CVE-2025-62495: Type confusion in string addition in QuickJS

An integer overflow vulnerability exists in the QuickJS regular expression engine (libregexp) due to an inconsistent representation of the bytecode buffer size.

The regular expression bytecode is stored in a DynBuf structure, which correctly uses a $\text{size}\\text{t}$ (an unsigned type, typically 64-bit) for its size member.

However, several functions, such as reemitopu32 and other internal parsing routines, incorrectly cast or store this DynBuf $\text{size}\\text{t}$ value into a signed int (typically 32-bit).

When a large or complex regular expression (such as those generated by a recursive pattern in a Proof-of-Concept) causes the bytecode size to exceed $2^{31}$ bytes (the maximum positive value for a signed 32-bit integer), the size value wraps around, resulting in a negative integer when stored in the int variable (Integer Overflow).

This negative value is subsequently used in offset calculations. For example, within functions like reparsedisjunction, the negative size is used to compute an offset (pos) for patching a jump instruction.

This negative offset is then incorrectly added to the buffer pointer (s->byte\code.buf + pos), leading to an out-of-bounds write on the first line of the snippet below:

putu32(s->bytecode.buf + pos, len);