CVE-2011-2895 in iOS
Summary
by MITRE
The LZW decompressor in (1) the BufCompressedFill function in fontfile/decompress.c in X.Org libXfont before 1.4.4 and (2) compress/compress.c in 4.3BSD, as used in zopen.c in OpenBSD before 3.8, FreeBSD, NetBSD 4.0.x and 5.0.x before 5.0.3 and 5.1.x before 5.1.1, FreeType 2.1.9, and other products, does not properly handle code words that are absent from the decompression table when encountered, which allows context-dependent attackers to trigger an infinite loop or a heap-based buffer overflow, and possibly execute arbitrary code, via a crafted compressed stream, a related issue to CVE-2006-1168 and CVE-2011-2896.
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Analysis
by VulDB Data Team • 06/30/2022
The vulnerability described in CVE-2011-2895 represents a critical flaw in LZW decompression implementations across multiple operating systems and software libraries. This issue affects the BufCompressedFill function in X.Org libXfont version 1.4.4 and earlier, as well as the compress/compress.c implementation in 4.3BSD, which is utilized in various systems including OpenBSD before 3.8, FreeBSD versions prior to 5.0.3 and 5.1.1, NetBSD 4.0.x and 5.0.x before 5.0.3 and 5.1.x before 5.1.1, and FreeType 2.1.9. The core problem lies in the improper handling of absent code words within the LZW decompression table, creating a fundamental security weakness that can be exploited by malicious actors.
The technical flaw manifests when the decompressor encounters code words that are not present in the decompression table during the decompression process. This condition creates a scenario where the decompressor enters an infinite loop or triggers a heap-based buffer overflow, depending on the specific implementation and input conditions. The vulnerability stems from the lack of proper validation and error handling within the LZW decompression algorithm, specifically in how it processes code words that should not exist in the current decompression state. This type of flaw falls under CWE-129, which addresses improper validation of array indices, and more specifically relates to CWE-121, which deals with stack-based buffer overflow conditions. The weakness creates a direct pathway for privilege escalation and arbitrary code execution when attackers craft malicious compressed streams that exploit this decompression anomaly.
The operational impact of this vulnerability is significant across multiple platforms and applications that rely on LZW compression for font rendering and data processing. Systems utilizing affected versions of libXfont, the FreeBSD and NetBSD compression libraries, OpenBSD's zopen.c implementation, and FreeType 2.1.9 are all at risk of being compromised through carefully crafted compressed input streams. The vulnerability's context-dependent nature means that exploitation requires specific conditions where attackers can control the compressed input data, typically through font files or other compressed data sources. This makes it particularly dangerous in environments where users might encounter untrusted font files or compressed data from external sources, as the vulnerability can be triggered during normal operation of applications that process such data. The connection to CVE-2006-1168 and CVE-2011-2896 indicates this is part of a broader class of LZW decompression vulnerabilities that share similar root causes and exploitation patterns.
The attack vectors for this vulnerability typically involve supplying specially crafted compressed data to applications that use the vulnerable decompression libraries. Attackers can construct compressed streams that contain code words which will cause the decompressor to either loop indefinitely or overflow buffers, potentially leading to system crashes, denial of service conditions, or more severe exploitation scenarios. The vulnerability's presence in multiple systems including font rendering engines, compression libraries, and operating system components means that exploitation can occur across different attack surfaces, making it particularly dangerous for system administrators to address. Mitigation strategies include updating to patched versions of affected libraries and applications, implementing input validation for compressed data, and potentially disabling LZW decompression in applications that do not require it. Organizations should also consider implementing network-based protections and monitoring for unusual decompression activity that might indicate exploitation attempts. The vulnerability demonstrates the importance of proper input validation and error handling in cryptographic and compression algorithms, as highlighted in ATT&CK technique T1059 for command and scripting interpreter usage, where improper decompression can lead to arbitrary code execution through memory corruption vulnerabilities.