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’).

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⚠️ 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))

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

• GHSA Advisory
• NVD Record

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