CVE-2026-6104
Global buffer over-read in mb_convert_encoding() with attacker-supplied encoding
Description
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.
How to fix CVE-2026-6104
To remediate CVE-2026-6104, upgrade the affected package to a fixed version below.
- —upgrade to 8.4.21 or later
- —upgrade to 8.4.21 or later
- —upgrade to 8.4.21 or later
- —upgrade to 8.4.21-1~deb13u1 or later
Is CVE-2026-6104 being exploited?
Low — EPSS is 0.0%, meaning exploitation activity has not been observed at scale.
Affected packages (4)
- >= 8.4.0, < 8.4.21, >= 8.5.0, < 8.5.6
- >= 8.4.0, < 8.4.21, >= 8.5.0, < 8.5.6
- >= 8.4.0, < 8.4.21, >= 8.5.0, < 8.5.6
- from 0, < 8.4.21-1~deb13u1
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 4.0 | — | CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:L/SC:L/SI:N/SA:L/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:M/U:Amber |
| osv | CVSS 3.1 | CRITICAL9.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H |