CVE-2007-4770 in International Components for Unicode
Summary
by MITRE
libicu in International Components for Unicode (ICU) 3.8.1 and earlier attempts to process backreferences to the nonexistent capture group zero (aka \0), which might allow context-dependent attackers to read from, or write to, out-of-bounds memory locations, related to corruption of REStackFrames.
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Analysis
by VulDB Data Team • 08/04/2019
The vulnerability identified as CVE-2007-4770 affects the International Components for Unicode (ICU) library version 3.8.1 and earlier, specifically within the regular expression processing functionality. This issue represents a critical memory safety flaw that arises during the handling of backreferences in regular expression patterns. The problem occurs when the library attempts to process backreferences to capture group zero, which is a special group that should not exist in standard regular expression syntax, creating a scenario where memory operations become unpredictable and potentially exploitable.
The technical flaw manifests in the improper handling of regex patterns containing backreferences to group zero, which is not a valid capture group in standard regex implementations. When ICU processes such patterns, it attempts to access memory locations that are not properly allocated or validated, leading to potential out-of-bounds memory reads or writes. This vulnerability specifically impacts the REStackFrames data structures used internally by the ICU library to manage regular expression execution states, where the stack frame management fails to properly validate the existence of capture groups before attempting to reference them. The issue falls under the CWE-125 vulnerability category, which describes out-of-bounds read conditions that can lead to information disclosure or system compromise.
The operational impact of this vulnerability is significant as it can be exploited by context-dependent attackers who craft malicious regular expression patterns to trigger memory corruption. Attackers can potentially leverage this flaw to read sensitive data from memory locations, write arbitrary data to memory, or even execute arbitrary code depending on the system configuration and memory layout. The vulnerability is particularly concerning because it can be triggered through regular expression processing functions that are commonly used in text processing applications, web applications, and system utilities that rely on ICU for internationalization support. This makes the attack surface quite broad across various software systems that utilize ICU for text handling and pattern matching operations.
Mitigation strategies for CVE-2007-4770 involve immediate upgrading to ICU version 3.9.0 or later, where the vulnerability has been addressed through proper validation of capture group references and improved memory management in the regular expression engine. Organizations should also implement input validation measures to prevent untrusted regular expression patterns from being processed by ICU libraries, particularly in web applications where user input may be used in regex operations. Additionally, system administrators should consider implementing runtime protections such as address space layout randomization and stack canaries to reduce the exploitability of memory corruption vulnerabilities. The ATT&CK framework categorizes this vulnerability under the T1059.007 technique for command and scripting interpreter, as it can be exploited through malicious regex patterns that bypass normal input validation and execute unintended code through memory corruption.