CVE-2009-0023 in HTTP Server
Summary
by MITRE
The apr_strmatch_precompile function in strmatch/apr_strmatch.c in Apache APR-util before 1.3.5 allows remote attackers to cause a denial of service (daemon crash) via crafted input involving (1) a .htaccess file used with the Apache HTTP Server, (2) the SVNMasterURI directive in the mod_dav_svn module in the Apache HTTP Server, (3) the mod_apreq2 module for the Apache HTTP Server, or (4) an application that uses the libapreq2 library, which triggers a heap-based buffer underflow.
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Analysis
by VulDB Data Team • 09/06/2019
The vulnerability described in CVE-2009-0023 represents a critical heap-based buffer underflow condition within the Apache Portable Runtime Utility library, specifically affecting the apr_strmatch_precompile function. This flaw exists in APR-util versions prior to 1.3.5 and demonstrates how seemingly benign input processing can lead to catastrophic system failures. The vulnerability manifests when processing regular expression patterns through the strmatch functionality, creating a scenario where attacker-controlled data can manipulate memory allocation and access patterns in ways that fundamentally compromise system stability. The affected components operate at a foundational level within the Apache ecosystem, making this issue particularly dangerous as it can impact multiple modules and applications that depend on APR-util for string matching operations.
The technical implementation of this vulnerability stems from inadequate input validation and memory boundary checking within the apr_strmatch_precompile function. When processing crafted regular expression patterns, the function fails to properly validate the length and structure of input data, leading to memory access violations that result in heap corruption. This buffer underflow occurs during the precompilation phase of string matching operations, where the system attempts to optimize pattern matching algorithms by pre-processing input patterns. The flaw specifically affects the handling of complex regular expressions that contain malformed or specially constructed sequences, causing the underlying memory allocator to access regions outside of allocated buffers. The vulnerability operates through multiple attack vectors including .htaccess files, SVNMasterURI directives, mod_apreq2 module configurations, and applications utilizing libapreq2, demonstrating the widespread impact across different Apache modules and components.
The operational impact of this vulnerability extends beyond simple denial of service conditions to potentially enable more sophisticated attacks. When exploited, the heap-based buffer underflow can cause daemon crashes, application termination, and system instability that affects not only individual services but entire server operations. The vulnerability's presence in core Apache components means that successful exploitation can lead to cascading failures across multiple services running on the same server infrastructure. Network-based attackers can leverage this flaw to remotely disrupt services without requiring authentication or elevated privileges, making it particularly attractive for malicious actors seeking to cause disruption. The vulnerability's classification as a heap-based buffer underflow aligns with CWE-121, which specifically addresses heap-based buffer overflow conditions, while its exploitation pattern corresponds to ATT&CK technique T1499.004 related to network denial of service attacks. The impact is amplified by the fact that many Apache installations rely on these vulnerable components for basic functionality, creating widespread potential for service disruption.
Mitigation strategies for CVE-2009-0023 focus primarily on immediate patching and version updates to APR-util 1.3.5 and later releases where the buffer underflow has been addressed through proper input validation and memory boundary checks. Organizations should prioritize updating their Apache HTTP Server installations and any applications that utilize libapreq2 or mod_apreq2 modules to ensure they are running patched versions of the affected libraries. Additionally, implementing input sanitization measures and regular expression validation within .htaccess files and SVN configurations can provide additional defensive layers. Network administrators should consider implementing intrusion detection systems that can identify patterns associated with exploitation attempts, while application developers should review their code for similar buffer handling patterns that might be susceptible to similar vulnerabilities. The fix implemented in APR-util 1.3.5 specifically addresses the memory allocation and boundary checking procedures within the strmatch functions, ensuring that input data is properly validated before processing and that memory access operations remain within allocated buffer boundaries.