smallwebwaf/internal/waf/waf.go

// Package waf runs the OWASP Core Rule Set 4.25.0, through Coraza, on the
// method, the URL with its query and the headers of a request, and on its
// body while SWWAF_WAF_BODY_LIMIT is set, with the six changes smallwebwaf
// makes to it, as "Attack detection" under "Configuration surface" in
// SPEC.md describes them. It reads no response.
//
// smallwebwaf writes only to its state directory, so Coraza is built with
// its no_fs_access tag, as the Dockerfile and script/build build it: of a
// file in a multipart body, Coraza then counts the bytes instead of
// writing them to the system's temporary directory.
package waf

import (
	"fmt"
	"io"
	"net/http"
	"net/netip"
	"slices"
	"strconv"
	"strings"

	coreruleset "github.com/corazawaf/coraza-coreruleset/v4"
	"github.com/corazawaf/coraza/v3"
	"github.com/corazawaf/coraza/v3/experimental/plugins/plugintypes"
	"github.com/corazawaf/coraza/v3/types"
)

// directives are the Core Rule Set as smallwebwaf runs it, with the
// paranoia level for %d, and bodyDirectives for %s while
// SWWAF_WAF_BODY_LIMIT is set. Each rule smallwebwaf adds has an id from
// 900000 to 900999, the ids the Core Rule Set keeps for the rules that set
// it up, which SWWAF_WAF_DISABLED_RULES refuses, so that no setting
// switches one off. Coraza joins a line ending in \ to the next, without
// the spaces at the start of the next.
const directives = `
# The engine only detects. smallwebwaf compares the request's anomaly
# score with SWWAF_WAF_ANOMALY_THRESHOLD itself, in block and detect mode
# alike. It reads no body, unless bodyDirectives switch that on.
SecRuleEngine DetectionOnly
SecRequestBodyAccess Off
SecResponseBodyAccess Off

Include @crs-setup.conf.example

SecAction "id:900000,phase:1,pass,nolog,\
    setvar:tx.blocking_paranoia_level=%d"

# The first change: PUT, PATCH and DELETE are allowed besides GET, HEAD,
# POST and OPTIONS.
SecAction "id:900200,phase:1,pass,nolog,\
    setvar:'tx.allowed_methods=GET HEAD POST OPTIONS PUT PATCH DELETE'"

# The second: Expect and Content-Encoding are taken off the Core Rule Set's
# list of the headers it refuses. Content-Encoding goes back on it for a
# body the Core Rule Set reads (900260 in bodyDirectives).
SecAction "id:900250,phase:1,pass,nolog,\
    setvar:'tx.restricted_headers_basic=/proxy/ /lock-token/ /content-range/ \
    /if/ /x-http-method-override/ /x-http-method/ /x-method-override/ \
    /x-middleware-subrequest/'"
%s
# Coraza keeps the first 1000 query parameters of a request, and the first
# 1000 fields of a form data or JSON body, and drops the rest, which no
# rule then reads, so a request with more adds 5 to the score, as a rule
# the Core Rule Set rates critical does. Coraza's recommended
# configuration refuses such a request in its rules 200004 and 200005.
# This rule runs once the body is read, and before the Core Rule Set adds
# up the score in the same phase.
SecArgumentsLimit 1000
SecRule ARGUMENTS_LIMIT_REACHED "@eq 1" "id:900300,phase:2,pass,\
    severity:'CRITICAL',setvar:'tx.inbound_anomaly_score_pl1=+5'"

# The sixth: only the rules for requests are loaded, and no response is
# inspected.
Include @owasp_crs/REQUEST-*.conf

# The third: redirect_uri is not checked for a URL naming an IP address or
# localhost. Coraza matches a parameter name here, and in the fourth,
# without regard to case. ARGS holds the fields of a form data or multipart
# body Coraza reads as well as the query parameters, so a field of one of
# these names is left out too.
SecRuleUpdateTargetById 931100 "!ARGS:redirect_uri"
SecRuleUpdateTargetById 934110 "!ARGS:redirect_uri"

# The fourth: the query parameters in which gitea sends names within a
# repository or its own records, or a page of its own site, are not
# checked against the lists of system files, shell paths and command
# names. Coraza takes one rule id per directive.
SecRuleUpdateTargetById 930120 "!ARGS:path|!ARGS:files|!ARGS:skip-to|\
    !ARGS:sub_path|!ARGS:ref|!ARGS:sha|!ARGS:branch|!ARGS:workflow|\
    !ARGS:artifactName|!ARGS:redirect_to"
SecRuleUpdateTargetById 932160 "!ARGS:path|!ARGS:files|!ARGS:skip-to|\
    !ARGS:sub_path|!ARGS:ref|!ARGS:sha|!ARGS:branch|!ARGS:workflow|\
    !ARGS:artifactName|!ARGS:redirect_to"
SecRuleUpdateTargetById 932260 "!ARGS:path|!ARGS:files|!ARGS:skip-to|\
    !ARGS:sub_path|!ARGS:ref|!ARGS:sha|!ARGS:branch|!ARGS:workflow|\
    !ARGS:artifactName|!ARGS:redirect_to"

# The fifth, for Referer: it is not checked for a Unix command without
# arguments, or for Java starting a process. The cookies are left out in
# Inspect.
SecRuleUpdateTargetById 932340 "!REQUEST_HEADERS:Referer"
SecRuleUpdateTargetById 944110 "!REQUEST_HEADERS:Referer"
`

// bodyDirectives have the Core Rule Set read the part of a request body
// Inspect gives it, which is at most one byte longer than the limit, up to
// the limit, %d bytes, and read JSON and XML as Coraza's recommended
// configuration has it in its rules 200000, 200001 and 200006, with
// text/json, and any application or text type ending in +xml or +json,
// besides; form data and multipart Coraza knows by itself. %% stands for
// a % Coraza reads.
const bodyDirectives = `
SecRequestBodyAccess On
SecRequestBodyLimit %d
SecRequestBodyLimitAction ProcessPartial

SecRule REQUEST_HEADERS:Content-Type \
    "@rx ^(?:application|text)/(?:[a-z0-9.-]+[+])?xml" \
    "id:900410,phase:1,pass,nolog,t:none,t:lowercase,ctl:requestBodyProcessor=XML"
SecRule REQUEST_HEADERS:Content-Type \
    "@rx ^(?:application|text)/(?:[a-z0-9.-]+[+])?json" \
    "id:900420,phase:1,pass,nolog,t:none,t:lowercase,ctl:requestBodyProcessor=JSON"

# The rest of the second change: Content-Encoding is refused again on a
# body of a kind the Core Rule Set reads, since a compressed body cannot be
# inspected.
SecRule REQBODY_PROCESSOR "@rx ^(?:URLENCODED|MULTIPART|JSON|XML)$" \
    "id:900260,phase:1,pass,nolog,\
    setvar:'tx.restricted_headers_basic=%%{tx.restricted_headers_basic} \
    /content-encoding/'"

# A body Coraza fails to parse (900440), and a multipart body that fails
# its strict checks (900450), each add 5 to the score, as a rule the Core
# Rule Set rates critical does: no rule reads what comes after the fault,
# which the app may still read. Coraza's recommended configuration refuses
# them in its rules 200002 and 200003. A multipart body the limit cuts
# before the colon of a part's header line, or between the carriage return
# and the line feed that end a part's header line or the empty line after
# its headers, adds 5 too, since Coraza takes the line the limit cuts for a
# malformed header. Coraza parses any form data body.
SecRule REQBODY_ERROR "!@eq 0" "id:900440,phase:2,pass,severity:'CRITICAL',\
    setvar:'tx.inbound_anomaly_score_pl1=+5'"
SecRule MULTIPART_STRICT_ERROR "!@eq 0" "id:900450,phase:2,pass,\
    severity:'CRITICAL',setvar:'tx.inbound_anomaly_score_pl1=+5'"
`

// cookiesNotRead are the cookies the Core Rule Set reads a request
// without, the rest of the fifth change.
//
//nolint:gochecknoglobals // a constant cannot be a list
var cookiesNotRead = []string{"gitea_flash", "redirect_to"}

// Params are what New needs.
type Params struct {
	// ParanoiaLevel is SWWAF_WAF_PARANOIA_LEVEL, from 1 to 4.
	ParanoiaLevel int
	// DisabledRules are the ids of the rules switched off
	// (SWWAF_WAF_DISABLED_RULES).
	DisabledRules []int
	// BodyLimit is the most of a request body the Core Rule Set reads
	// (SWWAF_WAF_BODY_LIMIT), 0 while it is off and it reads none.
	BodyLimit int64
}

// CoreRuleSet is the Core Rule Set, ready to inspect requests. It is safe
// for concurrent use.
type CoreRuleSet struct {
	waf       coraza.WAF
	bodyLimit int64
}

// New returns the Core Rule Set with the six changes, at params'
// paranoia level, without the rules it switches off, and reading request
// bodies up to params' limit.
func New(params Params) (*CoreRuleSet, error) {
	body := ""
	if params.BodyLimit > 0 {
		body = fmt.Sprintf(bodyDirectives, params.BodyLimit)
	}

	text := fmt.Sprintf(directives, params.ParanoiaLevel, body)

	if len(params.DisabledRules) > 0 {
		ids := make([]string, len(params.DisabledRules))
		for i, id := range params.DisabledRules {
			ids[i] = strconv.Itoa(id)
		}

		text += "SecRuleRemoveById " + strings.Join(ids, " ") + "\n"
	}

	waf, err := coraza.NewWAF(coraza.NewWAFConfig().
		WithRootFS(coreruleset.FS).
		WithDirectives(text))
	if err != nil {
		return nil, fmt.Errorf("load the Core Rule Set: %w", err)
	}

	return &CoreRuleSet{waf: waf, bodyLimit: params.BodyLimit}, nil
}

// Result is what the Core Rule Set found in a request.
type Result struct {
	// RuleIDs are the ids of the rules that matched, in the order they
	// ran.
	RuleIDs []int
	// Score is the request's anomaly score: what those rules add up to.
	Score int
}

// Inspect runs the Core Rule Set on r, a request from client: on its
// method, its URL with the query, and its headers, the Cookie header
// without the cookies in cookiesNotRead, and on body, r's body as the
// caller has it, as readBody reads it. It returns what it found, what it
// read of body, which the app is still to be sent, and the error that
// ended the reading early, if one did.
func (c *CoreRuleSet) Inspect(
	r *http.Request, client netip.Addr, body io.Reader,
) (Result, []byte, error) {
	tx := c.waf.NewTransaction()
	// Closing would remove the files Coraza wrote, and it writes none.
	defer func() { _ = tx.Close() }()

	tx.ProcessConnection(client.String(), 0, "", 0)
	tx.ProcessURI(r.URL.String(), r.Method, r.Proto)

	for name, values := range r.Header {
		for _, value := range values {
			if name == "Cookie" {
				value = withoutCookiesNotRead(value)
				if value == "" {
					continue // it held those cookies alone
				}
			}

			tx.AddRequestHeader(name, value)
		}
	}

	// Go's server takes these two out of the headers.
	tx.AddRequestHeader("Host", r.Host)

	for _, encoding := range r.TransferEncoding {
		tx.AddRequestHeader("Transfer-Encoding", encoding)
	}

	tx.ProcessRequestHeaders()

	read, err := c.readBody(tx, body)

	// This reads the body in memory and runs the rest of the rules, and
	// cannot fail.
	_, _ = tx.ProcessRequestBody()

	var ids []int

	for _, matched := range tx.MatchedRules() {
		// The rules that look for attacks have a severity; the others set
		// the Core Rule Set up and add up the score.
		rule := matched.Rule()
		if rule.Severity() != types.RuleSeverityUnset {
			ids = append(ids, rule.ID())
		}
	}

	return Result{RuleIDs: ids, Score: score(tx)}, read, err
}

// readBody reads body, the body of the request in tx, which has run on
// the request's headers, while SWWAF_WAF_BODY_LIMIT is set and the body is
// of a kind the Core Rule Set reads: form data and multipart, of which it
// reads the first c.bodyLimit bytes, and JSON and XML, which it reads only
// when they are no longer than that, since they cannot be read in part.
// readBody reads one byte past the limit, to tell which they are, gives
// the Core Rule Set what it reads, and returns what it read and the error
// that ended the reading early, if one did.
func (c *CoreRuleSet) readBody(tx types.Transaction, body io.Reader) ([]byte, error) {
	if c.bodyLimit == 0 {
		return nil, nil
	}

	inPart := false
	// How the body is read is a variable of the transaction, which only
	// Coraza's interface for plugins reads.
	state := tx.(plugintypes.TransactionState) //nolint:forcetypeassert // every one is

	switch state.Variables().RequestBodyProcessor().Get() {
	case "URLENCODED", "MULTIPART":
		inPart = true
	case "JSON", "XML":
	default:
		return nil, nil
	}

	read, err := io.ReadAll(io.LimitReader(body, c.bodyLimit+1))

	if inPart || (err == nil && int64(len(read)) <= c.bodyLimit) {
		// Coraza holds what it reads of the body in memory, up to the
		// limit, so this cannot fail.
		_, _, _ = tx.WriteRequestBody(read)
	}

	return read, err
}

// score returns the anomaly score the Core Rule Set added up in tx, a
// transaction it has run, or 0 if a rule that adds it up is switched off.
func score(tx types.Transaction) int {
	// The score is in a variable of the transaction, which only Coraza's
	// interface for plugins reads.
	state := tx.(plugintypes.TransactionState) //nolint:forcetypeassert // every one is

	values := state.Variables().TX().Get("blocking_inbound_anomaly_score")
	if len(values) == 0 {
		return 0
	}

	n, _ := strconv.Atoi(values[0])

	return n
}

// withoutCookiesNotRead returns value, a Cookie header's, without the
// cookies in cookiesNotRead.
func withoutCookiesNotRead(value string) string {
	var kept []string

	for cookie := range strings.SplitSeq(value, ";") {
		name, _, _ := strings.Cut(strings.TrimSpace(cookie), "=")
		if !slices.Contains(cookiesNotRead, name) {
			kept = append(kept, cookie)
		}
	}

	return strings.Join(kept, ";")
}