CVE-2008-5557 in PHP
Summary
by MITRE
Heap-based buffer overflow in ext/mbstring/libmbfl/filters/mbfilter_htmlent.c in the mbstring extension in PHP 4.3.0 through 5.2.6 allows context-dependent attackers to execute arbitrary code via a crafted string containing an HTML entity, which is not properly handled during Unicode conversion, related to the (1) mb_convert_encoding, (2) mb_check_encoding, (3) mb_convert_variables, and (4) mb_parse_str functions.
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Analysis
by VulDB Data Team • 08/23/2019
This heap-based buffer overflow vulnerability exists within the mbstring extension of PHP versions 4.3.0 through 5.2.6, specifically in the file ext/mbstring/libmbfl/filters/mbfilter_htmlent.c. The flaw occurs when processing HTML entities within strings that undergo Unicode conversion, creating a condition where attacker-controlled input can corrupt heap memory. The vulnerability is triggered through context-dependent attacks that leverage the improper handling of HTML entities during encoding conversion operations. This represents a classic buffer overflow scenario where insufficient bounds checking allows data to be written beyond allocated memory boundaries, potentially enabling arbitrary code execution.
The technical implementation of this vulnerability involves the mbstring functions including mb_convert_encoding, mb_check_encoding, mb_convert_variables, and mb_parse_str which all utilize the same underlying code path for HTML entity processing. When a malicious string containing crafted HTML entities is processed, the Unicode conversion routines fail to properly validate the size of the output buffer, leading to memory corruption. The vulnerability is particularly dangerous because it can be exploited through multiple API endpoints, amplifying its attack surface. This flaw aligns with CWE-121 Heap-based Buffer Overflow, which specifically addresses buffer overflows occurring in heap memory regions.
The operational impact of this vulnerability is severe as it allows remote attackers to execute arbitrary code on affected systems with the privileges of the web server process. Attackers can craft malicious HTML entity sequences that, when processed by any of the affected mbstring functions, cause the buffer overflow to occur. This creates a significant risk for web applications that process user input through these encoding functions, particularly in environments where PHP handles untrusted data from web forms, API requests, or file uploads. The vulnerability is especially concerning in the context of the ATT&CK framework's privilege escalation techniques, as successful exploitation could lead to full system compromise.
Mitigation strategies for this vulnerability include immediate upgrading to PHP versions 5.2.7 or later where the issue has been resolved through proper bounds checking and memory management. Organizations should also implement input validation and sanitization measures to prevent malformed HTML entities from reaching the vulnerable functions. Additionally, deploying web application firewalls and implementing proper error handling can help detect and prevent exploitation attempts. The fix implemented by PHP developers involved strengthening the buffer size calculations and adding proper boundary checks during HTML entity processing, directly addressing the root cause of the heap corruption. System administrators should also consider implementing runtime protections such as stack canaries or address space layout randomization to make exploitation more difficult, though these are secondary defenses against this specific vulnerability.