CVE-2026-93599: rustls-webpki before 0.103.13 Panic via empty BIT STRING
rustls-webpki through 0.103.12 (and 0.104.0-alpha releases before 0.104.0-alpha.7) contains a reachable panic in bitstringflags() in src/der.rs. The input guard fails to reject a named-bit BIT STRING whose content is exactly [0x00] (zero padding bits and no data bytes), so rawbits.len() - 1 underflows on the empty slice and the subsequent index operation panics (subtract-with-overflow in debug, index-out-of-bounds in release). The condition is reachable through the public API BorrowedCertRevocationList::fromder() when a CRL contains an issuingDistributionPoint extension with such an onlySomeReasons value. Exploitation requires an application that explicitly opts in to CRL revocation checking by passing RevocationOptions to verifyforusage() and that parses CRL bytes obtained from a source the attacker can influence; the default rustls configuration, which does not use RevocationOptions, is unaffected. A crafted CRL causes a denial of service via the panic. Fixed in 0.103.13 and 0.104.0-alpha.7.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
rustls-webpkito a version that resolves this vulnerability.Fixed in 0.103.13 - Upgrade
Upgrade
rustls-webpkito a version that resolves this vulnerability.Fixed in 0.104.0-alpha.7
Event History
Frequently Asked Questions
Which deployments are exposed to this denial of service?
Only applications that explicitly enable CRL revocation checking by supplying RevocationOptions to verify_for_usage() are exposed. The application must also parse CRL data from an attacker-influenced source; the default rustls configuration does not use RevocationOptions and is unaffected.
What input is required to trigger the panic?
An attacker must provide a CRL containing an issuingDistributionPoint extension whose onlySomeReasons value is a named-bit BIT STRING with content exactly [0x00]. Processing that CRL through BorrowedCertRevocationList::from_der() can trigger the panic.
What should teams do if they cannot update immediately?
Avoid parsing attacker-influenced CRLs while CRL revocation checking is enabled, or disable the explicit RevocationOptions-based CRL checking path. Updating to 0.103.13 or 0.104.0-alpha.7 fixes the issue.