critical CVSS 9.1

CVE-2026-6104·PHP vulnerability

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.

Severity
critical
Software
PHP
Fixed in
8.5.6
Published
2026-05-10

Affected versions

From: 8.5.0

Until: 8.5.6

Fixed in: 8.5.6

How to fix this CVE

Update PHP to version 8.5.6 or later to address a critical memory safety issue in the mbstring extension. This vulnerability allows attackers to trigger out-of-bounds memory reads through specially crafted encoding parameters passed to mbstring functions, potentially leading to information disclosure or denial of service. Immediate patching is recommended for all production environments running affected PHP versions.

sudo dnf update php

Defensia detects this vulnerability

How to check if you are affected

  1. Step 1: Check installed PHP version with: php -v
  2. Step 2: Verify if mbstring extension is enabled by running: php -m | grep mbstring
  3. Step 3: Search application logs for calls to mb_convert_encoding(), mb_detect_encoding(), mb_convert_variables(), or mb_detect_order() functions with unusual encoding parameters
  4. Step 4: After patching, confirm the fix by running: php -v and verifying it reports 8.5.6 or later

FAQ

What is CVE-2026-6104?

This is a critical memory safety vulnerability in PHP's mbstring extension where NUL byte-embedded encoding names bypass string length validation in comparison operations, allowing out-of-bounds memory reads that can leak sensitive data or crash the application.

Is CVE-2026-6104 being actively exploited?

No, this vulnerability is not currently listed on the CISA Known Exploited Vulnerabilities catalog, and no public exploits are available. However, the critical CVSS score of 9.1 warrants prompt patching.

What versions of PHP are affected by CVE-2026-6104?

PHP 8.5.0 through 8.5.5 are vulnerable. PHP 8.4.0 through 8.4.20 are also affected per the NVD advisory. All users on these versions must upgrade to 8.5.6 or apply vendor patches.

How do I check if my server is vulnerable to CVE-2026-6104?

Run: php -v to check your version. If it reports 8.5.x before 8.5.6, your server is vulnerable. Additionally, confirm mbstring is loaded with: php -m | grep mbstring

Does Defensia detect CVE-2026-6104?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If PHP is installed on a monitored server, CVE-2026-6104 will appear in your dashboard with remediation steps.

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References

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