CVE-2018-1336 in Tomcat
Summary
by MITRE
An improper handing of overflow in the UTF-8 decoder with supplementary characters can lead to an infinite loop in the decoder causing a Denial of Service. Versions Affected: Apache Tomcat 9.0.0.M9 to 9.0.7, 8.5.0 to 8.5.30, 8.0.0.RC1 to 8.0.51, and 7.0.28 to 7.0.86.
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Analysis
by VulDB Data Team • 04/29/2023
The vulnerability identified as CVE-2018-1336 represents a critical flaw in Apache Tomcat's UTF-8 decoder implementation that can be exploited to cause denial of service conditions through infinite loop scenarios. This issue specifically affects the handling of UTF-8 encoded supplementary characters, which are four-byte sequences representing Unicode characters outside the basic multilingual plane. The improper overflow handling within the decoder creates a condition where certain malformed input sequences can trigger an infinite loop during the decoding process, effectively consuming system resources and rendering the application unavailable to legitimate users.
The technical root cause of this vulnerability lies in the decoder's failure to properly validate input boundaries when processing UTF-8 supplementary characters. When the decoder encounters specific byte sequences that should be rejected or properly handled according to UTF-8 standards, it instead enters an infinite loop due to inadequate boundary checking mechanisms. This flaw operates at the level of the application server's input processing pipeline, where HTTP request data is decoded before being processed by servlet applications. The vulnerability is particularly dangerous because it can be triggered through normal HTTP request parameters, making it accessible to attackers without requiring special privileges or complex exploitation techniques.
The operational impact of CVE-2018-1336 extends beyond simple service disruption to encompass significant system resource exhaustion and potential application instability. An attacker can craft malicious HTTP requests containing specifically constructed UTF-8 sequences that will cause the Tomcat server to enter an infinite loop, consuming CPU cycles and memory resources indefinitely. This behavior can be leveraged to perform distributed denial of service attacks where multiple concurrent requests can overwhelm server resources, leading to complete service unavailability. The vulnerability affects multiple major versions of Apache Tomcat, including the widely deployed 7.x and 8.x series, making it particularly concerning for enterprise environments that rely on these server versions.
From a cybersecurity perspective, this vulnerability maps directly to CWE-129, which addresses improper handling of buffer overflow conditions in input validation, and aligns with ATT&CK technique T1499.004 related to network denial of service. The attack surface is broad since any application running on affected Tomcat versions that processes user-supplied HTTP parameters is potentially vulnerable. Organizations should implement immediate mitigations including upgrading to patched versions of Apache Tomcat, implementing input validation at the application level, and deploying web application firewalls to filter suspicious UTF-8 sequences. The vulnerability demonstrates the critical importance of proper input validation and boundary checking in server-side applications, particularly when handling character encoding standards that involve multi-byte sequences. Security teams should also consider implementing monitoring and alerting mechanisms to detect unusual CPU usage patterns that might indicate exploitation attempts.