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//------------------------------------------------------------------------------
// <copyright file="CrossSiteScriptingValidation.cs" company="Microsoft">
// Copyright (c) Microsoft Corporation. All rights reserved.
// </copyright>
//------------------------------------------------------------------------------
/*
* Detection of unsafe strings from the client (aee ASURT 122278 for details)
*
*/
namespace System.Web {
using System;
using System.Globalization;
internal static class CrossSiteScriptingValidation {
private static bool IsAtoZ(char c) {
return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z');
}
// Per VSWhidbey 362133, we no longer check for those cases
#if OBSOLETE
// Detect strings like "OnFocus="
private static bool IsDangerousOnString(string s, int index) {
// If the next character is not an 'n', it's safe
if (s[index+1] != 'n' && s[index+1] != 'N') return false;
// If the previous character is a letter, it's safe (e.g. "won")
if (index > 0 && IsAtoZ(s[index-1])) return false;
int len = s.Length;
// Skip any number of letters, then any number of white spaces
index += 2;
while (index < len && IsAtoZ(s[index])) index++;
while (index < len && Char.IsWhiteSpace(s[index])) index++;
// If there is an equal, it's unsafe
return (index < len && s[index] == '=');
}
// Detect strings like "javascript:"
private static bool IsDangerousScriptString(string s, int index) {
int len = s.Length;
// Check for end of string case
if (index+6 >= len) return false;
// If the 's' is not followed by "cript", it's safe
// We avoid calling String.Compare for perf reasons.
if ((s[index+1] != 'c' && s[index+1] != 'C') ||
(s[index+2] != 'r' && s[index+2] != 'R') ||
(s[index+3] != 'i' && s[index+3] != 'I') ||
(s[index+4] != 'p' && s[index+4] != 'P') ||
(s[index+5] != 't' && s[index+5] != 'T')) return false;
// Skip any number of white spaces
index += 6;
while (index < len && Char.IsWhiteSpace(s[index])) index++;
// If there is a colon, it's unsafe
return (index < len && s[index] == ':');
}
// Detect "expression(". (as in style="qqq:expression(alert('Attack!'))", see ASURT 127079)
private static bool IsDangerousExpressionString(string s, int index) {
// Check for end of string case
if (index+10 >= s.Length) return false;
// If the 'e' is not followed by an "x", it's safe.
// This avoids calling String.Compare in most cases ("ex?" is rare)
if (s[index+1] != 'x' && s[index+1] != 'X') return false;
// Check the rest of the string
return (String.Compare(
s, index+2, "pression(", 0, 9, true /*ignoreCase*/,
CultureInfo.InvariantCulture) == 0);
}
#endif // OBSOLETE
// Detect constructs that look like HTML tags
#if OBSOLETE
private static char[] startingChars = new char[] { '<', '&', '/', '*', 'o', 'O', 's', 'S' , 'e', 'E' };
#endif // OBSOLETE
private static char[] startingChars = new char[] { '<', '&' };
// Only accepts http: and https: protocols, and protocolless urls.
// Used by web parts to validate import and editor input on Url properties.
// Review: is there a way to escape colon that will still be recognized by IE?
// %3a does not work with IE.
internal static bool IsDangerousUrl(string s) {
if (String.IsNullOrEmpty(s)) {
return false;
}
// Trim the string inside this method, since a Url starting with whitespace
// is not necessarily dangerous. This saves the caller from having to pre-trim
// the argument as well.
s = s.Trim();
int len = s.Length;
if ((len > 4) &&
((s[0] == 'h') || (s[0] == 'H')) &&
((s[1] == 't') || (s[1] == 'T')) &&
((s[2] == 't') || (s[2] == 'T')) &&
((s[3] == 'p') || (s[3] == 'P'))) {
if ((s[4] == ':') ||
((len > 5) && ((s[4] == 's') || (s[4] == 'S')) && (s[5] == ':'))) {
return false;
}
}
int colonPosition = s.IndexOf(':');
if (colonPosition == -1) {
return false;
}
return true;
}
internal static bool IsValidJavascriptId(string id) {
return (String.IsNullOrEmpty(id) || System.CodeDom.Compiler.CodeGenerator.IsValidLanguageIndependentIdentifier(id));
}
internal static bool IsDangerousString(string s, out int matchIndex) {
//bool inComment = false;
matchIndex = 0;
for (int i=0;;) {
// Look for the start of one of our patterns
int n = s.IndexOfAny(startingChars, i);
// If not found, the string is safe
if (n<0) return false;
// If it's the last char, it's safe
if (n == s.Length-1) return false;
matchIndex = n;
switch (s[n]) {
case '<':
// If the < is followed by a letter or '!', it's unsafe (looks like a tag or HTML comment)
if (IsAtoZ(s[n+1]) || s[n+1] == '!' || s[n+1] == '/' || s[n+1] == '?') return true;
break;
case '&':
// If the & is followed by a #, it's unsafe (e.g. S)
if (s[n+1] == '#') return true;
break;
#if OBSOLETE
case '/':
// Look for a starting C style comment (i.e. "/*")
if (s[n+1] == '*') {
// Remember that we're inside a comment
inComment = true;
n++;
}
break;
case '*':
// If we're not inside a comment, we don't care about finding "*/".
if (!inComment) break;
// Look for the end of a C style comment (i.e. "*/"). If we found one,
// we found a full comment, which we don't allow (VSWhidbey 228396).
if (s[n+1] == '/') return true;
break;
case 'o':
case 'O':
if (IsDangerousOnString(s, n))
return true;
break;
case 's':
case 'S':
if (IsDangerousScriptString(s, n))
return true;
break;
case 'e':
case 'E':
if (IsDangerousExpressionString(s, n))
return true;
break;
#endif // OBSOLETE
}
// Continue searching
i=n+1;
}
}
}
}
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