PT-2026-42645 — Insufficient Verification of Data Authenticity in Crates.Io P3-Challenger

PT-2026-42645 · Crates.Io · P3-Challenger

Published

2026-05-21

Updated

2026-05-21

CVSS v4.0

8.9

High

Vector

AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:H/SA:N

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:

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.
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.
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.

Included in v0.4.3 and v0.5.3

Fix

Insufficient Verification of Data Authenticity

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GHSA-VJ64-RJF3-W3V7

P3-Challenger