) ConvertToNative(typeDesc reflect.Type) (any, error) {
+	return nil, ErrNotImplemented
+}
+
+func (h HTTPHeaders) ConvertToType(typeVal ref.Type) ref.Val {
+	switch typeVal {
+	case types.MapType:
+		return h
+	case types.TypeType:
+		return types.MapType
+	}
+
+	return types.NewErr("can't convert from %q to %q", types.MapType, typeVal)
+}
+
+func (h HTTPHeaders) Equal(other ref.Val) ref.Val {
+	return types.Bool(false) // We don't want to compare header maps
+}
+
+func (h HTTPHeaders) Type() ref.Type {
+	return types.MapType
+}
+
+func (h HTTPHeaders) Value() any { return h }
+
+func (h HTTPHeaders) Find(key ref.Val) (ref.Val, bool) {
+	k, ok := key.(types.String)
+	if !ok {
+		return nil, false
+	}
+
+	if _, ok := h.Header[string(k)]; !ok {
+		return nil, false
+	}
+
+	return types.String(strings.Join(h.Header.Values(string(k)), ",")), true
+}
+
+func (h HTTPHeaders) Contains(key ref.Val) ref.Val {
+	_, ok := h.Find(key)
+	return types.Bool(ok)
+}
+
+func (h HTTPHeaders) Get(key ref.Val) ref.Val {
+	result, ok := h.Find(key)
+	if !ok {
+		return types.ValOrErr(result, "no such key: %v", key)
+	}
+	return result
+}
+
+func (h HTTPHeaders) Iterator() traits.Iterator { panic("TODO(Xe): implement me") }
+
+func (h HTTPHeaders) IsZeroValue() bool {
+	return len(h.Header) == 0
+}
+
+func (h HTTPHeaders) Size() ref.Val { return types.Int(len(h.Header)) }

lib/policy/expressions/http_headers_test.go

@@ -0,0 +1,52 @@
+package expressions
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/google/cel-go/common/types"
+)
+
+func TestHTTPHeaders(t *testing.T) {
+	headers := HTTPHeaders{
+		Header: http.Header{
+			"Content-Type": {"application/json"},
+			"Cf-Worker":    {"true"},
+			"User-Agent":   {"Go-http-client/2"},
+		},
+	}
+
+	t.Run("contains-existing-header", func(t *testing.T) {
+		resp := headers.Contains(types.String("User-Agent"))
+		if !bool(resp.(types.Bool)) {
+			t.Fatal("headers does not contain User-Agent")
+		}
+	})
+
+	t.Run("not-contains-missing-header", func(t *testing.T) {
+		resp := headers.Contains(types.String("Xxx-Random-Header"))
+		if bool(resp.(types.Bool)) {
+			t.Fatal("headers does not contain User-Agent")
+		}
+	})
+
+	t.Run("get-existing-header", func(t *testing.T) {
+		val := headers.Get(types.String("User-Agent"))
+		switch val.(type) {
+		case types.String:
+			// ok
+		default:
+			t.Fatalf("result was wrong type %T", val)
+		}
+	})
+
+	t.Run("not-get-missing-header", func(t *testing.T) {
+		val := headers.Get(types.String("Xxx-Random-Header"))
+		switch val.(type) {
+		case *types.Err:
+			// ok
+		default:
+			t.Fatalf("result was wrong type %T", val)
+		}
+	})
+}

lib/policy/expressions/join.go

@@ -0,0 +1,104 @@
+package expressions
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+
+	"github.com/google/cel-go/cel"
+)
+
+// JoinOperator is a type wrapper for and/or operators.
+//
+// This is a separate type so that validation can be done at the type level.
+type JoinOperator string
+
+// Possible values for JoinOperator
+const (
+	JoinAnd JoinOperator = "&&"
+	JoinOr  JoinOperator = "||"
+)
+
+// Valid ensures that JoinOperator is semantically valid.
+func (jo JoinOperator) Valid() error {
+	switch jo {
+	case JoinAnd, JoinOr:
+		return nil
+	default:
+		return ErrWrongJoinOperator
+	}
+}
+
+var (
+	ErrWrongJoinOperator = errors.New("expressions: invalid join operator")
+	ErrNoExpressions     = errors.New("expressions: cannot join zero expressions")
+	ErrCantCompile       = errors.New("expressions: can't compile one expression")
+)
+
+// JoinClauses joins a list of compiled clauses into one big if statement.
+//
+// Imagine the following two clauses:
+//
+//	ball.color == "red"
+//	ball.shape == "round"
+//
+// JoinClauses would emit one "joined" clause such as:
+//
+//	( ball.color == "red" ) && ( ball.shape == "round" )
+func JoinClauses(env *cel.Env, operator JoinOperator, clauses ...*cel.Ast) (*cel.Ast, error) {
+	if err := operator.Valid(); err != nil {
+		return nil, fmt.Errorf("%w: wanted && or ||, got: %q", err, operator)
+	}
+
+	switch len(clauses) {
+	case 0:
+		return nil, ErrNoExpressions
+	case 1:
+		return clauses[0], nil
+	}
+
+	var exprs []string
+	var errs []error
+
+	for _, clause := range clauses {
+		clauseStr, err := cel.AstToString(clause)
+		if err != nil {
+			errs = append(errs, err)
+			continue
+		}
+
+		exprs = append(exprs, "( "+clauseStr+" )")
+	}
+
+	if len(errs) != 0 {
+		return nil, fmt.Errorf("errors while decompiling statements: %w", errors.Join(errs...))
+	}
+
+	statement := strings.Join(exprs, " "+string(operator)+" ")
+	result, iss := env.Compile(statement)
+	if iss != nil {
+		return nil, iss.Err()
+	}
+
+	return result, nil
+}
+
+func Join(env *cel.Env, operator JoinOperator, clauses ...string) (*cel.Ast, error) {
+	var statements []*cel.Ast
+	var errs []error
+
+	for _, clause := range clauses {
+		stmt, iss := env.Compile(clause)
+		if iss != nil && iss.Err() != nil {
+			errs = append(errs, fmt.Errorf("%w: %q gave: %w", ErrCantCompile, clause, iss.Err()))
+			continue
+		}
+		statements = append(statements, stmt)
+	}
+
+	if len(errs) != 0 {
+		return nil, fmt.Errorf("errors while joining clauses: %w", errors.Join(errs...))
+	}
+
+	return JoinClauses(env, operator, statements...)
+}

lib/policy/expressions/join_test.go

@@ -0,0 +1,90 @@
+package expressions
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/google/cel-go/cel"
+)
+
+func TestJoin(t *testing.T) {
+	env, err := NewEnvironment()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, tt := range []struct {
+		name      string
+		clauses   []string
+		op        JoinOperator
+		err       error
+		resultStr string
+	}{
+		{
+			name:    "no-clauses",
+			clauses: []string{},
+			op:      JoinAnd,
+			err:     ErrNoExpressions,
+		},
+		{
+			name:      "one-clause-identity",
+			clauses:   []string{`remoteAddress == "8.8.8.8"`},
+			op:        JoinAnd,
+			err:       nil,
+			resultStr: `remoteAddress == "8.8.8.8"`,
+		},
+		{
+			name: "multi-clause-and",
+			clauses: []string{
+				`remoteAddress == "8.8.8.8"`,
+				`host == "anubis.techaro.lol"`,
+			},
+			op:        JoinAnd,
+			err:       nil,
+			resultStr: `remoteAddress == "8.8.8.8" && host == "anubis.techaro.lol"`,
+		},
+		{
+			name: "multi-clause-or",
+			clauses: []string{
+				`remoteAddress == "8.8.8.8"`,
+				`host == "anubis.techaro.lol"`,
+			},
+			op:        JoinOr,
+			err:       nil,
+			resultStr: `remoteAddress == "8.8.8.8" || host == "anubis.techaro.lol"`,
+		},
+		{
+			name: "git-user-agent",
+			clauses: []string{
+				`userAgent.startsWith("git/") || userAgent.contains("libgit")`,
+				`"Git-Protocol" in headers && headers["Git-Protocol"] == "version=2"`,
+			},
+			op:  JoinAnd,
+			err: nil,
+			resultStr: `(userAgent.startsWith("git/") || userAgent.contains("libgit")) && "Git-Protocol" in headers &&
+headers["Git-Protocol"] == "version=2"`,
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			result, err := Join(env, tt.op, tt.clauses...)
+			if !errors.Is(err, tt.err) {
+				t.Errorf("wanted error %v but got: %v", tt.err, err)
+			}
+
+			if tt.err != nil {
+				return
+			}
+
+			program, err := cel.AstToString(result)
+			if err != nil {
+				t.Fatalf("can't decompile program: %v", err)
+			}
+
+			if tt.resultStr != program {
+				t.Logf("wanted: %s", tt.resultStr)
+				t.Logf("got: %s", program)
+				t.Error("program did not compile as expected")
+			}
+		})
+	}
+}

lib/policy/expressions/url_values.go

@@ -0,0 +1,78 @@
+package expressions
+
+import (
+	"errors"
+	"net/url"
+	"reflect"
+	"strings"
+
+	"github.com/google/cel-go/common/types"
+	"github.com/google/cel-go/common/types/ref"
+	"github.com/google/cel-go/common/types/traits"
+)
+
+var ErrNotImplemented = errors.New("expressions: not implemented")
+
+// URLValues is a type wrapper to expose url.Values into CEL programs.
+type URLValues struct {
+	url.Values
+}
+
+func (u URLValues) ConvertToNative(typeDesc reflect.Type) (any, error) {
+	return nil, ErrNotImplemented
+}
+
+func (u URLValues) ConvertToType(typeVal ref.Type) ref.Val {
+	switch typeVal {
+	case types.MapType:
+		return u
+	case types.TypeType:
+		return types.MapType
+	}
+
+	return types.NewErr("can't convert from %q to %q", types.MapType, typeVal)
+}
+
+func (u URLValues) Equal(other ref.Val) ref.Val {
+	return types.Bool(false) // We don't want to compare header maps
+}
+
+func (u URLValues) Type() ref.Type {
+	return types.MapType
+}
+
+func (u URLValues) Value() any { return u }
+
+func (u URLValues) Find(key ref.Val) (ref.Val, bool) {
+	k, ok := key.(types.String)
+	if !ok {
+		return nil, false
+	}
+
+	if _, ok := u.Values[string(k)]; !ok {
+		return nil, false
+	}
+
+	return types.String(strings.Join(u.Values[string(k)], ",")), true
+}
+
+func (u URLValues) Contains(key ref.Val) ref.Val {
+	_, ok := u.Find(key)
+	return types.Bool(ok)
+}
+
+func (u URLValues) Get(key ref.Val) ref.Val {
+	result, ok := u.Find(key)
+	if !ok {
+		return types.ValOrErr(result, "no such key: %v", key)
+	}
+	return result
+}
+
+func (u URLValues) Iterator() traits.Iterator { panic("TODO(Xe): implement me") }
+
+func (u URLValues) IsZeroValue() bool {
+	return len(u.Values) == 0
+}
+
+func (u URLValues) Size() ref.Val { return types.Int(len(u.Values)) }

lib/policy/expressions/url_values_test.go

@@ -0,0 +1,50 @@
+package expressions
+
+import (
+	"net/url"
+	"testing"
+
+	"github.com/google/cel-go/common/types"
+)
+
+func TestURLValues(t *testing.T) {
+	headers := URLValues{
+		Values: url.Values{
+			"format": {"json"},
+		},
+	}
+
+	t.Run("contains-existing-key", func(t *testing.T) {
+		resp := headers.Contains(types.String("format"))
+		if !bool(resp.(types.Bool)) {
+			t.Fatal("headers does not contain User-Agent")
+		}
+	})
+
+	t.Run("not-contains-missing-key", func(t *testing.T) {
+		resp := headers.Contains(types.String("not-there"))
+		if bool(resp.(types.Bool)) {
+			t.Fatal("headers does not contain User-Agent")
+		}
+	})
+
+	t.Run("get-existing-key", func(t *testing.T) {
+		val := headers.Get(types.String("format"))
+		switch val.(type) {
+		case types.String:
+			// ok
+		default:
+			t.Fatalf("result was wrong type %T", val)
+		}
+	})
+
+	t.Run("not-get-missing-key", func(t *testing.T) {
+		val := headers.Get(types.String("not-there"))
+		switch val.(type) {
+		case *types.Err:
+			// ok
+		default:
+			t.Fatalf("result was wrong type %T", val)
+		}
+	})
+}

lib/policy/policy.go

@@ -94,6 +94,15 @@ func ParseConfig(fin io.Reader, fname string, defaultDifficulty int) (*ParsedCon
 			}
 		}
 
+		if b.Expression != nil {
+			c, err := NewCELChecker(b.Expression)
+			if err != nil {
+				validationErrs = append(validationErrs, fmt.Errorf("while processing rule %s expressions: %w", b.Name, err))
+			} else {
+				cl = append(cl, c)
+			}
+		}
+
 		if b.Challenge == nil {
 			parsedBot.Challenge = &config.ChallengeRules{
 				Difficulty: defaultDifficulty,