Integer overflow in js_bigint_from_string in QuickJS

Summary

A vulnerability exists in the QuickJS engine's BigInt string parsing logic (js_bigint_from_string) when attempting to create a BigInt from a string with an excessively large number of digits. The function calculates the necessary number of bits (n_bits) required to store the BigInt using the formula: $$\text{n\_bits} = (\text{n\_digits} \times 27 + 7) / 8 \quad (\text{for radix 10})$$ • For large input strings (e.g., $79,536,432$ digits or more for base 10), the intermediate calculation $(\text{n\_digits} \times 27 + 7)$ exceeds the maximum value of a standard signed 32-bit integer, resulting in an Integer Overflow. • The resulting n_bits value becomes unexpectedly small or even negative due to this wrap-around. • This flawed n_bits is then used to compute n_limbs, the number of memory "limbs" needed for the BigInt object. Since n_bits is too small, the calculated n_limbs is also significantly underestimated. • The function proceeds to allocate a JSBigInt object using this underestimated n_limbs. • When the function later attempts to write the actual BigInt data into the allocated object, the small buffer size is quickly exceeded, leading to a Heap Out-of-Bounds Write as data is written past the end of the allocated r->tab array.

Timeline

1. Oct 16, 2025, 3:52 PM UTCCVE published
2. Oct 16, 2025, 5:26 PM UTCCISA SSVC assessment

   Exploitation: poc · Automatable: no · Technical impact: total

3. Oct 16, 2025, 5:26 PM UTCRecord last updated

Affected versions

• js_bigint_from_string

  From 2025-04-26 · fixed in 2025-09-13

References 2