DEV Community

CVE Reports
CVE Reports

Posted on • Originally published at cvereports.com

CVE-2026-23886: CVE-2026-23886: The Case of the Fatal Uppercase

#security #cve #cybersecurity

CVE-2026-23886: The Case of the Fatal Uppercase

Vulnerability ID: CVE-2026-23886
CVSS Score: 5.3
Published: 2026-01-21

A denial-of-service vulnerability in Swift OTel and W3C TraceContext libraries caused by strict input validation that triggers a process crash via preconditionFailure(). A single HTTP request with an uppercase character in the traceparent header is enough to kill the server.

TL;DR

The Swift OpenTelemetry implementation took the W3C spec too literally. By assuming all traceparent headers would be lowercase hex, the parser used a fatal error for anything else. Attackers can crash the entire backend process by sending a single header containing an uppercase letter (e.g., 'A').

⚠️ Exploit Status: POC

Technical Details

  • CWE ID: CWE-20
  • Attack Vector: Network (HTTP)
  • CVSS Score: 5.3 (Medium)
  • Impact: Denial of Service (Process Crash)
  • Exploit Status: Trivial / PoC Available
  • EPSS Score: 0.00099

Affected Systems

  • Swift applications using OpenTelemetry
  • Swift applications using W3C Trace Context headers
  • Server-side Swift middleware
  • swift-w3c-trace-context: < 1.0.0-beta.5 (Fixed in: 1.0.0-beta.5)
  • swift-otel: < 1.0.4 (Fixed in: 1.0.4)

Code Analysis

Commit: 5da9b14

Replace preconditionFailure with throwing error for invalid hex chars

- preconditionFailure()
+ throw TraceParentDecodingError(.invalidCharacter(major))
Enter fullscreen mode Exit fullscreen mode

Exploit Details

  • GitHub Advisory: Official advisory containing reproduction steps and malformed string examples.

Mitigation Strategies

  • Input Sanitization at the Edge
  • Dependency Updates
  • Disable Tracing Middleware (Temporary)

Remediation Steps:

  1. Open your Package.swift file.
  2. Update swift-w3c-trace-context dependency to at least 1.0.0-beta.5.
  3. Update swift-otel dependency to at least 1.0.4.
  4. Run swift package update to resolve the new versions.
  5. Rebuild and redeploy the application.

References

Read the full report for CVE-2026-23886 on our website for more details including interactive diagrams and full exploit analysis.

Top comments (0)