CVE-2026-93599 in rustls-webpki
Summary
by MITRE • 09/18/2026
rustls-webpki through 0.103.12 (and 0.104.0-alpha releases before 0.104.0-alpha.7) contains a reachable panic in bit_string_flags() 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 raw_bits.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::from_der() 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 verify_for_usage() 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.
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Analysis
by VulDB Data Team • 09/18/2026
The vulnerability identified in rustls-webpki versions through 0.103.12 and pre-release versions prior to 0.104.0-alpha.7 constitutes a critical denial of service flaw rooted in improper input validation within the DER parsing logic. Specifically, the defect resides in the bit_string_flags function located in src/der.rs, where the implementation fails to adequately guard against malformed BIT STRING structures. The technical root cause is an integer underflow that occurs when processing a named-bit BIT STRING containing exactly one byte with the value 0x00. This specific byte sequence represents zero padding bits and no actual data bytes, which should be rejected as invalid according to standard DER encoding rules for this context. However, the existing input guard does not detect this edge case, allowing the parsing logic to proceed incorrectly.
When the parser encounters this malformed structure, it attempts to calculate the length of raw_bits by subtracting one from its current length. Since the slice contains only a single byte representing padding, subtracting one results in an underflow condition for unsigned integers. In debug builds, this manifests as a panic due to arithmetic overflow checks being active. In release builds, where such safety checks are typically disabled for performance reasons, the operation leads to an index-out-of-bounds error because the resulting length is interpreted as a very large positive number or wraps around incorrectly, causing subsequent array access operations to fail catastrophically. This behavior results in an immediate termination of the process handling the certificate validation, effectively creating a denial of service condition for any application relying on this library instance.
The exploitability of this vulnerability is contingent upon specific configuration choices within the consuming application. The flaw is reachable through the public API method BorrowedCertRevocationList::from_der() when processing Certificate Revocation Lists that include an issuingDistributionPoint extension containing a malformed onlySomeReasons value derived from the vulnerable BIT STRING structure. Crucially, exploitation requires that the target application has explicitly opted into CRL revocation checking by passing specific RevocationOptions to the verify_for_usage function. Applications using the default rustls configuration, which does not utilize these advanced revocation options, remain unaffected as they do not trigger the code path containing the vulnerability. Furthermore, the attacker must have control over or influence on the source of the CRL bytes being parsed, allowing them to inject the crafted malicious payload into the validation pipeline.
From a security classification perspective, this issue aligns with CWE-190 Integer Overflow or Wraparound and CWE-20 Improper Input Validation, as it stems from failing to validate the structural integrity of encoded data before performing arithmetic operations on its length. In terms of attack vectors, this vulnerability facilitates Denial of Service against services that perform certificate revocation checking, which is a critical component in maintaining trust chains for TLS connections. This maps to MITRE ATT&CK technique T1499 Endpoint Denial of Service, where the attacker leverages software vulnerabilities to disrupt service availability rather than compromising confidentiality or integrity directly. The impact is severe for services that rely on strict CRL validation and are exposed to untrusted certificate authorities or compromised distribution points, as a single malformed packet can crash the TLS handshake process.
Mitigation strategies primarily involve upgrading rustls-webpki to version 0.103.13 or later, which includes patches addressing this underflow condition by properly rejecting BIT STRING structures that do not meet expected length requirements after padding removal. For applications unable to upgrade immediately, a temporary workaround involves disabling CRL revocation checking if the security posture permits it, thereby avoiding the execution of the vulnerable code path entirely. However, disabling revocation checks reduces overall security and should only be considered as an emergency measure in controlled environments where alternative risk mitigation strategies are in place. Developers integrating rustls-webpki into their projects must ensure that dependency management systems automatically pull these patched versions to prevent exposure to this denial of service vector.