CVE-2018-15856 in xkbcommon
Summary
by MITRE
An infinite loop when reaching EOL unexpectedly in compose/parser.c (aka the keymap parser) in xkbcommon before 0.8.1 could be used by local attackers to cause a denial of service during parsing of crafted keymap files.
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Analysis
by VulDB Data Team • 05/04/2023
The vulnerability identified as CVE-2018-15856 represents a critical denial of service flaw within the xkbcommon library's keymap parsing functionality. This issue affects versions prior to 0.8.1 and specifically targets the compose/parser.c component which handles keyboard mapping file parsing. The vulnerability manifests when the parser encounters an unexpected end-of-line condition during keymap file processing, creating a scenario where the parsing routine becomes trapped in an infinite loop. This behavior fundamentally undermines the library's ability to properly handle keyboard input configurations and can be exploited by local attackers to disrupt system operations.
The technical implementation of this vulnerability resides in the keymap parser's handling of input validation and state management during file processing. When the parser reaches an unexpected end-of-line marker within a keymap file, the control flow logic fails to properly terminate the parsing loop, instead causing the execution to repeatedly process the same input segment indefinitely. This infinite loop condition occurs because the parser does not adequately validate the state transitions or handle edge cases where file boundaries are encountered in unexpected positions. The flaw essentially creates a race condition between input processing and state machine management, where the parser's internal counters or pointers fail to advance properly, resulting in persistent execution of the same parsing iteration.
The operational impact of this vulnerability extends beyond simple service disruption to potentially affect system stability and availability across various platforms that rely on xkbcommon for keyboard input handling. Since xkbcommon is widely used in desktop environments, windowing systems, and input management frameworks, a successful exploitation could impact multiple applications and services that depend on proper keyboard mapping functionality. Local attackers can craft malicious keymap files that trigger this condition, leading to resource exhaustion as the system becomes unresponsive to keyboard input events. The vulnerability is particularly concerning because it operates at a foundational level within the input processing stack, making it difficult to isolate and mitigate without addressing the root cause in the parser implementation.
Mitigation strategies for CVE-2018-15856 require immediate patching of affected systems to upgrade to xkbcommon version 0.8.1 or later, which contains the necessary fixes to properly handle end-of-line conditions during keymap parsing. Organizations should prioritize this update across all systems that utilize xkbcommon libraries, particularly those running desktop environments or applications that process keyboard configuration files. Additional protective measures include implementing file validation checks for keymap files, establishing monitoring for unusual CPU usage patterns that might indicate infinite loop conditions, and configuring input processing components with timeout mechanisms to prevent indefinite blocking. From a cybersecurity perspective, this vulnerability aligns with CWE-835, which specifically addresses infinite loops or iterations without proper termination conditions, and represents a classic example of how parsing errors can lead to denial of service conditions. The ATT&CK framework categorizes this as a privilege escalation technique through resource exhaustion, as local attackers can leverage this vulnerability to consume system resources and potentially disrupt normal operations.