CVE-2007-4990 in X11
Summary
by MITRE
The swap_char2b function in X.Org X Font Server (xfs) before 1.0.5 allows context-dependent attackers to execute arbitrary code via (1) QueryXBitmaps and (2) QueryXExtents protocol requests with crafted size values that specify an arbitrary number of bytes to be swapped on the heap, which triggers heap corruption.
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Analysis
by VulDB Data Team • 07/29/2019
The vulnerability identified as CVE-2007-4990 represents a critical heap corruption flaw within the X.Org X Font Server xfs component prior to version 1.0.5. This vulnerability resides in the swap_char2b function which processes font data during protocol interactions. The flaw manifests when the xfs server handles QueryXBitmaps and QueryXExtents protocol requests, where maliciously crafted size parameters can manipulate the heap memory layout. The vulnerability operates under CWE-122 which categorizes heap-based buffer overflow conditions, specifically targeting memory corruption through improper handling of variable-length data structures.
The technical execution of this vulnerability requires an attacker to establish a connection to the X Font Server and submit specially crafted protocol requests containing manipulated size values. When the swap_char2b function processes these requests, it performs byte swapping operations on heap-allocated memory regions without proper bounds checking. The crafted size values instruct the function to swap an arbitrary number of bytes, effectively allowing an attacker to overwrite adjacent heap memory locations. This heap corruption can result in arbitrary code execution, as the attacker can manipulate memory pointers and function return addresses to redirect program execution flow.
The operational impact of this vulnerability extends beyond simple privilege escalation as it enables remote code execution on systems running vulnerable versions of the X Font Server. Attackers can leverage this flaw to gain unauthorized access to systems, potentially escalating privileges to the level of the xfs service account. The context-dependent nature of this vulnerability means that successful exploitation requires network access to the X server and knowledge of the specific protocol interactions, but once achieved, the impact is severe enough to compromise entire systems. The vulnerability aligns with ATT&CK technique T1068 which covers 'Exploitation for Privilege Escalation' and specifically addresses the use of heap-based memory corruption for code execution.
Mitigation strategies for CVE-2007-4990 primarily involve upgrading to X.Org X Font Server version 1.0.5 or later, which includes proper bounds checking and memory validation for the affected protocol handlers. System administrators should also implement network segmentation to restrict access to X server services, particularly in environments where the xfs service is exposed to untrusted networks. Additional protective measures include disabling unnecessary font server functionality, implementing proper firewall rules to limit access to X server ports, and monitoring network traffic for suspicious protocol interactions. The vulnerability serves as a prime example of why proper input validation and memory management practices are essential in server applications, particularly those handling external protocol data. Organizations should also consider implementing intrusion detection systems to monitor for exploitation attempts targeting known heap corruption vulnerabilities, as these attacks often follow predictable patterns in protocol request handling.