CVE-2026-6104: PHP

Critical severity, CVSS 9.1. EPSS: 0.6% chance of exploitation in the next 30 days.

In PHP versions 8.4.* before 8.4.21 and 8.5.* before 8.5.6, when an encoding name containing an embedded NUL byte is passed to mb_convert_encoding() or related mbstring functions, the code incorrectly assumes that when strncasecmp() returns 0 it means the strings have the same length. This can lead to out-of-bounds read of global memory, potentially causing a crash or information disclosure or crash. Affected functions include mb_convert_encoding(), mb_detect_encoding(), mb_convert_variables(), and mb_detect_order(), as well as the mbstring.detect_order and mbstring.http_output INI settings.

Affected products

• PHP PHP: from 8.4.0, before 8.4.21 (fixed in 8.4.21); from 8.5.0, before 8.5.6 (fixed in 8.5.6)

Published 2026-05-10. Last modified 2026-07-24.