CVE-2019-17042 in rsyslog
Summary
by MITRE
An issue was discovered in Rsyslog v8.1908.0. contrib/pmcisconames/pmcisconames.c has a heap overflow in the parser for Cisco log messages. The parser tries to locate a log message delimiter (in this case, a space or a colon), but fails to account for strings that do not satisfy this constraint. If the string does not match, then the variable lenMsg will reach the value zero and will skip the sanity check that detects invalid log messages. The message will then be considered valid, and the parser will eat up the nonexistent colon delimiter. In doing so, it will decrement lenMsg, a signed integer, whose value was zero and now becomes minus one. The following step in the parser is to shift left the contents of the message. To do this, it will call memmove with the right pointers to the target and destination strings, but the lenMsg will now be interpreted as a huge value, causing a heap overflow.
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Analysis
by VulDB Data Team • 01/03/2024
The vulnerability identified as CVE-2019-17042 represents a critical heap overflow flaw within the rsyslog software version 8.1908.0, specifically within the contrib/pmcisconames/pmcisconames.c module. This issue stems from inadequate input validation during the parsing of Cisco log messages, creating a dangerous condition where the parser fails to properly handle malformed or unexpected message formats. The vulnerability manifests when the parser attempts to locate log message delimiters such as spaces or colons, but encounters strings that do not conform to these expected patterns, leading to unpredictable behavior in the parsing logic.
The technical execution of this vulnerability begins with the parser's failure to properly validate message structure before proceeding with processing. When the expected delimiter characters are not found in the log message, the variable lenMsg is set to zero, effectively bypassing crucial sanity checks designed to detect invalid log messages. This zero value then becomes a critical factor in the subsequent memory operations, as it allows the parser to incorrectly treat the message as valid despite its malformed nature. The parser continues processing without proper validation, leading to a cascade of operations that ultimately results in memory corruption.
The operational impact of this heap overflow vulnerability is severe and potentially exploitable in malicious scenarios. The vulnerability specifically affects the memmove function call that occurs after the parser attempts to shift message contents, where the previously zeroed lenMsg variable becomes negative (-1) due to decrement operations. This negative value is then interpreted by the memory management functions as an extremely large unsigned integer, causing the memmove operation to copy an enormous amount of data beyond the allocated heap boundaries. The resulting heap overflow creates opportunities for memory corruption that could potentially be leveraged for arbitrary code execution or denial of service conditions, particularly in environments where rsyslog processes untrusted log data from network devices.
This vulnerability aligns with CWE-121, Heap-based Buffer Overflow, and represents a classic example of improper input validation combined with integer overflow conditions. The flaw demonstrates characteristics consistent with ATT&CK technique T1059.007, which involves command and scripting interpreter usage, as the vulnerability could enable attackers to manipulate log processing behavior. The issue particularly affects systems that rely on rsyslog for processing Cisco network device logs, making it relevant to enterprise network monitoring and security operations. Organizations using affected rsyslog versions should prioritize patching and implementation of additional input validation measures to prevent exploitation of this memory corruption vulnerability.
The root cause of this vulnerability lies in the insufficient boundary checking and integer handling within the Cisco log message parser logic. The failure to properly validate message delimiters and handle edge cases where expected characters are absent creates a path for memory corruption. Security practitioners should note that this vulnerability impacts the integrity of log processing systems, potentially allowing attackers to disrupt normal operations or gain unauthorized access through memory corruption exploitation. The fix typically involves implementing proper input validation and ensuring that length variables are properly constrained before being used in memory operations, particularly in functions that manipulate heap memory through operations like memmove or memcpy.