CVE-2017-5356 in irssi
Summary
by MITRE
Irssi before 0.8.21 allows remote attackers to cause a denial of service (out-of-bounds read and crash) via a string containing a formatting sequence (%[) without a closing bracket (]).
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Analysis
by VulDB Data Team • 09/03/2020
The vulnerability identified as CVE-2017-5356 affects Irssi, a popular console-based IRC client that has been widely used for instant messaging and chat communications since its initial release in 1999. This particular flaw exists in versions prior to 0.8.21 and represents a critical buffer over-read condition that can be exploited remotely by attackers to cause system instability and potential service disruption. The vulnerability specifically targets Irssi's string formatting functionality, which is essential for displaying messages, handling user input, and processing various IRC protocol elements. When Irssi encounters a malformed string containing a formatting sequence beginning with a percent sign followed by an opening bracket without a corresponding closing bracket, the application fails to properly validate the input structure. This oversight creates an exploitable condition where the application attempts to read memory locations beyond the intended buffer boundaries, leading to unpredictable behavior and ultimately resulting in a crash or complete application termination. The issue stems from inadequate input sanitization within the client's formatting string parser, which does not properly handle malformed format specifiers that could originate from malicious IRC messages or user inputs. The vulnerability demonstrates characteristics consistent with CWE-129, which describes improper validation of length of input buffers, and aligns with ATT&CK technique T1499.004 for network denial of service attacks. The impact of this vulnerability extends beyond simple service interruption as it could be leveraged by attackers to disrupt communication channels for users relying on Irssi for critical messaging infrastructure. Remote exploitation is particularly concerning given that Irssi often operates in environments where users may connect to public or untrusted IRC networks where malicious actors could easily inject malformed messages. The technical flaw manifests when the application's internal formatting engine attempts to process the malformed sequence, causing an out-of-bounds memory access that triggers a segmentation fault or similar memory management exception. This type of vulnerability is classified as a remote code execution risk despite not directly enabling arbitrary code execution, as it can be used to systematically disrupt service availability and potentially create conditions for more sophisticated attacks. The vulnerability affects not only individual user sessions but also broader network communication stability, as IRC clients often serve as central communication points for collaborative work environments, gaming communities, and professional messaging systems. Organizations and individual users should immediately update to Irssi version 0.8.21 or later to mitigate this risk, as the fix involves implementing proper bounds checking and input validation for formatting sequences. Additional mitigations include implementing network-level filtering to detect and block malformed IRC messages, configuring client-side input validation, and monitoring for unusual crash patterns that may indicate exploitation attempts. The vulnerability serves as a reminder of the importance of robust input validation in client applications and highlights the need for continuous security assessments of widely-used open source software components. This issue represents a classic example of how seemingly minor parsing errors can lead to significant availability concerns, particularly in communication software where reliability is paramount for user experience and operational continuity.