A Use-After-Free (UAF) vulnerability exists in the QuickJS engine's standard library when iterating over the global list of unhandled rejected promises (ts->rejectedpromiselist).

The function jsstdpromiserejectioncheck attempts to iterate over the rejectedpromiselist to report unhandled rejections using a standard list loop.

The reason for a promise rejection is processed inside the loop, including calling jsstddumperror1(ctx, rp->reason).

If the promise rejection reason is an Error object that defines a custom property getter (e.g., via Object.defineProperty), this getter is executed during the error dumping process.

The malicious custom getter can execute JavaScript code that calls catch() on the same rejected promise being processed.

Calling catch() internally triggers jsstdpromiserejectiontracker, which then removes and frees the current promise entry (JSRejectedPromiseEntry) from the rejectedpromiselist.

Since the list iteration continues using the now-freed memory pointer (el), the subsequent loop access results in a Use-After-Free condition.

A vulnerability exists in the QuickJS engine's BigInt string conversion logic (jsbiginttostring1) due to an incorrect calculation of the required number of digits, which in turn leads to reading memory past the allocated BigInt structure.

The function determines the number of characters (ndigits) needed for the string representation by calculating:

$$ \\ \text{n\digits} = (\text{n\bits} + \text{log2\radix} - 1) / \text{log2\radix}$$

$$$$This formula is off-by-one in certain edge cases when calculating the necessary memory limbs. For instance, a 127-bit BigInt using radix 32 (where $\text{log2\radix}=5$) is calculated to need $\text{n\digits}=26$.

The maximum number of bits actually stored is $\text{n\bits}=127$, which requires only two 64-bit limbs ($\text{JS\LIMB\BITS}=64$).

The conversion loop iterates $\text{n\digits}=26$ times, attempting to read 5 bits in each iteration, totaling $26 \times 5 = 130$ bits.

In the final iterations of the loop, the code attempts to read data that spans two limbs:

C

c = (r->tab[pos] >> shift) | (r->tab[pos + 1] << (JSLIMBBITS - shift));

Since the BigInt was only allocated two limbs, the read operation for r->tab[pos + 1] becomes an Out-of-Bounds Read when pos points to the last valid limb (e.g., $pos=1$).

This vulnerability allows an attacker to cause the engine to read and process data from the memory immediately following the BigInt buffer. This can lead to Information Disclosure of sensitive data stored on the heap adjacent to the BigInt object.