Quick answers
- Is CVE-2026-86716 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-86716 being exploited in the wild.
- How severe is CVE-2026-86716?
- High severity, with a CVSS score of 7.3, published 8 September 2026.
- Which products and versions are affected?
- Cesanta mJS up to 1.26.
- Is there a fix for CVE-2026-86716?
- No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
- How is CVE-2026-86716 attacked?
- It is attacked over the network, in a software library.
- Am I affected by CVE-2026-86716?
- Check whether you run Cesanta mJS at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
Technical details
The vulnerability is a heap-based buffer over-read (CWE-125) in the skip_spaces_and_comments() function in src/mjs_tok.c. When mjs_exec() is called with input ending in whitespace, the function attempts to read one byte past the null terminator by checking p->pos[1] without first verifying p->pos[0] != '\0'. The flaw is triggered during lexical analysis by any code path that calls mjs_exec() with untrusted JavaScript input. This requires no authentication or special privileges—any application embedding mJS is vulnerable if it accepts external JavaScript. The issue causes an out-of-bounds read (not a write), resulting in a crash under memory sanitizers and potentially information disclosure. The vendor was informed early but has not yet patched the issue.
Affected products
- Cesanta mJS up to 1.26
Timeline
- 2026-07-30: disclosed: Issue publicly reported on GitHub
- 2026-09-08: advisory: CVE-2026-86716 assigned and published