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The lazy regex `<!--[\s\S]*?--!?>` only matched the innermost comment pair in nested inputs like `<!-- <!-- --> PAYLOAD -->`, leaving PAYLOAD visible in the sanitized output. The do-while loop did not help because no second `<!--` opener remained after the first pass. Replace with a character-by-character depth-tracking scan that increments a nesting counter on each `<!--` opener and decrements it on each `-->` or `--!>` closer, skipping all content at commentDepth > 0. This correctly handles arbitrary nesting depth. Add regression tests: - `<!-- <!-- --> PAYLOAD -->` → `""` - `before <!-- <!-- --> PAYLOAD --> after` → `"before after"` - `<!-- <!-- <!-- --> --> PAYLOAD -->` → `""` Fixes: nested HTML comment bypass (gh-aw-security#2066) Agent-Logs-Url: https://github.com/github/gh-aw/sessions/d33fff74-a28c-40d0-8636-53c99bda69ca Co-authored-by: szabta89 <1330202+szabta89@users.noreply.github.com>
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@copilot add fuzz tests + more unit tests |
- Add fuzz_remove_xml_comments_harness.cjs (JS harness for Go fuzzer)
- Add remove_xml_comments_fuzz_test.go (Go fuzz test with 33 seed cases)
- Validates: no crashes, output ≤ input length, output is a subsequence
of input (no chars synthesised), simple/nested comment bypass invariants
- Add 7 more unit tests in sanitize_content.test.cjs:
- Multiple independent comments
- Empty comment <!---->
- Unclosed comment (all content stripped to EOF)
- Adjacent comments with no text between
- Nested bypass with --!> outer closer
- Stray closer --> without opener (preserved as literal text)
Agent-Logs-Url: https://github.com/github/gh-aw/sessions/03e7d81d-149b-4b4f-b71c-aab59da2b609
Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>
Done in f56a127. Added: JS fuzz harness ( Go fuzz test (
7 more unit tests in
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🧪 Test Quality Sentinel ReportTest Quality Score: 95/100✅ Excellent test quality
Test Classification DetailsView All 11 Tests
Flagged Tests — Requires ReviewNone. All new tests are classified as high-value design tests. Language SupportTests analyzed:
Notable HighlightsThe test additions are exemplary for a security fix:
Verdict
📖 Understanding Test ClassificationsDesign Tests (High Value) verify what the system does:
Implementation Tests (Low Value) verify how the system does it:
Goal: Shift toward tests that describe the system's behavioral contract — the promises it makes to its users and collaborators. References:
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Pull request overview
Fixes an XML/HTML comment stripping bypass in removeXmlComments by replacing the previous lazy regex with a depth-tracking scanner, and adds regression + fuzz coverage.
Changes:
- Replace regex-based
removeXmlCommentswith a depth-tracking scan that correctly strips nested comment patterns. - Add unit tests covering nested comment bypasses, malformed closers, unclosed comments, and stray closers.
- Add a JS harness and a Go fuzz test to fuzz
removeXmlCommentsvia Node.
Show a summary per file
| File | Description |
|---|---|
| actions/setup/js/sanitize_content_core.cjs | Implements depth-tracking scanning to remove nested XML/HTML comments safely. |
| actions/setup/js/sanitize_content.test.cjs | Adds regression tests for nested comment bypass and related edge cases. |
| actions/setup/js/fuzz_remove_xml_comments_harness.cjs | Provides a Node/JS entrypoint to call removeXmlComments from Go fuzzing. |
| pkg/workflow/remove_xml_comments_fuzz_test.go | Adds a Go fuzz test that drives the JS harness and checks invariants. |
Copilot's findings
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Comments suppressed due to low confidence (4)
pkg/workflow/remove_xml_comments_fuzz_test.go:196
runRemoveXmlCommentsTesttreats any non-empty stderr as an “expected error” and returns(nil, nil), which can silently mask real failures (Node crash, syntax error, missing harness, etc.) and make the fuzz target effectively do nothing. It would be better to either return the error to the caller, or onlyt.Skipon clearly-detected environment issues (e.g., missingnode/missing file) while failing on other stderr-producing errors.
err = cmd.Run()
if err != nil {
if stderr.Len() > 0 {
return nil, nil // Expected error (e.g., harness not found)
}
pkg/workflow/remove_xml_comments_fuzz_test.go:138
- Similarly, the nested-bypass invariant wraps arbitrary fuzzed
textinside<!-- <!-- -->+ text +-->, but iftextcontains-->/--!>it can close the outer comment early and legitimately leave trailing characters, causing false positives. The invariant should either sanitize/escapetextto exclude closers, or only assert the bypass property for inputs that don’t contain closer sequences.
// The nested-opener bypass must always be stripped: wrapping the text in
// <!-- <!-- --> ... --> must produce no output
nestedBypass := "<!-- <!-- --> " + text + " -->"
nestedResult, nestedErr := runRemoveXmlCommentsTest(nestedBypass)
if nestedErr == nil && nestedResult != nil && nestedResult.Error == nil {
if nestedResult.Result != "" {
t.Errorf("Nested comment bypass not fully stripped: input=%q, output=%q",
nestedBypass, nestedResult.Result)
}
}
pkg/workflow/remove_xml_comments_fuzz_test.go:161
isSubsequenceOfiteratessubby rune but scanssby byte (len(s)andrune(s[si])). This will mis-handle any non-ASCII input (multi-byte UTF-8), which is especially relevant since the fuzz seeds include Unicode/emoji. Consider comparing using[]runefor both strings, or comparing byte slices consistently if you want a byte-level subsequence.
func isSubsequenceOf(sub, s string) bool {
si := 0
for _, c := range sub {
found := false
for si < len(s) {
if rune(s[si]) == c {
si++
found = true
break
}
si++
}
if !found {
return false
}
}
return true
pkg/workflow/remove_xml_comments_fuzz_test.go:186
- The harness path is likely incorrect: this test looks for
js/fuzz_remove_xml_comments_harness.cjsrelative to the package working directory, but the harness added in this PR is located atactions/setup/js/fuzz_remove_xml_comments_harness.cjs. As written, the fuzz test will fail to find/run the harness. Consider resolving the path relative to this Go file (viaruntime.Caller) or pointing toactions/setup/js/...explicitly, consistent with where the harness lives in this repo.
harnessPath := filepath.Join("js", "fuzz_remove_xml_comments_harness.cjs")
cmd := exec.Command("node", harnessPath)
cmd.Stdin = bytes.NewReader(inputJSON)
- Files reviewed: 4/4 changed files
- Comments generated: 2
| // A simple comment with no nested openers must be fully removed | ||
| simpleComment := "<!-- " + text + " -->" | ||
| simpleResult, simpleErr := runRemoveXmlCommentsTest(simpleComment) | ||
| if simpleErr == nil && simpleResult != nil && simpleResult.Error == nil { | ||
| if simpleResult.Result != "" { | ||
| t.Errorf("Simple comment not fully removed: input=%q, output=%q", | ||
| simpleComment, simpleResult.Result) | ||
| } | ||
| } |
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The “simple comment must be fully removed” invariant is tested by wrapping the fuzzed text inside <!-- + text + -->, but text may contain comment delimiters like -->/--!>/<!--, making this no longer a single “simple” comment. This will produce false failures (e.g., seed "-->" yields leftover text). Consider restricting/escaping text for this invariant, or using a fixed non-delimiter payload for the simple-comment check.
This issue also appears in the following locations of the same file:
- line 129
- line 145
- line 183
- line 192
| let result = ""; | ||
| let commentDepth = 0; | ||
| let position = 0; | ||
| while (position < s.length) { | ||
| const ch = s[position]; | ||
| if (ch === "<" && s.startsWith("<!--", position)) { | ||
| // Comment opener — increase nesting depth regardless of current depth | ||
| commentDepth++; | ||
| position += 4; | ||
| } else if (commentDepth > 0 && ch === "-" && s.startsWith("--!>", position)) { | ||
| // Malformed comment closer --!> (only meaningful inside an open comment) | ||
| commentDepth--; | ||
| position += 4; | ||
| } else if (commentDepth > 0 && ch === "-" && s.startsWith("-->", position)) { | ||
| // Normal comment closer --> (only meaningful inside an open comment) | ||
| commentDepth--; | ||
| position += 3; | ||
| } else { | ||
| // Include character in output only when outside all comment regions | ||
| if (commentDepth === 0) { | ||
| result += ch; | ||
| } | ||
| position++; | ||
| } |
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Building result via result += ch in a per-character loop can become quadratic and memory-heavy for large inputs (this sanitizer supports ~500KB strings). Consider accumulating into an array of chunks/characters and join("") at the end (or another linear-time builder pattern) to keep worst-case runtime predictable.
removeXmlCommentsused a lazy regex (<!--[\s\S]*?--!?>) that only consumed the innermost comment pair in nested inputs — leaving the outer payload visible. Thedo...whileloop didn't help because no second<!--opener remained after the first pass.Root cause
Fix
Replaced the lazy regex with a character-by-character depth-tracking scan:
commentDepthon each<!--opener-->/--!>closer (only when inside a comment)commentDepth === 0Tests added
Unit tests (
sanitize_content.test.cjs) — 10 new cases covering:<!-- <!-- --> PAYLOAD -->→""(original bypass)before <!-- <!-- --> PAYLOAD --> after→"before after"<!-- <!-- <!-- --> --> PAYLOAD -->→""(arbitrary nesting depth)<!---->--!>outer closer-->without opener preserved as literal textJS fuzz harness (
actions/setup/js/fuzz_remove_xml_comments_harness.cjs) — wrapsremoveXmlCommentsfor the Go fuzzer.Go fuzz test (
pkg/workflow/remove_xml_comments_fuzz_test.go) —FuzzRemoveXmlCommentswith 33 seed cases. Invariants verified per seed:<!-- text -->always strips to""<!-- <!-- --> text -->always strips to""