Skip to content

Global buffer over-read in mb_convert_encoding() with attacker-supplied encoding

Moderate
iluuu1994 published GHSA-74r9-qxhc-fx53 May 7, 2026

Package

ext-mbstring (PHP)

Affected versions

< 8.4.21
< 8.5.6

Patched versions

8.4.21
8.5.6

Description

Summary

A global buffer over-read exists in mbfl_name2encoding_ex() (ext/mbstring/libmbfl/mbfl/mbfl_encoding.c, line 352), called from mb_convert_encoding(...) and other mbstring functions, when parsing encoding names with embedded NUL bytes.

Root Cause

In mbfl_name2encoding_ex, the MIME name search loop (line 352) uses:

if (strncasecmp((*encoding)->mime_name, name, name_len) == 0 && (*encoding)->mime_name[name_len] == '\0') {

When name contains an embedded NUL byte (e.g., "UTF-8\x00AAAA..."), strncasecmp returns 0 after matching up to the NUL (since both strings have NUL at the same position). The subsequent check (*encoding)->mime_name[name_len] then reads at offset name_len (e.g., 22) from mime_name, which is only 6 bytes long ("UTF-8\0"). This reads 16+ bytes past the end of the global string into adjacent global memory.

The same issue exists on lines 362 for encoding aliases.

Minimal Reproducer

<?php
$encoding = "UTF-8\x00AAAAAAAAAAAAAAAA";
mb_convert_encoding('foo', $encoding, $encoding)

Running with ASan-instrumented PHP 8.4.20:

==552769==ERROR: AddressSanitizer: global-buffer-overflow on address 0x...
READ of size 1
    #0 in mbfl_name2encoding_ex mbfl_encoding.c:352
    #1 in php_mb_parse_encoding_list mbstring.c:350
    #2 in zif_mb_convert_encoding mbstring.c:2865

Attack Surface

  • Triggerable when a attacker-controlled encoding is passed to mb_convert_encoding() without sanitization
  • Also affected are mb_detect_encoding(), mb_convert_variables(), mb_detect_order(), as well as the mbstring.detect_order and mbstring.http_output INI settings.
  • Out-of-bounds read of global memory (CWE-125)
  • Information disclosure of adjacent global data; not directly exploitable for code execution

Environment

  • PHP 8.4.0+, -fsanitize=address,undefined

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability Low
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

CVE ID

CVE-2026-6104

Weaknesses

Out-of-bounds Read

The product reads data past the end, or before the beginning, of the intended buffer. Learn more on MITRE.

Credits