libcrux-chacha20poly1305: Potential Panic on Overlong Ciphertext Buffer

A vulnerability exists in libcrux-chacha20poly1305 versions prior to 0.0.8 where passing a ciphertext buffer with a length greater than ptxt.len() + TAG_LEN to the encrypt or xchacha20_poly1305::encrypt functions triggers a panic. If an attacker can control the length of the ciphertext buffer, this can be exploited to crash the application. The vulnerability was reported on May 19, 2026. The fix ensures that the encrypt functions no longer panic, instead writing the ciphertext and tag into the first ptxt.len() + TAG_LEN bytes of the provided buffer. This prevents denial-of-service attacks by crashing applications using the vulnerable library.

Attack Chain

1. The attacker identifies an application using a vulnerable version of libcrux-chacha20poly1305 (versions < 0.0.8).
2. The attacker locates the code where libcrux_chacha20poly1305::encrypt or libcrux_chacha20poly1305::xchacha20_poly1305::encrypt are called.
3. The attacker determines how to influence the length of the ciphertext buffer passed to the vulnerable function.
4. The attacker crafts a request or input that provides a ciphertext buffer length greater than ptxt.len() + TAG_LEN.
5. The application calls the vulnerable encrypt function with the attacker-controlled ciphertext buffer.
6. The encrypt function attempts to write beyond the bounds of the intended ciphertext, causing a panic.
7. The Rust runtime unwinds the stack and terminates the affected thread or process.
8. The application crashes, resulting in a denial-of-service.

Impact

Successful exploitation of this vulnerability results in a denial-of-service condition. An attacker can potentially crash any application utilizing a vulnerable version of the libcrux-chacha20poly1305 library if they can control the length of the ciphertext buffer. The number of affected applications is currently unknown, but any application using the vulnerable versions of the library is susceptible. This could impact services relying on the availability of applications utilizing this library, causing service interruptions.

Recommendation

• Upgrade the libcrux-chacha20poly1305 dependency to version 0.0.8 or later to remediate the vulnerability.
• Monitor application logs for unexpected crashes related to libcrux-chacha20poly1305 functions.