What is CVE-2015-8866?

CVE-2015-8866 is a critical-severity vulnerability in PHP, classified under Improper Restriction of XML External Entity Reference (XXE). CVSS score: 9.6/10. Published 2016-05-22.

How severe is CVE-2015-8866?

Critical severity. CVSS v3 base score is 9.6 out of 10.

XXE in PHP

CVE-2015-8866

ext/libxml/libxml.c in PHP before 5.5.22 and 5.6.x before 5.6.6, when PHP-FPM is used, does not isolate each thread from libxml_disable_entity_loader changes in other threads, which allows remote attackers to conduct XML External Entity (XXE) and XML Entity Expansion (XEE) attacks via a crafted XML document, a related issue to CVE-2015-5161.

Vulnerability class: XXE (XML External Entity)

Published 2016-05-22 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.6 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.042 (90.5th percentile), scored 2026-09-29.

Low probability of exploitation in the next 30 days, but still tracked. 91st percentile — 90.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.040 · newest: 0.042 · change: +0.001

Affected products

PHP
Canonical Ubuntu Linux — versions 12.04, 14.04, 15.10
openSUSE Leap — versions 42.1
openSUSE — versions 13.2
SUSE Linux Enterprise Module For Web Scripting — versions 12
SUSE Linux Enterprise Software Development Kit — versions 12

Weakness classification (CWE)

CWE-611 — Improper Restriction of XML External Entity Reference (XXE)

References

www.ubuntu.com/usn/USN-2952-1 (vendor-advisory, Third Party Advisory)
git.php.net/
bugs.launchpad.net/ubuntu/+source/php5/+bug/1509817 (Patch, Third Party Advisory, Issue Tracking)
rhn.redhat.com/errata/RHSA-2016-2750.html (vendor-advisory, Third Party Advisory)
www.ubuntu.com/usn/USN-2952-2 (vendor-advisory, Third Party Advisory)
www.php.net/ChangeLog-5.php (Release Notes, Vendor Advisory)
bugs.php.net/bug.php (Exploit, Patch, Issue Tracking, Vendor Advisory)
www.securityfocus.com/bid/87470 (VDB Entry, Third Party Advisory, vdb-entry)
lists.opensuse.org/opensuse-security-announce/2016-05/msg00031.html (vendor-advisory, Mailing List, Third Party Advisory)
lists.opensuse.org/opensuse-security-announce/2016-05/msg00033.html (vendor-advisory, Mailing List, Third Party Advisory)

Frequently asked questions

What is CVE-2015-8866?: CVE-2015-8866 is a critical-severity vulnerability in PHP, classified under Improper Restriction of XML External Entity Reference (XXE). CVSS score: 9.6/10. Published 2016-05-22.
How severe is CVE-2015-8866?: Critical severity. CVSS v3 base score is 9.6 out of 10.