Fix isAscii returning true for malformed surrogates #9261
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The previous implementation used `length == utf8Size()` to determine if a string was ASCII. This was incorrect because Okio's utf8Size() maps malformed surrogates (unpaired high/low surrogates) to the replacement character `?` which is 1 byte in UTF-8. This caused strings like "\uD800.com" (unpaired high surrogate) to incorrectly report as ASCII since: - length = 5 (one char for surrogate + 4 for ".com") - utf8Size() = 5 (surrogate mapped to '?' = 1 byte + 4 for ".com") The fix iterates through each character and checks if its code point is within the ASCII range (0-127). Fixes square#6357
yschimke
reviewed
Jan 25, 2026
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| /** Returns true if the [String] is ASCII encoded (0-127). */ | ||
| private fun String.isAscii() = length == utf8Size().toInt() | ||
| private fun String.isAscii() = all { it.code <= 127 } |
Collaborator
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I suspect we want to avoid Iterable.all to avoid an allocation.
yschimke
reviewed
Jan 25, 2026
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| // Unpaired high surrogate - should not match any hostname | ||
| assertThat(verifier.verify("\uD800.com", session)).isFalse() | ||
| assertThat(verifier.verify("foo\uD800.com", session)).isFalse() | ||
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| // Unpaired low surrogate - should not match any hostname | ||
| assertThat(verifier.verify("\uDC00.com", session)).isFalse() | ||
| assertThat(verifier.verify("foo\uDC00.com", session)).isFalse() |
Collaborator
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Would any of these have matched before the change? is there some case where length == utf8Size().toInt() would match because of offsetting issues, longer lower case, and malformed surrogate.
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Summary
The
isAscii()extension function inOkHostnameVerifierusedlength == utf8Size()to determine if a string was ASCII. This was incorrect because Okio'sutf8Size()maps malformed surrogates (unpaired high/low surrogates) to the replacement character?which is 1 byte in UTF-8.This caused strings like
"\uD800.com"(unpaired high surrogate) to incorrectly report as ASCII since:length = 5(one char for surrogate + 4 for ".com")utf8Size() = 5(surrogate mapped to '?' = 1 byte + 4 for ".com")The fix replaces the comparison with a simple character iteration check:
all { it.code <= 127 }Changes
isAscii()to iterate through characters and check code pointsTest Plan
malformedSurrogatesAreNotAsciitest that verifies unpaired high/low surrogates are rejectedHostnameVerifierTesttests continue to passFixes #6357