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Fixes in HTML Lexer to support HTML empty comment statements #327

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Fixes in HTML Lexer to support HTML empty comment statements #327

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9b25635
Fixes in parser to address bypass of the library and XSS attacks
jfbyers Dec 15, 2023
1ae3478
Fixes suggested by Mike Samuel
Mar 22, 2024
e8b68ef
Spacing
Mar 22, 2024
0d2f071
Merge remote-tracking branch 'origin' into bugfix/lexerHtmlComments
Sep 24, 2024
ce28bc9
Added new test
Sep 24, 2024
7408e06
Implemented change suggested by mikesamuel
Sep 24, 2024
ee341c9
Spacing
Sep 24, 2024
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18 changes: 17 additions & 1 deletion src/main/java/org/owasp/html/HtmlLexer.java
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Original file line number Diff line number Diff line change
Expand Up @@ -476,6 +476,7 @@ private static enum State {
COMMENT,
COMMENT_DASH,
COMMENT_DASH_DASH,
COMMENT_DASH_AFTER_BANG,
DIRECTIVE,
DONE,
BOGUS_COMMENT,
Expand Down Expand Up @@ -640,17 +641,30 @@ && canonicalElementName(start + 2, end)
case BANG:
if ('-' == ch) {
state = State.BANG_DASH;
} else if('>' == ch) { // <!> is a valid html comment
state = State.DONE;
type = HtmlTokenType.COMMENT;
} else {
state = State.DIRECTIVE;
}
break;
case BANG_DASH:
if ('-' == ch) {
state = State.COMMENT;
state = State.COMMENT_DASH_AFTER_BANG;
} else {
state = State.DIRECTIVE;
}
break;
case COMMENT_DASH_AFTER_BANG:
if ('>' == ch) { // <!--> is a valid html comment
state = State.DONE;
type = HtmlTokenType.COMMENT;
} else if ('-' == ch) { // <!---> is a valid html comment
state = State.COMMENT_DASH_AFTER_BANG;
} else {
state = State.COMMENT;
}
break;
case COMMENT:
if ('-' == ch) {
state = State.COMMENT_DASH;
Expand All @@ -665,6 +679,8 @@ && canonicalElementName(start + 2, end)
if ('>' == ch) {
state = State.DONE;
type = HtmlTokenType.COMMENT;
} else if ('!' == ch) { // --!> is also valid closing sequence
state = State.COMMENT_DASH_DASH;
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@mikesamuel mikesamuel Mar 25, 2024

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This would seem to suggest that<!-- --!-> is a whole comment tag.
iiuc, after <!-- --!- we should be in the comment_dash state.

Perhaps we need ! to transition to COMMENT_DASH_DASH_BANG here which transitions as does case COMMENT above.

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@mikesamuel mikesamuel Mar 25, 2024

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Does this look right?

flowchart TD
  BANG -- "-" --> BANG_DASH;
  BANG_DASH -- "-" --> COMMENT_DASH_AFTER_BANG;
  BANG_DASH -- "else" --> DIRECTIVE;
  COMMENT_DASH_AFTER_BANG -- "-" --> COMMENT_DASH_AFTER_BANG;
  COMMENT_DASH_AFTER_BANG -- ">" --> DONE;
  COMMENT_DASH_AFTER_BANG -- "else" --> COMMENT;
  COMMENT -- "-" --> COMMENT_DASH;
  COMMENT -- "else" --> COMMENT;
  COMMENT_DASH -- "-" --> COMMENT_DASH_DASH;
  COMMENT_DASH -- "else" --> COMMENT;
  COMMENT_DASH_DASH -- ">" --> DONE;
  COMMENT_DASH_DASH -- "-" --> COMMENT_DASH_DASH;
  COMMENT_DASH_DASH -- "!" --> COMMENT_DASH_DASH_BANG;
  COMMENT_DASH_DASH_BANG -- ">" --> DONE;
  COMMENT_DASH_DASH_BANG -- "-" --> COMMENT_DASH;
  COMMENT_DASH_DASH_BANG -- "else" --> COMMENT;
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@carlosvillasanchez carlosvillasanchez Apr 10, 2024

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Yes, this seems correct. The missing part is the COMMENT_DASH_AFTER_BANG case and the <!> scenario too. But the new COMMENT_DASH_DASH_BANG suggestion seems correct to me.

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@jfbyers jfbyers Sep 24, 2024 •
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Thanks @mikesamuel , this makes sense. I implemented your proposed changes and added new tests. All the use cases discussed so far pass the tests. Can you please validate / approve the MR? Thanks!

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@carlosvillasanchez carlosvillasanchez Sep 25, 2024

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@jfbyers Changes look good to me, let see what mike's thoughts are :)

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@jfbyers jfbyers Oct 4, 2024

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@subbudvk @mikesamuel are you ok moving forward with this PR ? What are the next steps? Thank you.

} else if ('-' == ch) {
state = State.COMMENT_DASH_DASH;
} else {
Expand Down
39 changes: 39 additions & 0 deletions src/test/java/org/owasp/html/HtmlLexerTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -121,6 +121,45 @@ public static final void testShortTags() {
"TAGEND: >");
}

@Test
public static final void testCommentDeclarationWith0CommentsAndXss() throws Exception
{
//check https://datatracker.ietf.org/doc/html/rfc1866#section-3.2.5
assertTokens("<!><img src=1 onError=alert(\"nice\")>",
"COMMENT: <!>",
"TAGBEGIN: <img",
"ATTRNAME: src",
"ATTRVALUE: 1",
"ATTRNAME: onError",
"ATTRVALUE: alert(\"nice\")",
"TAGEND: >"
);
}

@Test
public static final void testAbruptClosingOfEmptyComment() throws Exception
{
assertTokens("<!--><img>a<!--->b<!->c",
"COMMENT: <!-->",
"TAGBEGIN: <img",
"TAGEND: >",
"TEXT: a",
"COMMENT: <!--->",
"TEXT: b",
"SERVERCODE: <!->c"
);
}

@Test
public static final void testIncorrectlyClosedComment() throws Exception
{
assertTokens("<!-- Comment --!><img>",
"COMMENT: <!-- Comment --!>",
"TAGBEGIN: <img",
"TAGEND: >"
);
}

private static void lex(String input, Appendable out) throws Exception {
HtmlLexer lexer = new HtmlLexer(input);
int maxTypeLength = 0;
Expand Down