The following Rust program demonstrates some strangeness in AES encryption - if you have an immutable key slice and then operate on that slice, you get different encryption output than if you operate on a copy of that key.

For these functions, we expect that extending a 16 byte key to a 32 byte key by repeating it gives the same encrypted data, because the underlying rust-crypto functions repeat key data up to the necessary key size for the cipher.

use crypto::{
    aes, blockmodes, buffer,
    buffer::{BufferResult, ReadBuffer, WriteBuffer},
    symmetriccipher,
};

fn encrypt(
    key: &[u8],
    iv: &[u8],
    data: &str,
) -> Result<String, symmetriccipher::SymmetricCipherError> {
    let mut encryptor =
        aes::cbc_encryptor(aes::KeySize::KeySize256, key, iv, blockmodes::PkcsPadding);

    let mut encrypted_data = Vec::<u8>::new();
    let mut read_buffer = buffer::RefReadBuffer::new(data.as_bytes());
    let mut buffer = [0; 4096];
    let mut write_buffer = buffer::RefWriteBuffer::new(&mut buffer);

    loop {
        let result = encryptor.encrypt(&mut read_buffer, &mut write_buffer, true)?;

        encrypted_data.extend(
            write_buffer
                .take_read_buffer()
                .take_remaining()
                .iter()
                .copied(),
        );

        match result {
            BufferResult::BufferUnderflow => break,
            BufferResult::BufferOverflow => {}
        }
    }

    Ok(hex::encode(encrypted_data))
}

fn working() {
    let data = "data";
    let iv = [
        0xF0, 0xF1, 0xF2, 0xF3, 0xF4, 0xF5, 0xF6, 0xF7, 0xF8, 0xF9, 0xFA, 0xFB, 0xFC, 0xFD, 0xFE,
        0xFF,
    ];
    let key = [
        0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E,
        0x0F,
    ];
    // The copy here makes the code work.
    let key_copy = key;
    let key2: Vec<u8> = key_copy.iter().cycle().take(32).copied().collect();
    println!("key1:{} key2: {}", hex::encode(&key), hex::encode(&key2));

    let x1 = encrypt(&key, &iv, data).unwrap();
    println!("X1: {}", x1);

    let x2 = encrypt(&key2, &iv, data).unwrap();
    println!("X2: {}", x2);

    assert_eq!(x1, x2);
}

fn broken() {
    let data = "data";
    let iv = [
        0xF0, 0xF1, 0xF2, 0xF3, 0xF4, 0xF5, 0xF6, 0xF7, 0xF8, 0xF9, 0xFA, 0xFB, 0xFC, 0xFD, 0xFE,
        0xFF,
    ];
    let key = [
        0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E,
        0x0F,
    ];
    // This operation shouldn't affect the contents of key at all.
    let key2: Vec<u8> = key.iter().cycle().take(32).copied().collect();
    println!("key1:{} key2: {}", hex::encode(&key), hex::encode(&key2));

    let x1 = encrypt(&key, &iv, data).unwrap();
    println!("X1: {}", x1);

    let x2 = encrypt(&key2, &iv, data).unwrap();
    println!("X2: {}", x2);

    assert_eq!(x1, x2);
}

fn main() {
    working();
    broken();
}

The output from this program:

     Running `target/host/debug/rust-crypto-test`
key1:000102030405060708090a0b0c0d0e0f key2: 000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f
X1: 90462bbe32965c8e7ea0addbbed4cddb
X2: 90462bbe32965c8e7ea0addbbed4cddb
key1:000102030405060708090a0b0c0d0e0f key2: 000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f
X1: 26e847e5e7df1947bf82a650548a7d5b
X2: 90462bbe32965c8e7ea0addbbed4cddb
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `"26e847e5e7df1947bf82a650548a7d5b"`,
 right: `"90462bbe32965c8e7ea0addbbed4cddb"`', src/main.rs:83:5

Notably, the X1 key in the broken() test changes every time after rerunning the program.

The Rust Security Response WG was notified that the regex crate did not properly limit the complexity of the regular expressions (regex) it parses. An attacker could use this security issue to perform a denial of service, by sending a specially crafted regex to a service accepting untrusted regexes. No known vulnerability is present when parsing untrusted input with trusted regexes.

This issue has been assigned CVE-2022-24713. The severity of this vulnerability is "high" when the regex crate is used to parse untrusted regexes. Other uses of the regex crate are not affected by this vulnerability.

Overview

The regex crate features built-in mitigations to prevent denial of service attacks caused by untrusted regexes, or untrusted input matched by trusted regexes. Those (tunable) mitigations already provide sane defaults to prevent attacks. This guarantee is documented and it's considered part of the crate's API.

Unfortunately a bug was discovered in the mitigations designed to prevent untrusted regexes to take an arbitrary amount of time during parsing, and it's possible to craft regexes that bypass such mitigations. This makes it possible to perform denial of service attacks by sending specially crafted regexes to services accepting user-controlled, untrusted regexes.

Affected versions

All versions of the regex crate before or equal to 1.5.4 are affected by this issue. The fix is include starting from regex 1.5.5.

Mitigations

We recommend everyone accepting user-controlled regexes to upgrade immediately to the latest version of the regex crate.

Unfortunately there is no fixed set of problematic regexes, as there are practically infinite regexes that could be crafted to exploit this vulnerability. Because of this, we do not recommend denying known problematic regexes.

Acknowledgements

We want to thank Addison Crump for responsibly disclosing this to us according to the Rust security policy, and for helping review the fix.

We also want to thank Andrew Gallant for developing the fix, and Pietro Albini for coordinating the disclosure and writing this advisory.