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Reject a leading zero after the minus sign in JSON.parse #2322

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Reject a leading zero after the minus sign in JSON.parse #2322

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8835eab
Reject a leading zero after the minus sign in JSON numbers
0z5a Nov 13, 2026
0c87619
Check for a leading zero right after the minus sign
0z5a Nov 13, 2026
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17 changes: 7 additions & 10 deletions lib/VM/JSLib/JSONLexer.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -195,6 +195,13 @@ ExecutionStatus JSONLexer<Kind>::scanNumber() {
}
}

// The integer part cannot start with 0, unless it's 0.
if (*iter_.cur == '0') {
++iter_.cur;
if (LLVM_UNLIKELY(hasChar() && *iter_.cur >= '0' && *iter_.cur <= '9'))
return errorWithChar(u"Unexpected character in number: ", *iter_.cur);
}

bool isTrivialInteger = true;
uint64_t integer = 0;
constexpr uint64_t kFirstIntThatCanOverflow = (UINT64_MAX - 9) / 10;
Expand All @@ -221,16 +228,6 @@ ExecutionStatus JSONLexer<Kind>::scanNumber() {
numRef = iter_.cur.endCapture();
}

// Check for illegal leading 0.
size_t len = numRef.size();
assert(len > 0 && "scanNumber must be called on a number-looking char");
if (LLVM_UNLIKELY(
numRef[0] == '0' && len > 1 && numRef[1] >= '0' &&
numRef[1] <= '9')) {
// The integer part cannot start with 0, unless it's 0.
return errorWithChar(u"Unexpected character in number: ", numRef[1]);
}

if (isTrivialInteger) {
token_.setNumber((double)integer * multiplier);
return ExecutionStatus::RETURNED;
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18 changes: 18 additions & 0 deletions test/hermes/json.js
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Expand Up @@ -61,6 +61,24 @@ function parse(json) {
}
parse("015");
//CHECK-NEXT: SyntaxError: JSON Parse error: Unexpected character in number: 1
parse("-015");

@lavenzg lavenzg Nov 13, 2026

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Please also add test for unicode.

//CHECK-NEXT: SyntaxError: JSON Parse error: Unexpected character in number: 1
parse("-00");
//CHECK-NEXT: SyntaxError: JSON Parse error: Unexpected character in number: 0
parse("[-01.5]");
//CHECK-NEXT: SyntaxError: JSON Parse error: Unexpected character in number: 1
print(1 / JSON.parse("-0"), JSON.parse("-0.5"), JSON.parse("[-10]")[0]);
//CHECK-NEXT: -Infinity -0.5 -10
// A non-ASCII character in the input makes JSON.parse use the UTF-16 lexer.
parse("[\"\u4f60\", 015]");
//CHECK-NEXT: SyntaxError: JSON Parse error: Unexpected character in number: 1
parse("[\"\u4f60\", -015]");
//CHECK-NEXT: SyntaxError: JSON Parse error: Unexpected character in number: 1
parse("[\"\u4f60\", -00]");
//CHECK-NEXT: SyntaxError: JSON Parse error: Unexpected character in number: 0
var u = JSON.parse("[\"\u4f60\", -0, -0.5, -10]");
print(1 / u[1], u[2], u[3]);
//CHECK-NEXT: -Infinity -0.5 -10
parse("+5");
//CHECK-NEXT: SyntaxError: JSON Parse error: Unexpected character: +
parse("-");
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