# CVE-2026-77423: JLine regular expression denial of service in less viewer

Source: https://junglewise.ai/threats/cve-2026-77423-jline-is-a-java-library-for-handling-console-input-from-3-0-0  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-23. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-77423 · Severity: high · CVSS 7.5 · Published 2026-09-23

JLine is a Java library that provides console input handling for terminal applications and interactive shells. The built-in less viewer allows users to enter search patterns that are evaluated as regular expressions without safeguards, allowing an attacker to craft patterns with nested quantifiers that trigger catastrophic backtracking. This causes the session to hang indefinitely and consume CPU, and in server deployments using Telnet or SSH, repeated attacks can exhaust worker thread pools and deny service to legitimate users.

Technologies: [JLine](https://junglewise.ai/threats/technologies/jline). Vendors: [JLine](https://junglewise.ai/threats/vendors/jline).

## Quick answers

**Is CVE-2026-77423 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-77423 being exploited in the wild.

**How severe is CVE-2026-77423?** High severity, with a CVSS score of 7.5, published 23 September 2026.

**Which products and versions are affected?** JLine 3.0.0 to 3.30.14, 4.0.0 to 4.3.0.

**Is there a fix for CVE-2026-77423?** A patch was published on 2026-06-30: Fixed in versions 3.30.15 and 4.3.1 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-77423 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-77423?** Check whether you run JLine at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability is a regular expression denial of service (ReDoS) in the Less.java builtin where user-controlled search and display-filter patterns from getPattern() are passed directly to Java's backtracking regex engine without timeout protection. Nested quantifier patterns like (a+)+b can consume exponential CPU time. The fix introduces a SafeRegex utility that wraps regex matching in a timeout mechanism, checking elapsed time every 1024 character comparisons and throwing RegexTimeoutException if a deadline is exceeded.

## Affected products

- JLine JLine 3.0.0 to 3.30.14, 4.0.0 to 4.3.0

## Timeline

- 2026-09-23: disclosed
- 2026-06-30: patched: Fixed in versions 3.30.15 and 4.3.1

## References

- [https://github.com/jline/jline3/commit/1d5fc3099e77938b971e197211cad2d4fbb17541](https://github.com/jline/jline3/commit/1d5fc3099e77938b971e197211cad2d4fbb17541)
- [https://github.com/jline/jline3/commit/341ee69ccc57b7733c1b40d6993219b64b3206ae](https://github.com/jline/jline3/commit/341ee69ccc57b7733c1b40d6993219b64b3206ae)

## Related threats

- [JLine Nano editor ReDoS in regex search mode](https://junglewise.ai/threats/cve-2026-77421-jline-nano-editor-redos-in-regex-search-mode)
- [JLine HISTORY_IGNORE ReDoS via unescaped regex metacharacters](https://junglewise.ai/threats/cve-2026-77420-jline-history-ignore-redos-via-unescaped-regex-metacharacters)
- [JLine JLine3 denial of service in Telnet server NAWS handling](https://junglewise.ai/threats/cve-2026-56741-jline-jline3-denial-of-service-in-telnet-server-naws-handling)
- [JLine JLine3 denial of service in Telnet server NEW-ENVIRON handling](https://junglewise.ai/threats/cve-2026-56740-jline-jline3-denial-of-service-in-telnet-server-new-environ)