Is CVE-2026-6104 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-6104 being exploited in the wild.

How severe is CVE-2026-6104?

Critical severity, with a CVSS score of 9.1, published 10 May 2026.

Which products and versions are affected?

PHP Group PHP 8.4.* before 8.4.21, 8.5.* before 8.5.6; Red Hat Enterprise Linux AppStream 10.2.

Is there a fix for CVE-2026-6104?

A patch was published on 2026-06-02: Red Hat released security updates for RHEL 10 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-6104 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-6104?

Check whether you run PHP Group PHP or Red Hat Enterprise Linux AppStream at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-6104: PHP mbstring out-of-bounds read in mb_convert_encoding

CVE-2026-6104 · Severity: critical · CVSS 9.1 · Published 2026-05-10

PHP, a widely used programming language for web development, contains a security flaw in its multibyte string handling component. An attacker can exploit this by providing specially crafted text encoding names to a website, potentially causing the site to crash or leak sensitive internal memory data. This could lead to service disruptions or the exposure of information that should remain private.

Technologies: PHP Group PHP, Red Hat Enterprise Linux AppStream. Vendors: PHP Group, Red Hat.

Quick answers

Is CVE-2026-6104 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-6104 being exploited in the wild.
How severe is CVE-2026-6104?: Critical severity, with a CVSS score of 9.1, published 10 May 2026.
Which products and versions are affected?: PHP Group PHP 8.4.* before 8.4.21, 8.5.* before 8.5.6; Red Hat Enterprise Linux AppStream 10.2.
Is there a fix for CVE-2026-6104?: A patch was published on 2026-06-02: Red Hat released security updates for RHEL 10 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-6104 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-6104?: Check whether you run PHP Group PHP or Red Hat Enterprise Linux AppStream at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

An out-of-bounds read vulnerability (CWE-125) exists in PHP's mbstring extension, specifically within the mbfl_name2encoding_ex() function. The root cause is an incorrect assumption that a zero return from strncasecmp() implies equal string lengths when a NUL byte is embedded in the input encoding name. An attacker can trigger this by passing a malicious encoding string to functions like mb_convert_encoding(), mb_detect_encoding(), or via certain INI settings. This results in a global buffer over-read, allowing for the disclosure of adjacent global memory or a process crash. The issue is resolved in PHP versions 8.4.21 and 8.5.6.

Affected products

PHP Group PHP 8.4.* before 8.4.21, 8.5.* before 8.5.6
Red Hat Red Hat Enterprise Linux AppStream (v. 10) 10.2

Timeline

2026-05-07: advisory: PHP Group published GHSA-74r9-qxhc-fx53
2026-05-10: disclosed: CVE-2026-6104 published
2026-06-02: patched: Red Hat released security updates for RHEL 10

References

This page as Markdown. Free to reuse under CC BY 4.0 with a link back.