CVE-2025-62493

Full vulnerability record from the local CVENow database.

Description

A vulnerability exists in the QuickJS engine's BigInt string conversion logic (js_bigint_to_string1) 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 (n_digits) 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] << (JS_LIMB_BITS - 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.

Severity

MEDIUM

CVSS score

5.9 (v4.0)

Vulnerability status

Analyzed

Source identifier

[email protected]

Published

2025-10-16 16:15:39.797000

Last modified

2026-10-19 16:10:00.260000

View original CVE JSON

{
  "id": "CVE-2025-62493",
  "sourceIdentifier": "[email protected]",
  "published": "2025-10-16T16:15:39.797",
  "lastModified": "2026-10-19T16:10:00.260",
  "vulnStatus": "Analyzed",
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "A vulnerability exists in the QuickJS engine's BigInt string conversion logic (js_bigint_to_string1) due to an incorrect calculation of the required number of digits, which in turn leads to reading memory past the allocated BigInt structure.\n\n  *  The function determines the number of characters (n_digits) needed for the string representation by calculating:\n\n$$ \\\\ \\text{n\\_digits} = (\\text{n\\_bits} + \\text{log2\\_radix} - 1) / \\text{log2\\_radix}$$\n\n$$$$This formula is off-by-one in certain edge cases when calculating the necessary memory limbs. For instance, a 127-bit BigInt using radix 32 (where $\\text{log2\\_radix}=5$) is calculated to need $\\text{n\\_digits}=26$.\n\n\n  *  The maximum number of bits actually stored is $\\text{n\\_bits}=127$, which requires only two 64-bit limbs ($\\text{JS\\_LIMB\\_BITS}=64$).\n\n\n  *  The conversion loop iterates $\\text{n\\_digits}=26$ times, attempting to read 5 bits in each iteration, totaling $26 \\times 5 = 130$ bits.\n\n\n  *  In the final iterations of the loop, the code attempts to read data that spans two limbs:\n\nC\n\n\n\nc = (r->tab[pos] >> shift) | (r->tab[pos + 1] << (JS_LIMB_BITS - shift));\n\n\n\n\n\n\n\n\n\n  *  Since the BigInt was only allocated two limbs, the read operation for r->tab[pos + 1] becomes an Out-of-Bounds Read when pos points to the last valid limb (e.g., $pos=1$).\n\n\nThis vulnerability allows an attacker to cause the engine to read and process data from the memory immediately following the BigInt buffer. This can lead to Information Disclosure of sensitive data stored on the heap adjacent to the BigInt object."
    },
    {
      "lang": "es",
      "value": "Una vulnerabilidad existe en la lógica de conversión de cadena de BigInt del motor QuickJS (js_bigint_to_string1) debido a un cálculo incorrecto del número de dígitos requerido, lo que a su vez lleva a la lectura de memoria más allá de la estructura BigInt asignada.\n\n  *  La función determina el número de caracteres (n_digits) necesarios para la representación de cadena mediante el cálculo:\n\n$$ \\\\ \\text{n\\_digits} = (\\text{n\\_bits} + \\text{log2\\_radix} - 1) / \\text{log2\\_radix}$$\n\nEsta fórmula tiene un error de uno en ciertos casos límite al calcular los 'limbs' de memoria necesarios. Por ejemplo, se calcula que un BigInt de 127 bits que usa una base 32 (donde $\\text{log2\\_radix}=5$) necesita $\\text{n\\_digits}=26$.\n\n  *  El número máximo de bits realmente almacenados es $\\text{n\\_bits}=127$, lo que requiere solo dos 'limbs' de 64 bits ($\\text{JS\\_LIMB\\_BITS}=64$).\n\n  *  El bucle de conversión itera $\\text{n\\_digits}=26$ veces, intentando leer 5 bits en cada iteración, sumando un total de $26 \\times 5 = 130$ bits.\n\n  *  En las iteraciones finales del bucle, el código intenta leer datos que abarcan dos 'limbs':\n\nC\n\nc = (r -> tab[pos] >> shift) | (r -> tab[pos + 1] << (JS_LIMB_BITS - shift));\n\nDado que el BigInt solo se asignó con dos 'limbs', la operación de lectura para r -> tab[pos + 1] se convierte en una lectura fuera de límites (Out-of-Bounds Read) cuando 'pos' apunta al último 'limb' válido (por ejemplo, $pos=1$).\n\nEsta vulnerabilidad permite a un atacante hacer que el motor lea y procese datos de la memoria inmediatamente posterior al búfer BigInt. Esto puede llevar a la revelación de información de datos sensibles almacenados en el 'heap' adyacente al objeto BigInt."
    }
  ],
  "affected": [
    {
      "source": "[email protected]",
      "affectedData": [
        {
          "vendor": "QuickJS",
          "product": "QuickJS",
          "defaultStatus": "unaffected",
          "collectionURL": "https://bellard.org/quickjs/",
          "packageName": "js_bigint_to_string1",
          "versions": [
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              "version": "2025-04-26",
              "lessThan": "2025-09-13",
              "versionType": "date",
              "status": "affected"
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        }
      ]
    }
  ],
  "metrics": {
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          "modifiedSubAvailabilityImpact": "NOT_DEFINED",
          "Safety": "NOT_DEFINED",
          "Automatable": "NOT_DEFINED",
          "Recovery": "NOT_DEFINED",
          "valueDensity": "NOT_DEFINED",
          "vulnerabilityResponseEffort": "NOT_DEFINED",
          "providerUrgency": "NOT_DEFINED"
        }
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  "weaknesses": [
    {
      "source": "[email protected]",
      "type": "Secondary",
      "description": [
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          "lang": "en",
          "value": "CWE-125"
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  "references": [
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      "url": "https://bellard.org/quickjs/Changelog",
      "source": "[email protected]",
      "tags": [
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    },
    {
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}