webhooker/internal/delivery/target_redact.go

package delivery

import (
	"net/url"
	"slices"
	"strings"

	"sneak.berlin/go/webhooker/internal/database"
)

// RedactionMarker stands in for a target credential found in
// text the target's remote peer chose.
const RedactionMarker = "(redacted)"

// Redactor removes one target's own credential material from
// text that target's remote peer chose: a delivery response
// body, or a delivery error stored before the delivery path
// learned to mask the URLs it embeds.
//
// It removes byte-identical echoes of strings taken from the
// target's stored configuration, and nothing else. Anything
// the remote re-encodes survives: JSON "\/" escaping (what
// PHP's json_encode emits by default), percent-encoding, HTML
// entities, and an echo of only part of a path. It cannot
// remove a secret the remote invented.
//
// The zero Redactor removes nothing, which is what a caller
// holding no target for a delivery gets.
type Redactor struct {
	secrets []string
}

// NewRedactor builds the redactor for one target.
func NewRedactor(t *database.Target) Redactor {
	// Drop empty strings here rather than at the site that
	// produced one. strings.ReplaceAll with an empty old string
	// inserts the marker at every byte boundary, so a single
	// empty secret destroys every body and error the target
	// renders; filtering at the collection point means no field
	// added to targetSecrets later can reintroduce that.
	// url.Parse("https://@example.com/in") is the known
	// producer: a non-nil User whose String is "".
	secrets := slices.DeleteFunc(
		targetSecrets(t),
		func(s string) bool { return s == "" },
	)

	// Longest first, so replacing a secret that is contained
	// in a longer one cannot leave a fragment of the longer
	// one behind. Configured headers arrive in map order, so
	// the sort is also what makes the result deterministic.
	slices.SortFunc(secrets, func(a, b string) int {
		if d := len(b) - len(a); d != 0 {
			return d
		}

		return strings.Compare(a, b)
	})

	return Redactor{secrets: secrets}
}

// Redact replaces every occurrence of the target's credential
// material in s.
func (r Redactor) Redact(s string) string {
	if s == "" {
		return s
	}

	for _, secret := range r.secrets {
		s = strings.ReplaceAll(s, secret, RedactionMarker)
	}

	return s
}

// RedactCut redacts s, which its caller has already cut to a
// byte budget, and additionally drops any tail of s that is a
// proper prefix of a secret.
//
// The cut lands wherever the remote's padding puts it, so the
// remote chooses where inside the credential it falls. The
// severed prefix left behind equals no secret, so plain
// Redact would render it verbatim.
func (r Redactor) RedactCut(s string) string {
	s = r.Redact(s)

	if n := r.secretPrefixSuffix(s); n > 0 {
		return s[:len(s)-n] + RedactionMarker
	}

	return s
}

// secretPrefixSuffix returns the length of the longest suffix
// of s that is a proper prefix of one of the secrets, or 0
// when there is none.
func (r Redactor) secretPrefixSuffix(s string) int {
	longest := 0

	for _, secret := range r.secrets {
		// Proper prefixes only: a whole secret at the tail was
		// already replaced by Redact.
		n := min(len(secret)-1, len(s))
		for ; n > longest; n-- {
			if strings.HasSuffix(s, secret[:n]) {
				longest = n

				break
			}
		}
	}

	return longest
}

// targetSecrets returns the credential-bearing strings a
// target's configuration carries.
//
// The destination URL contributes. Its path, query and
// userinfo are the credential for both target types that have
// one — an incoming-webhook URL is a bearer token, which is
// why MaskURL elides exactly those parts — and they are the
// material this service actually sends, so a remote that
// echoes the request back echoes them.
//
// Configured request headers contribute their values, but
// only for the credential-shaped names isCredentialHeaderName
// picks out. That is the same class-based rule applied to
// URLs: an echoed Accept or User-Agent still renders, an
// echoed Authorization does not.
func targetSecrets(t *database.Target) []string {
	if t == nil {
		return nil
	}

	switch t.Type {
	case database.TargetTypeSlack:
		cfg, err := parseSlackConfig(t.Config)
		if err != nil {
			return nil
		}

		return urlSecrets(cfg.WebhookURL)
	case database.TargetTypeHTTP:
		cfg, err := parseHTTPConfig(t.Config)
		if err != nil {
			return nil
		}

		return append(
			urlSecrets(cfg.URL),
			headerSecrets(cfg.Headers)...,
		)
	case database.TargetTypeDatabase, database.TargetTypeLog:
		// Neither has a destination URL, so neither has
		// anything to redact.
		return nil
	default:
		return nil
	}
}

// urlSecrets returns the substrings of a destination URL that
// must not survive into a rendered page: the whole URL, the
// parts of it MaskURL elides, and any userinfo.
//
// No length floor is applied to the path, and none to the
// userinfo. A short path or a four-byte username is treated as
// a credential exactly like a long one, because the field takes
// an arbitrary URL and no part of it can be assumed non-secret —
// the same rule MaskURL applies. headerSecrets does carry a
// floor, and the difference is deliberate: a header is picked
// out by a name-shaped guess and its value may be ordinary
// text, whereas a URL's path and userinfo are credential
// material by position.
func urlSecrets(raw string) []string {
	raw = strings.TrimSpace(raw)
	if raw == "" {
		return nil
	}

	secrets := []string{raw}

	parsed, err := url.Parse(raw)
	if err != nil {
		return secrets
	}

	if parsed.Path != "" && parsed.Path != "/" {
		requestURI := parsed.RequestURI()
		secrets = append(secrets, requestURI)

		if escaped := parsed.EscapedPath(); escaped != requestURI {
			secrets = append(secrets, escaped)
		}
	}

	if parsed.User != nil {
		secrets = append(secrets, parsed.User.String())

		if pw, ok := parsed.User.Password(); ok && pw != "" {
			secrets = append(secrets, pw)
		}
	}

	return secrets
}

// minHeaderSecretBytes is the shortest header value treated as
// a credential. Unlike a URL path, a header value can be a
// couple of bytes long, and redacting those would scatter the
// marker through ordinary response text for no gain.
const minHeaderSecretBytes = 4

// headerSecrets returns the values of the configured headers
// whose names are credential-shaped.
func headerSecrets(headers map[string]string) []string {
	var secrets []string

	for name, value := range headers {
		value = strings.TrimSpace(value)
		if len(value) < minHeaderSecretBytes {
			continue
		}

		if isCredentialHeaderName(name) {
			secrets = append(secrets, value)
		}
	}

	return secrets
}

// isCredentialHeaderName classifies a header by its name. The
// value is never inspected, so the rule is the same
// class-based one MaskURL applies to a destination URL.
//
// The fragments are short on purpose, and match anywhere in
// the name, so abbreviations an operator might use are covered
// too: X-Sig, X-Pass, X-HMAC. That over-matches — a header
// named X-Design contains "sig" — and over-matching is the
// safe direction here: the cost is a marker where an echoed
// header value would have rendered.
func isCredentialHeaderName(name string) bool {
	name = strings.ToLower(strings.TrimSpace(name))

	// Names that carry a credential by definition.
	switch name {
	case "authorization", "proxy-authorization", "cookie":
		return true
	}

	// What operators call their own credential headers:
	// X-Api-Key, X-Hub-Signature, X-Auth-Token.
	for _, fragment := range []string{
		"auth",
		"credential",
		"hmac",
		"key",
		"pass",
		"secret",
		"sig",
		"token",
	} {
		if strings.Contains(name, fragment) {
			return true
		}
	}

	return false
}