CVE-2025-62493 — Heap out-of-bounds read in js_bigint_to_string1 in QuickJS
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.
Who exploits it and how urgent it is
A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.
How the score was calculated
- EPSS 0.4% (30-day exploitation probability)+5
- Technical severity (medium)+3
- CISA SSVC: exploitation "poc"+5
The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).
Triage
Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.
CISA SSVC assessment CISA
Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.
Remediation
Fixes and mitigations documented by the sources — linked to the original advisory.
No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.
First 24 hours
Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.
- Identify exposed assets running the affected vendor/product/version combinations (QuickJS QuickJS, Quickjs Project Quickjs).
- No fix documented in the sources: reduce exposure and monitor the vendor advisory.
- Search available logs for exploit probes matching the vulnerability class (memory corruption), errors and authentication anomalies.
Affected scope
Vendor, product and versions as the sources word them.
| Vendor | Product | Versions | Status | Source |
|---|---|---|---|---|
| QuickJS | QuickJS | ≥ 2025-04-26 and < 2025-09-13 | Vulnerable | CVE Program (cve.org) e CISA ADP |
| Quickjs Project | Quickjs | < 2025-09-13 | Vulnerable | NIST National Vulnerability Database |
Attack characteristics
The CVSS vector, decoded. It describes the attack, not your exposure to it.
Every base score collected
Sources score independently and may disagree; each row says who scored it and under which version.
| Score | Version | Severity | Expl. | Impact | Scored by |
|---|---|---|---|---|---|
| 5.9 | CVSS 4.0 | Medium | — | — | cve-coordination@google.com via NIST National Vulnerability Database |
| 5.9 | CVSS 4.0 | Medium | — | — | Google via CVE Program (cve.org) e CISA ADP |
| 6.5 | CVSS 3.1 | Medium | 2.8 | 3.6 | nvd@nist.gov via NIST National Vulnerability Database |
| 6.5 | CVSS 3.1 | Medium | — | — | GitHub, Inc. via GitHub Security Advisories |
CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N (score shown at the top: nvd@nist.gov) Weakness MITRE
Weakness class according to MITRE’s CWE catalog.
EPSS over time FIRST
Oct 13, 2026 → Oct 15, 2026 · 3 reading(s) · max 0.38%
Detection
Read off the CVSS vector and the weakness class. Starting points, not tested rules.
- Correlate service crashes/restarts with inbound traffic; repeated crashes may indicate attempts.
Analysis and videos
CVE-2025-62493: QuickJS BigInt-to-string heap out-of-bounds read
CVE-2025-62493 is a heap out-of-bounds read in the js_bigint_to_string1 function of the QuickJS engine, caused by an off-by-one error when calculating the number of digits needed for a BigInt string conversion. The record classifies it as CWE-125 with a NVD CVSS 3.1 base score of 6.5 and a Google CVSS 4.0 score of 5.9. The sources consulted show no confirmed active exploitation and no listing in CISA KEV.
Timeline
What happened to this CVE, newest first.
- 2026
- Affected products added · quickjs/quickjs
- SSVC assessment updated
- Record published
- CVSS changed · 6.5
- Source status changed · → Analyzed
- Record created in this database
- 2025
- CVE published
Source activity
Repeated readings are counted rather than listed.
- 1× References added · Oct 13, 2026 – Oct 13, 2026
- 1× Official description updated · Oct 13, 2026 – Oct 13, 2026
References
4 on the record
Elsewhere on this site
Related CVEs (same products)
- High CVE-2025-69654 Uncontrolled Resource Consumption in Quickjs Project Quickjs Mar 6, 2026
- Medium CVE-2025-69653 A crafted JavaScript input can trigger an internal assertion failure in QuickJS release… Mar 6, 2026
- High CVE-2025-62495 Type confusion in string addition in QuickJS Oct 16, 2025
- High CVE-2025-62496 Integer overflow in js_bigint_from_string in QuickJS Oct 16, 2025
- High CVE-2025-62490 Use-after-free in js_print_object in QuickJS Oct 16, 2025
- High CVE-2025-62491 Use-after-free in js_std_promise_rejection_check in QuickJS Oct 16, 2025
Not in any source we poll
Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.
- No fixed version, patch or mitigation documented.
- No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
- No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
- No exploit packet captures, log samples, or incident case IDs supplied.
Answered from this record
Text assembled only from the fields above — no additional claims.
What should defenders know first?
CVE-2025-62493 is "Heap out-of-bounds read in js_bigint_to_string1 in QuickJS", rated Medium. It affects QuickJS (QuickJS); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 6.5 describes technical severity. EPSS 0.38% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.
Comments
Moderated technical discussion. Read the community guidelines.
No comments yet.
Sign in or create an account to comment and suggest corrections.