CVE-2021-21703
CWE-284Published: October 25, 2021· Updated: Jun 17, 2026
Official Description
In PHP versions 7.3.x up to and including 7.3.31, 7.4.x below 7.4.25 and 8.0.x below 8.0.12, when running PHP FPM SAPI with main FPM daemon process running as root and child worker processes running as lower-privileged users, it is possible for the child processes to access memory shared with the main process and write to it, modifying it in a way that would cause the root process to conduct invalid memory reads and writes, which can be used to escalate privileges from local unprivileged user to the root user.
Technical Analysis
CVE-2021-21703 requires local access, meaning attackers must already have a foothold on the target system.
Exploitation requires low privileges, which limits the exposure to scenarios where an attacker has already gained initial access.
A successful exploit results in complete confidentiality breach (data exposure), full integrity compromise (data manipulation), availability disruption (denial of service), with a CVSS base score of 7.0.
CVSS v3.1 Vector Breakdown
Affected Vendors & Products
Exploit & PoC Resources
Official Patches & Advisories
All References (24)
Quick Facts
Related CVEs (CWE-284)
Recommended Actions
- →Apply vendor patches immediately
- →Monitor CVE-2021-21703 in threat intel feeds
- →Review IDS/IPS signatures for exploitation attempts