int EVP_DecryptUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out, int *outl,
                      const unsigned char *in, int inl)
{
    int fix_len;
    unsigned int b;

    if (ctx->cipher->flags & EVP_CIPH_FLAG_CUSTOM_CIPHER) {
        fix_len = ctx->cipher->do_cipher(ctx, out, in, inl);
        if (fix_len < 0) {
            *outl = 0;
            return 0;
        } else
            *outl = fix_len;
        return 1;
    }

    if (inl <= 0) {
        *outl = 0;
        return inl == 0;
    }

    if (ctx->flags & EVP_CIPH_NO_PADDING)
        return EVP_EncryptUpdate(ctx, out, outl, in, inl);

    b = ctx->cipher->block_size;
    OPENSSL_assert(b <= sizeof ctx->final);

    if (ctx->final_used) {
        memcpy(out, ctx->final, b);
        out += b;
        fix_len = 1;
    } else
        fix_len = 0;

    if (!EVP_EncryptUpdate(ctx, out, outl, in, inl))
        return 0;

    /*
     * if we have 'decrypted' a multiple of block size, make sure we have a
     * copy of this last block
     */
    if (b > 1 && !ctx->buf_len) {
        *outl -= b;
        ctx->final_used = 1;
        memcpy(ctx->final, &out[*outl], b);
    } else
        ctx->final_used = 0;

    if (fix_len)
        *outl += b;

    return 1;
}

This weakness involves numeric computation errors, such as integer overflows, underflows, or precision losses, which can lead to miscalculations and exploitable behaviors in software.