CVE-2026-46654

Plonky3 MultiField32Challenger: transcript malleability and challenge entropy loss

Included in v0.4.3 and v0.5.3

• Key: challenger/src/multi_field_challenger.rs | MultiField32Challenger::duplexing | transcript_malleability

• Affected files: challenger/src/multi_field_challenger.rs, field/src/helpers.rs

• Violated invariant: The Fiat-Shamir sponge must bind challenges to the exact sequence of observed field elements. Specifically: (1) absorption must be injective, distinct observation streams must produce distinct sponge states, (2) squeezing must be injective, distinct PF rate cells must yield distinct F challenge sequences, and (3) all bits of each absorbed PF element must influence the sponge state.

• Exploit scenario: An attacker controlling prover-side observations can craft distinct transcripts that produce identical challenges, breaking the binding property of Fiat-Shamir. Three independent attack vectors exist:

  1. Partial-chunk aliasing (absorb): duplexing() packs input_buffer.chunks(num_f_elms) via reduce_32 (base 2^32) with no length marker and no zeroing of unused rate slots. Observing [x] followed by a sample yields the same sponge state as [x, 0, ..., 0] (padded to num_f_elms) followed by a sample, since reduce_32 treats missing high limbs identically to explicit zeros. The attacker can extend or truncate the tail of any observation batch without changing future challenges.

  2. Non-injective squeeze (squeeze): split_32 decomposes each PF rate cell into base-2^64 digits and maps each through TF::from_u64, which reduces mod F::ORDER (~2^31). Two distinct PF values whose base-2^64 digits differ only in their upper 33 bits produce identical F challenge sequences. This weakens the entropy of sampled challenges and can enable selective forgery when the attacker can influence the sponge state pre-squeeze.

  3. High-bit truncation (observe Hash/MerkleCap): num_f_elms = PF::bits() / 64 computes the number of F limbs per PF element. For BN254 (254-bit field), this yields 3 limbs covering 192 bits, the top 62 bits of every digest word are silently discarded. An attacker can find two distinct BN254 hash digests that differ only in bits 192–253 and observe them interchangeably without affecting challenges.

• Evidence: In duplexing(), the absorb path (reduce_32 with base 2^32) and the squeeze path (split_32 with base 2^64) use incompatible radices with no length domain separation. reduce_32 is a plain Horner fold acc * 2^32 + digit with no padding or tag, so trailing zeros are free. split_32 extracts u64 digits and casts each via TF::from_u64, which performs modular reduction, collapsing the top bits. The limb count PF::bits() / 64 is a floor division that silently drops all bits beyond 64 * num_f_elms for fields whose bit-width is not a multiple of 64.

Kodem intelligence

Already deployed Kodem?

1. What is CVE-2026-46654? CVE-2026-46654 is a high-severity security vulnerability in p3-challenger (rust), affecting versions < 0.4.3. It is fixed in 0.4.3, 0.5.3.
2. Which versions of p3-challenger are affected by CVE-2026-46654? p3-challenger (rust) versions < 0.4.3 is affected.
3. Is there a fix for CVE-2026-46654? Yes. CVE-2026-46654 is fixed in 0.4.3, 0.5.3. Upgrade to this version or later.
4. Is CVE-2026-46654 exploitable, and should I be worried? Whether CVE-2026-46654 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-46654 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-46654?
   • Upgrade p3-challenger to 0.4.3 or later
   • Upgrade p3-challenger to 0.5.3 or later