url.go

Documentation: html/template

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package template
     6  
     7  import (
     8  	"fmt"
     9  	"strings"
    10  )
    11  
    12  // urlFilter returns its input unless it contains an unsafe scheme in which
    13  // case it defangs the entire URL.
    14  //
    15  // Schemes that cause unintended side effects that are irreversible without user
    16  // interaction are considered unsafe. For example, clicking on a "javascript:"
    17  // link can immediately trigger JavaScript code execution.
    18  //
    19  // This filter conservatively assumes that all schemes other than the following
    20  // are unsafe:
    21  //   - http:   Navigates to a new website, and may open a new window or tab.
    22  //     These side effects can be reversed by navigating back to the
    23  //     previous website, or closing the window or tab. No irreversible
    24  //     changes will take place without further user interaction with
    25  //     the new website.
    26  //   - https:  Same as http.
    27  //   - mailto: Opens an email program and starts a new draft. This side effect
    28  //     is not irreversible until the user explicitly clicks send; it
    29  //     can be undone by closing the email program.
    30  //
    31  // To allow URLs containing other schemes to bypass this filter, developers must
    32  // explicitly indicate that such a URL is expected and safe by encapsulating it
    33  // in a template.URL value.
    34  func urlFilter(args ...any) string {
    35  	s, t := stringify(args...)
    36  	if t == contentTypeURL {
    37  		return s
    38  	}
    39  	if !isSafeURL(s) {
    40  		return "#" + filterFailsafe
    41  	}
    42  	return s
    43  }
    44  
    45  // isSafeURL is true if s is a relative URL or if URL has a protocol in
    46  // (http, https, mailto).
    47  func isSafeURL(s string) bool {
    48  	if protocol, _, ok := strings.Cut(s, ":"); ok && !strings.Contains(protocol, "/") {
    49  		if !strings.EqualFold(protocol, "http") && !strings.EqualFold(protocol, "https") && !strings.EqualFold(protocol, "mailto") {
    50  			return false
    51  		}
    52  	}
    53  	return true
    54  }
    55  
    56  // urlEscaper produces an output that can be embedded in a URL query.
    57  // The output can be embedded in an HTML attribute without further escaping.
    58  func urlEscaper(args ...any) string {
    59  	return urlProcessor(false, args...)
    60  }
    61  
    62  // urlNormalizer normalizes URL content so it can be embedded in a quote-delimited
    63  // string or parenthesis delimited url(...).
    64  // The normalizer does not encode all HTML specials. Specifically, it does not
    65  // encode '&' so correct embedding in an HTML attribute requires escaping of
    66  // '&' to '&amp;'.
    67  func urlNormalizer(args ...any) string {
    68  	return urlProcessor(true, args...)
    69  }
    70  
    71  // urlProcessor normalizes (when norm is true) or escapes its input to produce
    72  // a valid hierarchical or opaque URL part.
    73  func urlProcessor(norm bool, args ...any) string {
    74  	s, t := stringify(args...)
    75  	if t == contentTypeURL {
    76  		norm = true
    77  	}
    78  	var b strings.Builder
    79  	if processURLOnto(s, norm, &b) {
    80  		return b.String()
    81  	}
    82  	return s
    83  }
    84  
    85  // processURLOnto appends a normalized URL corresponding to its input to b
    86  // and reports whether the appended content differs from s.
    87  func processURLOnto(s string, norm bool, b *strings.Builder) bool {
    88  	b.Grow(len(s) + 16)
    89  	written := 0
    90  	// The byte loop below assumes that all URLs use UTF-8 as the
    91  	// content-encoding. This is similar to the URI to IRI encoding scheme
    92  	// defined in section 3.1 of  RFC 3987, and behaves the same as the
    93  	// EcmaScript builtin encodeURIComponent.
    94  	// It should not cause any misencoding of URLs in pages with
    95  	// Content-type: text/html;charset=UTF-8.
    96  	for i, n := 0, len(s); i < n; i++ {
    97  		c := s[i]
    98  		switch c {
    99  		// Single quote and parens are sub-delims in RFC 3986, but we
   100  		// escape them so the output can be embedded in single
   101  		// quoted attributes and unquoted CSS url(...) constructs.
   102  		// Single quotes are reserved in URLs, but are only used in
   103  		// the obsolete "mark" rule in an appendix in RFC 3986
   104  		// so can be safely encoded.
   105  		case '!', '#', '$', '&', '*', '+', ',', '/', ':', ';', '=', '?', '@', '[', ']':
   106  			if norm {
   107  				continue
   108  			}
   109  		// Unreserved according to RFC 3986 sec 2.3
   110  		// "For consistency, percent-encoded octets in the ranges of
   111  		// ALPHA (%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D),
   112  		// period (%2E), underscore (%5F), or tilde (%7E) should not be
   113  		// created by URI producers
   114  		case '-', '.', '_', '~':
   115  			continue
   116  		case '%':
   117  			// When normalizing do not re-encode valid escapes.
   118  			if norm && i+2 < len(s) && isHex(s[i+1]) && isHex(s[i+2]) {
   119  				continue
   120  			}
   121  		default:
   122  			// Unreserved according to RFC 3986 sec 2.3
   123  			if 'a' <= c && c <= 'z' {
   124  				continue
   125  			}
   126  			if 'A' <= c && c <= 'Z' {
   127  				continue
   128  			}
   129  			if '0' <= c && c <= '9' {
   130  				continue
   131  			}
   132  		}
   133  		b.WriteString(s[written:i])
   134  		fmt.Fprintf(b, "%%%02x", c)
   135  		written = i + 1
   136  	}
   137  	b.WriteString(s[written:])
   138  	return written != 0
   139  }
   140  
   141  // Filters and normalizes srcset values which are comma separated
   142  // URLs followed by metadata.
   143  func srcsetFilterAndEscaper(args ...any) string {
   144  	s, t := stringify(args...)
   145  	switch t {
   146  	case contentTypeSrcset:
   147  		return s
   148  	case contentTypeURL:
   149  		// Normalizing gets rid of all HTML whitespace
   150  		// which separate the image URL from its metadata.
   151  		var b strings.Builder
   152  		if processURLOnto(s, true, &b) {
   153  			s = b.String()
   154  		}
   155  		// Additionally, commas separate one source from another.
   156  		return strings.ReplaceAll(s, ",", "%2c")
   157  	}
   158  
   159  	var b strings.Builder
   160  	written := 0
   161  	for i := 0; i < len(s); i++ {
   162  		if s[i] == ',' {
   163  			filterSrcsetElement(s, written, i, &b)
   164  			b.WriteString(",")
   165  			written = i + 1
   166  		}
   167  	}
   168  	filterSrcsetElement(s, written, len(s), &b)
   169  	return b.String()
   170  }
   171  
   172  // Derived from https://play.golang.org/p/Dhmj7FORT5
   173  const htmlSpaceAndASCIIAlnumBytes = "\x00\x36\x00\x00\x01\x00\xff\x03\xfe\xff\xff\x07\xfe\xff\xff\x07"
   174  
   175  // isHTMLSpace is true iff c is a whitespace character per
   176  // https://infra.spec.whatwg.org/#ascii-whitespace
   177  func isHTMLSpace(c byte) bool {
   178  	return (c <= 0x20) && 0 != (htmlSpaceAndASCIIAlnumBytes[c>>3]&(1<<uint(c&0x7)))
   179  }
   180  
   181  func isHTMLSpaceOrASCIIAlnum(c byte) bool {
   182  	return (c < 0x80) && 0 != (htmlSpaceAndASCIIAlnumBytes[c>>3]&(1<<uint(c&0x7)))
   183  }
   184  
   185  func filterSrcsetElement(s string, left int, right int, b *strings.Builder) {
   186  	start := left
   187  	for start < right && isHTMLSpace(s[start]) {
   188  		start++
   189  	}
   190  	end := right
   191  	for i := start; i < right; i++ {
   192  		if isHTMLSpace(s[i]) {
   193  			end = i
   194  			break
   195  		}
   196  	}
   197  	if url := s[start:end]; isSafeURL(url) {
   198  		// If image metadata is only spaces or alnums then
   199  		// we don't need to URL normalize it.
   200  		metadataOk := true
   201  		for i := end; i < right; i++ {
   202  			if !isHTMLSpaceOrASCIIAlnum(s[i]) {
   203  				metadataOk = false
   204  				break
   205  			}
   206  		}
   207  		if metadataOk {
   208  			b.WriteString(s[left:start])
   209  			processURLOnto(url, true, b)
   210  			b.WriteString(s[end:right])
   211  			return
   212  		}
   213  	}
   214  	b.WriteString("#")
   215  	b.WriteString(filterFailsafe)
   216  }
   217
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