CVE-2026-4926: Regular Expression DoS Vulnerability

CVE-2026-4926 Overview

CVE-2026-4926 is a Regular Expression Denial of Service (ReDoS) vulnerability that occurs when processing route patterns containing multiple sequential optional groups. A bad regular expression is generated any time you have multiple sequential optional groups (curly brace syntax), such as {a}{b}{c}:z. The generated regex grows exponentially with the number of groups, causing denial of service through CPU exhaustion.

Critical Impact

Attackers can exploit this vulnerability to cause denial of service by triggering exponential regex computation, rendering the affected application unresponsive.

Affected Products

• Route pattern parsing components using curly brace syntax for optional groups
• Web frameworks and routing libraries utilizing the vulnerable regex generation logic
• Applications accepting user-controlled route patterns

Discovery Timeline

• 2026-03-26 - CVE CVE-2026-4926 published to NVD
• 2026-03-26 - Last updated in NVD database

Technical Details for CVE-2026-4926

Vulnerability Analysis

This vulnerability is classified under CWE-400 (Uncontrolled Resource Consumption) and stems from inefficient regular expression generation when processing route patterns with sequential optional groups. When multiple optional groups are defined consecutively using curly brace syntax (e.g., {a}{b}{c}:z), the resulting regular expression exhibits exponential growth characteristics.

The core issue lies in how the regex engine handles backtracking when evaluating these patterns. Each additional optional group multiplies the possible matching paths, creating a computational complexity that grows exponentially. This allows an attacker to craft route patterns that consume excessive CPU resources, leading to application unavailability.

The vulnerability can be exploited remotely over the network without requiring authentication or user interaction. While confidentiality and integrity are not impacted, the availability impact is significant as affected services can become completely unresponsive.

Root Cause

The root cause is the algorithm used to generate regular expressions from route pattern definitions. When processing sequential optional groups using the curly brace syntax, the generator fails to optimize or limit the resulting regex complexity. Each optional group adds multiplicative branches to the regex state machine, and the lack of bounds checking allows patterns that produce exponentially complex regular expressions.

Attack Vector

The attack vector is network-based and requires no privileges or user interaction. An attacker can exploit this vulnerability by:

1. Identifying an endpoint or configuration interface that accepts route pattern definitions
2. Submitting a route pattern containing multiple sequential optional groups (e.g., {a}{b}{c}{d}{e}:endpoint)
3. The application generates an exponentially complex regex from this pattern
4. When the regex is evaluated against incoming requests, CPU resources are exhausted
5. The application becomes unresponsive, resulting in denial of service

The attack is particularly dangerous when user-controlled input can influence route pattern definitions, as this gives attackers direct control over the regex generation process.

Detection Methods for CVE-2026-4926

Indicators of Compromise

• Sudden CPU spikes correlated with route processing or pattern matching operations
• Application threads stuck in regex evaluation states for extended periods
• Increased response times or timeouts on routing-related operations
• Log entries showing unusually long route pattern strings with multiple curly brace groups

Detection Strategies

• Monitor for route patterns containing three or more consecutive optional groups ({...}{...}{...})
• Implement application performance monitoring (APM) to detect regex evaluation latency anomalies
• Review incoming requests or configuration changes for patterns matching \{[^}]+\}\{[^}]+\} sequences
• Set up alerts for sustained high CPU utilization in web server or routing processes

Monitoring Recommendations

• Configure resource utilization thresholds to alert on abnormal CPU consumption patterns
• Implement request timeout monitoring to detect slow route resolution
• Enable detailed logging for route pattern compilation and evaluation
• Establish baseline metrics for regex processing time to identify deviations

How to Mitigate CVE-2026-4926

Immediate Actions Required

• Upgrade to version 8.4.0 or later where this vulnerability has been patched
• Audit existing route patterns for multiple sequential optional groups and refactor if necessary
• Implement input validation to reject route patterns with excessive optional group sequences
• Consider implementing timeouts for regex evaluation operations

Patch Information

The vulnerability has been fixed in version 8.4.0. Organizations should upgrade to this version or later to remediate the vulnerability. For detailed information about the security patch, refer to the OpenJS Foundation Security Advisories.

Workarounds

• Limit the number of sequential optional groups in route patterns to no more than two consecutive groups
• Avoid passing user-controlled input as route patterns entirely
• Implement server-side validation to reject route patterns containing more than a defined threshold of optional groups
• Apply resource limits (CPU time, memory) to regex evaluation processes to prevent complete service unavailability

The vulnerability mechanism involves exponential regex generation from sequential optional groups. When a pattern like {a}{b}{c}:z is processed, the resulting regular expression contains branches that multiply with each additional optional group, leading to catastrophic backtracking during evaluation. Consult the OpenJS Foundation Security Advisories for additional technical details and guidance.