The WebGL `DrawElementsInstanced` method was susceptible to a heap buffer overflow when used on systems with the Mesa VM driver. This issue could allow an attacker to perform remote code execution and sandbox escape. This vulnerability affects Firefox ESR < 115.6, Thunderbird < 115.6, and Firefox < 121.
Conclusion & alert: CVE-2023-6856 is rated High Risk (74.2/100): CVSS High severity, with high exploitation likelihood (EPSS 20.47%, 97th percentile). Core evidence: EPSS ranks this CVE among the most likely to be exploited in the near term. EPSS rose +10.00% over the last day, indicating growing attacker interest. Mandatory action: High exploitation likelihood—assess exposure and prioritize remediation.
Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.
EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).
| # | Date | Old EPSS score | New EPSS score | Delta (New - Old) |
|---|---|---|---|---|
| 1 | 2026-06-15 | 10.47% | 20.47% | +10.00% |
| 2 | 2026-04-19 | 10.99% | 10.47% | -0.52% |
| 3 | 2026-03-02 | — | 10.99% | — |
Full EPSS history (24 records total)
CVSS metrics for this CVE.
| Base score | Version | Severity | Vector | Exploitability | Impact | Score source |
|---|---|---|---|---|---|---|
| 8.8 | 3.1 | HIGH |
|
2.8 | 5.9 | [email protected] |
| vendor | priority | summary | link |
|---|---|---|---|
alpine
|
— | CVE-2023-6856: 3 source package rows (firefox, firefox-esr, thunderbird); 143 state rows across 5 repos (3.19-community, 3.20-community, 3.21-community, 3.22-community, edge-community); fixed 18, open 125. | https://security.alpinelinux.org/vuln/CVE-2023-6856 |
debian
|
not yet assigned | CVE-2023-6856 not yet assigned priority: Debian including 3 source packages (firefox, firefox-esr, thunderbird), 11 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 11. | https://security-tracker.debian.org/tracker/CVE-2023-6856 |
gentoo
|
high | CVE-2023-6856: 2 GLSA(s) (202401-10, 202402-25), 4 atom(s) (mail-client/thunderbird, mail-client/thunderbird-bin, www-client/firefox, www-client/firefox-bin); latest impact high. | https://bugs.gentoo.org/buglist.cgi?quicksearch=CVE-2023-6856 |
redhat
|
high | — | https://access.redhat.com/security/cve/CVE-2023-6856 |
ubuntu
|
medium | CVE-2023-6856 medium priority: Ubuntu including 8 source packages (firefox, mozjs102, …), 72 status rows across 9 suites (bionic, focal, jammy, lunar, mantic, noble, trusty, upstream, xenial): DNE 37, ignored 24, released 6, not-affected 3, needs-triage 2. | https://ubuntu.com/security/CVE-2023-6856 |
| Vendor | Product | Version | Raw CPE |
|---|---|---|---|
| mozilla | firefox | < 121.0 | cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:* |
| mozilla | firefox_esr | < 115.6 | cpe:2.3:a:mozilla:firefox_esr:*:*:*:*:*:*:*:* |
| mozilla | thunderbird | < 115.6 | cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:* |
| debian | debian_linux | 10.0 | cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:* |
| debian | debian_linux | 11.0 | cpe:2.3:o:debian:debian_linux:11.0:*:*:*:*:*:*:* |
| debian | debian_linux | 12.0 | cpe:2.3:o:debian:debian_linux:12.0:*:*:*:*:*:*:* |
| URL | Tags |
|---|---|
| https://bugzilla.mozilla.org/show_bug.cgi?id=1843782 | Issue Tracking Permissions Required |
| https://lists.debian.org/debian-lts-announce/2023/12/msg00020.html | Mailing List Third Party Advisory |
| https://lists.debian.org/debian-lts-announce/2023/12/msg00021.html | Mailing List Third Party Advisory |
| https://security.gentoo.org/glsa/202401-10 | Third Party Advisory |
| https://www.debian.org/security/2023/dsa-5581 | Third Party Advisory |
| https://www.debian.org/security/2023/dsa-5582 | Third Party Advisory |
| https://www.mozilla.org/security/advisories/mfsa2023-54/ | Vendor Advisory |
| https://www.mozilla.org/security/advisories/mfsa2023-55/ | Vendor Advisory |
| https://www.mozilla.org/security/advisories/mfsa2023-56/ | Vendor Advisory |