Critical Vulnerabilities Discovered in rustls-webpki Cryptography Library: Panic and URI Validation Flaws Exposed

A security audit has identified three vulnerabilities in rustls-webpki, a widely deployed Rust cryptography library critical to TLS implementations. The most severe issue—cataloged as RUSTSEC-2026-0104—allows a reachable panic during certificate revocation list (CRL) parsing, occurring before signature verification completes. This creates a denial-of-service vector that attackers could exploit by delivering malformed CRL data to affected applications.

The flaw specifically stems from mishandling syntactically valid empty BIT STRING structures within the `onlySomeReasons` element of the `IssuingDistributionPoint` CRL extension. Any application that processes CRLs becomes vulnerable, though those relying solely on OCSP stapling or other revocation methods remain unaffected. The second vulnerability, RUSTSEC-2026-0098, involves incorrect acceptance of name constraints for URI names—effectively bypassing a security boundary meant to restrict certificate authority authorization scopes. A third undisclosed vulnerability rounds out the audit findings.

Maintainers have released patched versions: >=0.103.13, <0.104.0-alpha.1, and >=0.104.0-alpha.7 address the CRL parsing panic. The URI constraint flaw and third vulnerability patch status remain unspecified in current documentation. Organizations deploying rustls-webpki in production TLS stacks should verify their dependency versions immediately, particularly in environments where external parties can trigger CRL validation. The library's role as a foundational component in numerous Rust-based secure communications frameworks amplifies the potential blast radius of these findings.