CVE-2026-77421 in JLine
Summary
by MITRE • 09/23/2026
JLine is a Java library for handling console input. From 3.0.0 until 3.30.15 and 4.3.1, the JLine built-in nano editor's regex search mode passes a user-controlled search term from doSearch(String text) in builtins/src/main/java/org/jline/builtins/Nano.java to Java's backtracking regular expression engine without a timeout or backtracking bound. A nested-quantifier expression evaluated against non-matching buffer content can consume excessive CPU and indefinitely block the editor session thread, and remote multi-user deployments can lose a worker thread for each affected session. This issue is fixed in versions 3.30.15 and 4.3.1.
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Analysis
by VulDB Data Team • 09/23/2026
The JLine library serves as a foundational component for Java-based console input handling, providing robust support for command-line interfaces and interactive shells. Within this ecosystem, the built-in nano editor implementation offers text editing capabilities that are frequently utilized in terminal applications. A critical security flaw exists within the regex search functionality of this editor module, specifically affecting versions ranging from 3.0.0 up to 3.30.15 for the legacy branch and up to version 4.3.1 for the current development line. This vulnerability stems from an improper handling of regular expression patterns during user-initiated searches, where a search term provided by the end-user is passed directly to Java's backtracking regular expression engine without any protective constraints such as execution timeouts or complexity bounds.
The technical core of this flaw lies in the interaction between unvalidated user input and the underlying regex engine. When a user performs a search operation via the doSearch method, the supplied text is treated as a pattern string. If an attacker crafts a maliciously constructed regular expression containing nested quantifiers, such as multiple levels of grouping with repetition operators like (a+)+ or ((.).), they can trigger catastrophic backtracking when this pattern is evaluated against buffer content that does not match the desired sequence. Because Java's regex engine lacks built-in safeguards to limit the depth of its recursive matching attempts in this context, it will exhaust available computational resources attempting to resolve all possible combinations before concluding a non-match. This behavior effectively transforms a simple text search operation into a mechanism for resource exhaustion.
The operational impact of this vulnerability is significant, particularly regarding availability and system stability. For local users interacting with the editor, the consequence is an indefinite blockage of the editor session thread. The application becomes unresponsive as it consumes excessive CPU cycles attempting to process the malicious pattern, effectively resulting in a denial-of-service condition for that specific user instance. In more complex environments, such as remote multi-user deployments or servers hosting multiple concurrent JLine-based sessions, the implications are even more severe. Each affected session can consume an entire worker thread indefinitely, leading to thread pool exhaustion. This cascading failure can render the broader application or service unavailable to legitimate users, disrupting critical workflows and potentially causing system-wide instability depending on how threads are managed by the hosting environment.
This vulnerability is classified under CWE-400, which covers Uncontrolled Resource Consumption, as it allows an attacker to cause a denial of service through excessive resource usage without requiring authentication in many default configurations. Furthermore, from a tactical perspective aligned with MITRE ATT&CK frameworks, this behavior aligns with techniques related to Impact and potentially Initial Access if the vulnerability can be triggered remotely via network services that expose JLine interfaces. The lack of input validation on regex patterns is a common pitfall in software development, often overlooked because regular expressions are typically assumed to be safe for text processing tasks. However, as demonstrated here, they require strict sanitization or constraint enforcement when accepting external data.
Mitigation strategies must focus on both immediate patching and long-term defensive coding practices. The primary remediation is to upgrade the JLine library to version 3.30.15 or later for legacy systems, or version 4.3.1 and above for modern deployments, where this issue has been addressed by implementing safeguards against catastrophic backtracking. For organizations unable to immediately patch their dependencies, temporary mitigations include restricting the length of search inputs, disabling regex-based search features in favor of simple string matching if available, or deploying a Web Application Firewall that can detect and block anomalous CPU usage patterns associated with such attacks. Additionally, developers should adopt secure coding standards that mandate timeout mechanisms for all regular expression evaluations involving untrusted input, ensuring that no single operation can monopolize system resources indefinitely.